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LTC3412 2.5A, 4MHz, Monolithic Synchronous Step-Down Regulator
FEATURES

DESCRIPTIO
High Efficiency: Up to 95% 2.5A Output Current Low Quiescent Current: 62A Low RDS(ON) Internal Switches: 85m Programmable Frequency: 300kHz to 4MHz No Schottky Diode Required 2% Output Voltage Accuracy 0.8V Reference Allows Low Output Voltage Selectable Forced Continuous/Burst Mode Operation with Adjustable Burst Clamp Synchronizable Switching Frequency Low Dropout Operation: 100% Duty Cycle Power Good Output Voltage Monitor Overtemperature Protection Available in 16-Lead Thermally Enhanced TSSOP and QFN Packages
The LTC(R)3412 is a high efficiency monolithic synchronous, step-down DC/DC converter utilizing a constant frequency, current mode architecture. It operates from an input voltage range of 2.625V to 5.5V and provides an adjustable regulated output voltage from 0.8V to 5V while delivering up to 2.5A of output current. The internal synchronous power switch with 85m on-resistance increases efficiency and eliminates the need for an external Schottky diode. Switching frequency is set by an external resistor or can be sychronized to an external clock. 100% duty cycle provides low dropout operation extending battery life in portable systems. OPTI-LOOP(R) compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The LTC3412 can be configured for either Burst Mode(R) operation or forced continuous operation. Forced continuous operation reduces noise and RF interference while Burst Mode operation provides high efficiency by reducing gate charge losses at light loads. In Burst Mode operation, external control of the burst clamp level allows the output voltage ripple to be adjusted according to the requirements of the application. To further maximize battery life, the P-channel MOSFET is turned on continuously in dropout (100% duty cycle).
, LT, LTC, LTM, Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

Portable Instruments Battery-Powered Equipment Notebook Computers Distributed Power Systems Cellular Telephones Digital Cameras
TYPICAL APPLICATIO
VIN 2.7V TO 5.5V SVIN RT 4.7M 309k
Efficiency vs Load Current
22F PVIN PGOOD SW LTC3412 RUN/SS 470pF 15k 1000pF 100pF PGND SGND ITH SYNC/MODE VFB 110k 75k 392k
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100 VOUT 2.5V 2.5A EFFICIENCY (%)
1H 100F
80 Burst Mode OPERATION 60 FORCED CONTINUOUS 40
20 VIN = 3.3V VOUT = 2.5V 0.01 0.1 1 LOAD CURRENT (A) 10
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0 0.001
Figure 1. 2.5V, 2.5A Step-Down Regulator
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LTC3412
ABSOLUTE
AXI U RATI GS (Note 1)
Operating Temperature Range (Note 2) ....................................... - 40C to 85C Junction Temperature (Note 5) ............................. 125C Lead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300C
Input Supply Voltage ...................................- 0.3V to 6V ITH, RUN, VFB Voltages ............................... - 0.3V to VIN SYNC/MODE Voltages ................................ - 0.3V to VIN SW Voltage ................................... - 0.3V to (VIN + 0.3V) Peak SW Sink and Source Current ......................... 6.5A
PACKAGE/ORDER I FOR ATIO
TOP VIEW SVIN PGOOD ITH VFB RT SYNC/MODE RUN/SS SGND 1 2 3 4 5 6 7 8 17 16 PVIN 15 SW 14 SW 13 PGND 12 PGND 11 SW 10 SW 9 PVIN
SYNC/MODE
VFB
16 15 14 13 RUN/SS 1 SGND 2 PVIN 3 SW 4 5 SW 6 PGND 7 PGND 8 SW 17 12 PGOOD 11 SVIN 10 PVIN 9 SW
FE PACKAGE 16-LEAD PLASTIC TSSOP EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB TJMAX = 125C, JA = 37.6C/W, JC = 10C/W
UF PACKAGE 16-LEAD (4mm x 4mm) PLASTIC QFN EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB TJMAX = 125C, JA = 34C/W, JC = 1C/W
ORDER PART NUMBER LTC3412EFE LTC3412IFE
FE PART MARKING 3412EFE 3412IFE
ORDER PART NUMBER LTC3412EUF
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V unless otherwise specified.
SYMBOL SVIN VFB IFB VFB VLOADREG VPGOOD RPGOOD IQ PARAMETER Signal Input Voltage Range Regulated Feedback Voltage Voltage Feedback Leakage Current Reference Voltage Line Regulation Output Voltage Load Regulation Power Good Range Power Good Pull-Down Resistance Input DC Bias Current Active Current Sleep Shutdown CONDITIONS (Note 3) VIN = 2.7V to 5.5V (Note 3) Measured in Servo Loop, VITH = 0.36V Measured in Servo Loop, VITH = 0.84V

ELECTRICAL CHARACTERISTICS
MIN 2.625 0.784
ITH
RT
(Note 4) VFB = 0.78V, VITH = 1V VFB = 1V, VITH = 0V VRUN = 0V, VMODE = 0V
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TOP VIEW
UF PART MARKING 3412
TYP 0.800 0.1 0.04 0.02 - 0.02 7.5 120 250 62 0.02
MAX 5.5 0.816 0.4 0.2 0.2 - 0.2 9 200 330 80 1
UNITS V V A %/V % % % A A A
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LTC3412
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V unless otherwise specified.
SYMBOL fOSC fSYNC RPFET RNFET ILIMIT VUVLO ILSW VRUN IRUN PARAMETER Switching Frequency Switching Frequency Range SYNC Capture Range RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET Peak Current Limit Undervoltage Lockout Threshold SW Leakage Current RUN Threshold RUN/SS Leakage Current CONDITIONS ROSC = 309k (Note 6) (Note 6) ISW = 1A (Note 7) ISW = -1A (Note 7) MIN 0.88 0.3 0.3 TYP 0.95 MAX 1.1 4 4 110 90 2.625 1 0.8 1 UNITS MHz MHz MHz m m A V A V A
ELECTRICAL CHARACTERISTICS
4 2.375 VRUN = 0V, VIN = 5.5V 0.5
85 65 5.4 2.500 0.1 0.65
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3412E is guaranteed to meet performance specifications from 0C to 85C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3412I is guaranteed to meet specified performance over the - 40C to 85C temperature range.
Note 3: The LTC3412 is tested in a feedback loop that adjusts VFB to achieve a specified error amplifier output voltage (ITH). Note 4: Dynamic supply current is higher due to the internal gate charge being delivered at the switching frequency. Note 5: TJ is calculated from the ambient temperature TA and power dissipation as follows: LTC3412: TJ = TA + PD (37.6C/W). Note 6: 4MHz operation is guaranteed by design and not production tested. Note 7: Switch on resistance is guaranteed by design and test correlation in the UF package and by production test in the FE package.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100 100 90 80 Burst Mode OPERATION 80
EFFICIENCY (%)
EFFICIENCY (%)
60 FORCED CONTINUOUS 40
60 50 40 30
EFFICIENCY (%)
20 VIN = 3.3V VOUT = 2.5V 0.01 0.1 1 LOAD CURRENT (A) 10
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0 0.001
UW
Efficiency vs Load Current
100 90 VIN = 3.3V VIN = 5V 80 70 60 50 40 30 VOUT = 2.5V 1MHz Burst Mode OPERATION 0.01 0.1 1 LOAD CURRENT (A) 10
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Efficiency vs Load Current
70
VIN = 3.3V
VIN = 5V
20 10
20 10 0 0.001
0 0.001
VOUT = 2.5V 1MHz FORCED CONTINUOUS 0.01 0.1 1 LOAD CURRENT (A) 10
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LTC3412 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage
98 LOAD = 100mA 96
EFFICIENCY (%)
EFFICIENCY (%)
1H 0.47H 2.2H VIN = 3.3V VOUT = 2.5V LOAD = 1A Burst Mode OPERATION 800 1300 1800 2300 2800 3300 3800 FREQUENCY (kHz)
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%VOUT/VOUT
94 92 90 88 LOAD = 2.5A
LOAD = 1A 95 94 93 92
86 2.55
VOUT = 2.5V 1MHz Burst Mode OPERATION 3.05 3.55 4.05 4.55 INPUT VOLTAGE (V) 5.05
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Burst Mode Operation
VOUT 100mV/DIV
VOUT 20mV/DIV
IL 1A/DIV
IL 200mA/DIV
4s/DIV VIN = 3.3V, VOUT = 2.5V LOAD = 50mA
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20s/DIV
IL 1A/DIV
VOUT 100mV/DIV
Start-Up, Burst Mode Operation
0.7960 VOUT 1V/DIV 0.7955
REFERENCE VOLTAGE (V)
ON-RESISTANCE (m)
VRUN 1V/DIV
IL 1A/DIV
1ms/DIV VIN = 3.3V, VOUT = 2.5V LOAD = 1
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Efficiency vs Frequency
97 96 0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 -0.14 -0.16 -0.18
Load Regulation
VIN = 3.3V VOUT = 2.5V
91 300
0
0.5
1 1.5 LOAD CURRENT (A)
2
2.5
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Load Step Transient Forced Continuous
Load Step Transient Burst Mode Operation
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VIN = 3.3V, VOUT = 2.5V LOAD STEP = NO LOAD TO 2.5A
20s/DIV VIN = 3.3V, VOUT = 2.5V LOAD STEP = 50mA TO 2.5A
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Reference Voltage vs Temperature
120 VIN = 3.3V 100 80 60 40 20 0 15 35 55 75 95 115 120 TEMPERATURE (C)
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Switch On-Resistance vs Input Voltage
0.7950 0.7945 0.7940 0.7935 0.7930 0.7925 0.7920 -45 -25 -5
PFET ON-RESISTANCE
NFET ON-RESISTANCE
2.5
3
3.5 4 INPUT VOLTAGE (V)
4.5
5
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LTC3412 TYPICAL PERFOR A CE CHARACTERISTICS
Switch On-Resistance vs Temperature
120 110 100
ON-RESISTANCE (m)
VIN = 3.3V
FREQUENCY (kHz)
90 80 70 60 50 40 30 20 -40 -20 NFET ON-RESISTANCE
LEAKAGE CURRENT (nA)
PFET ON-RESISTANCE
0
20 40 60 80 TEMPERATURE (C)
Frequency vs Input Voltage
1050 1040
FREQUENCY (kHz)
R = 309k
1030 1020 1010 1000 990 2.5
FREQUENCY (kHz)
1004 1002 1000 998 996 994 992
DC SUPPLY CURRENT (A)
3
3.5 4 4.5 INPUT VOLTAGE (V)
DC Supply Current vs Temperature
VIN = 3.3V 300
SUPPLY CURRENT (A)
ACTIVE
MINIMUM PEAK INDUCTOR CURRENT (mA)
350
2500 2000 1500 1000 500 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 BURST CLAMP VOLTAGE (V) 1
CURRENT LIMIT (A)
250 200 150 100 SLEEP 50 0 -40 -20
0
20 40 60 80 TEMPERATURE (C)
UW
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Switch Leakage vs Input Voltage
2.5 4500 4000 2.0 3500 3000 2500 2000 1500 1000 500 5 5.5
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Frequency vs ROSC
VIN = 3.3V
1.5
1.0 SYNCHRONOUS SWITCH 0.5 MAIN SWITCH 0
100 120
2.5
3
3.5 4 4.5 INPUT VOLTAGE (V)
0 50 150 250 350 450 550 650 750 850 950 ROSC (k)
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Switching Frequency vs Temperature
1010 1008 1006 VIN = 3.3V 300 250 200 150 100 350
DC Supply Current vs Input Voltage
ACTIVE
SLEEP 50 0 20 40 60 80 TEMPERATURE (C) 100 120
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5
5.5
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990 -40 -20
2.5
3
3.5 4 4.5 INPUT VOLTAGE (V)
5
5.5
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Minimum Peak Inductor Current vs Burst Clamp Voltage
4000 3500 3000 VIN = 3.3V
Current Limit vs Input Voltage
6.8 6.6 6.4 6.2 6.0 5.8 5.6 5.4 2.75
100 120
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3.25
4.75 4.25 3.75 INPUT VOLTAGE (V)
5.25
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LTC3412
PI FU CTIO S
SVIN (Pin 1/Pin 11): Signal Input Supply. Decouple this pin to SGND with a capacitor. Normally SVIN is equal to PVIN. SVIN can be greater than PVIN but keep the voltage difference between SVIN and PVIN less than 0.5V. PGOOD (Pin 2/Pin 12): Power Good Output. Open-drain logic output that is pulled to ground when the output voltage is not within 7.5% of regulation point. ITH (Pin 3/Pin 13): Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is from 0.2V to 1.4V with 0.2V corresponding to the zero-sense voltage (zero current). VFB (Pin 4/Pin 14): Feedback Pin. Receives the feedback voltage from a resistive divider connected across the output. RT (Pin 5/Pin 15): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. SYNC/MODE (Pin 6/Pin 16): Mode Select and External Clock Synchronization Input. To select forced continuous, tie to SVIN. Connecting this pin to a voltage between 0V and 1V selects Burst Mode operation with the burst clamp set to the pin voltage.
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(FE/UH Package)
RUN/SS (Pin 7/Pin 1): Run Control and Soft-Start Input. Forcing this pin below 0.5V shuts down the LTC3412. In shutdown all functions are disabled drawing < 1A of supply current. A capacitor to ground from this pin sets the ramp time to full output current. SGND (Pin 8/Pin 2): Signal Ground. All small-signal components, compensation components and the exposed pad on the bottom side of the IC should connect to this ground, which in turn connects to PGND at one point. PVIN (Pins 9, 16/Pins 3, 10): Power Input Supply. Decouple this pin to PGND with a capacitor. SW (Pins 10, 11, 14, 15/Pins 4, 5, 8, 9): Switch Node Connection to the Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. PGND (Pins 12, 13/Pins 6, 7): Power Ground. Connect this pin close to the (-) terminal of CIN and COUT. Exposed Pad (Pin 17/Pin 17): Signal Ground. Must be soldered to PCB for electrical connection and thermal performance.
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LTC3412
FU CTIO AL BLOCK DIAGRA
SVIN 1 SGND 8 ITH 3
VOLTAGE REFERENCE 0.8V
+ -
VFB 4
-
ERROR AMPLIFIER SYNC/MODE
0.74V
+ -
+ -
+
+
RUN/SS 7 RUN 0.86V PGOOD 2 NMOS CURRENT COMPARATOR N-CH
-
OPERATIO
Main Control Loop The LTC3412 is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation, the internal top power switch (P-channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the current comparator trips and turns off the top power MOSFET. The peak inductor current at which the current comparator shuts off the top power switch is controlled by the voltage on the ITH pin. The error amplifier adjusts the voltage on the ITH pin by comparing the feedback signal from a resistor divider on the VFB pin with an internal 0.8V reference. When the load
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PVIN 9 16 SLOPE COMPENSATION RECOVERY BCLAMP PMOS CURRENT COMPARATOR
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+ -
P-CH
BURST COMPARATOR 10 SLOPE COMPENSATION 11 SW 14 15
OSCILLATOR
LOGIC
+ - - +
REVERSE CURRENT COMPARATOR 5 RT 6 SYNC/MODE
12 PGND 13
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current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the ITH voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-channel MOSFET) turns on until either the bottom current limit is reached or the beginning of the next clock cycle. The bottom current limit is set at -2A for forced continuous mode and 0A for Burst Mode operation. The operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 4MHz.
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LTC3412
OPERATIO
Overvoltage and undervoltage comparators will pull the PGOOD output low if the output voltage comes out of regulation by 7.5%. In an overvoltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the overvoltage condition clears or the bottom MOSFET's current limit is reached. Forced Continuous Mode Connecting the SYNC/MODE pin to SVIN will disable Burst Mode operation and force continuous current operation. At light loads, forced continuous mode operation is less efficient than Burst Mode operation but may be desirable in some applications where it is necessary to keep switching harmonics out of a signal band. The output voltage ripple is minimized in this mode. Burst Mode Operation Connecting the SYNC/MODE pin to a voltage between 0V to 1V enables Burst Mode operation. In Burst Mode operation, the internal power MOSFETs operate intermittently at light loads. This increases efficiency by minimizing switching losses. During Burst Mode operation, the minimum peak inductor current is externally set by the voltage on the SYNC/MODE pin and the voltage on the ITH pin is monitored by the burst comparator to determine when sleep mode is enabled and disabled. When the average inductor current is greater than the load current, the voltage on the ITH pin drops. As the ITH voltage falls below 150mV, the burst comparator trips and enables sleep mode. During sleep mode, the top MOSFET is held off and the ITH pin is disconnected from the output of the error amplifier. The majority of the internal circuitry is also turned off to reduce the quiescent current to 62A while the load current is solely supplied by the output capacitor. When the output voltage drops, the ITH pin is reconnected to the output of the error amplifier and the top power MOSFET along with all the internal circuitry is switched back on. This process repeats at a rate that is dependent on the load demand. Pulse skipping operation can be implemented by connecting the SYNC/MODE pin to ground. This forces the burst clamp level to be at 0V. As the load current decreases, the Frequency Synchronization The internal oscillator of the LTC3412 can be synchronized to an external clock connected to the SYNC/MODE pin. The frequency of the external clock can be in the range of 300kHz to 4MHz. For this application, the oscillator timing resistor should be chosen to correspond to a frequency that is 25% lower than the synchronization frequency. During synchronization, the burst clamp is set to 0V and each switching cycle begins at the falling edge of the external clock signal. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-channel MOSFET and the inductor. Low Supply Operation The LTC3412 is designed to operate down to an input supply voltage of 2.625V. One important consideration at low input supply voltages is that the RDS(ON) of the Pchannel and N-channel power switches increases. The user should calculate the power dissipation when the LTC3412 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at duty cycles greater than 50%. It is accomplished
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peak inductor current will be determined by the voltage on the ITH pin until the ITH voltage drops below 200mV. At this point, the peak inductor current is determined by the minimum on-time of the current comparator. If the load demand is less than the average of the minimum on-time inductor current, switching cycles will be skipped to keep the output voltage in regulation.
LTC3412
OPERATIO
internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the LTC3412, however, slope compensation recovery is implemented to keep the maximum inductor peak current constant throughout the range of duty cycles. This keeps the maximum output current relatively constant regardless of duty cycle.
APPLICATIO S I FOR ATIO
The basic LTC3412 application circuit is shown in Figure 1. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge and switching losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency of the LTC3412 is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation:
ROSC = 3.23 * 1011 f(Hz) () - 10k
Although frequencies as high as 4MHz are possible, the minimum on-time of the LTC3412 imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 110ns. Therefore, the minimum duty cycle is equal to 100 * 110ns * f(Hz).
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Short-Circuit Protection When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. To prevent current runaway from occurring, a secondary current limit is imposed on the inductor current. If the inductor valley current increases larger than 4.8A, the top power MOSFET will be held off and switching cycles will be skipped until the inductor current falls to a safe level. Inductor Selection For a given input and output voltage, the inductor value and operating frequency determine the ripple current. The ripple current IL increases with higher VIN and decreases with higher inductance.
V V IL = OUT 1 - OUT VIN fL
Having a lower ripple current reduces the ESR losses in the output capacitors and the output voltage ripple. Highest efficiency operation is achieved at low frequency with small ripple current. This, however, requires a large inductor. A reasonable starting point for selecting the ripple current is IL = 0.4(IMAX). The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation:
V L = OUT fIL(MAX) VOUT 1- V IN(MAX)
The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase.
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LTC3412
APPLICATIO S I FOR ATIO
Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, mollypermalloy, or Kool M(R) cores. Actual core loss is independent of core size for a fixed inductor value but it is very dependent on the inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate energy but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko and Sumida. CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoidal current at the source of the top MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. RMS current is given by:
IRMS = IOUT (MAX)
VOUT VIN
VIN -1 VOUT
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not
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offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple and load step transients, as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, VOUT, is determined by:
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1 VOUT IL ESR + 8fC OUT
The output ripple is highest at maximum input voltage since IL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and
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LTC3412
APPLICATIO S I FOR ATIO
the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. Output Voltage Programming The output voltage is set by an external resistive divider according to the following equation: R2 VOUT = 0.8V 1 + R1 The resistive divider allows the VFB pin to sense a fraction of the output voltage as shown in Figure 2.
VOUT R2 VFB LTC3412 SGND
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R1
Figure 2. Setting the Output Voltage
Burst Clamp Programming If the voltage on the SYNC/MODE pin is less than VIN by 1V, Burst Mode operation is enabled. During Burst Mode operation, the voltage on the SYNC/MODE pin determines the burst clamp level which sets the minimum peak inductor current, IBURST, for each switching cycle according to the following equation: 3.75A IBURST = (VBURST - 0.2V ) 0.8V VBURST is the voltage on the SYNC/MODE pin. IBURST can be programmed in the range of 0A to 3.75A. For values of VBURST greater than 1V, IBURST is set at 3.75A. For values of VBURST less than 0.2V, IBURST is set at 0A. As the output load current drops, the peak inductor current decreases to keep the output voltage in regulation. When the output load current demands a peak inductor current that is less than IBURST, the burst clamp will force the peak inductor
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current to remain equal to IBURST regardless of further reductions in the load current. Since the average inductor current is greater than the output load current, the voltage on the ITH pin will decrease. When the ITH voltage drops to 150mV, sleep mode is enabled in which both power MOSFETs are shut off along with most of the circuitry to minimize power consumption. All circuitry is turned back on and the power MOSFETs begin switching again when the output voltage drops out of regulation. The value for IBURST is determined by the desired amount of output voltage ripple. As the value of IBURST increases, the sleep period between pulses and the output voltage ripple increase. The burst clamp voltage, VBURST, can be set by a resistor divider from the VFB pin to the SGND pin as shown in Figure 1. Pulse skipping, which is a compromise between low output voltage ripple and efficiency, can be implemented by connecting the SYNC/MODE pin to ground. This sets IBURST to 0A. In this condition, the peak inductor current is limited by the minimum on-time of the current comparator, and the lowest output voltage ripple is achieved while still operating discontinuously. During very light output loads, pulse skipping allows only a few switching cycles to be skipped while maintaining the output voltage in regulation. Frequency Synchronization The LTC3412's internal oscillator can be synchronized to an external clock signal. During synchronization, the top MOSFET turn-on is locked to the falling edge of the external frequency source. The synchronization frequency range is 300kHz to 4MHz. Synchronization only occurs if the external frequency is greater than the frequency set by the external resistor. Because slope compensation is generated by the oscillator's RC circuit, the external frequency should be set 25% higher than the frequency set by the external resistor to ensure that adequate slope compensation is present. Soft-Start The RUN/SS pin provides a means to shut down the LTC3412 as well as a timer for soft-start. Pulling the RUN/SS pin below 0.5V places the LTC3412 in a low quiescent current shutdown state (IQ < 1A).
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LTC3412
APPLICATIO S I FOR ATIO
The LTC3412 contains an internal soft-start clamp that gradually raises the clamp on ITH after the RUN/SS pin is pulled above 2V. The full current range becomes available on ITH after 1024 switching cycles. If a longer soft-start period is desired, the clamp on ITH can be set externally with a resistor and capacitor on the RUN/SS pin as shown in Figure 1. The soft-start duration can be calculated by using the following formula:
VIN tSS = RSSC SS ln (Seconds) VIN - 1.8V
Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence. 1. The VIN quiescent current is due to two components: the DC bias current as given in the electrical characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. In continuous mode, IGATECHG=f(QT + QB) where QT and QB are the gate charges of the internal top and bottom
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switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW and external inductor RL. In continuous mode the average output current flowing through inductor L is "chopped" between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% of the total loss. Thermal Considerations In most applications, the LTC3412 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3412 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3412 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(JA) where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature.
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LTC3412
APPLICATIO S I FOR ATIO
The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3412 in dropout at an input voltage of 3.3V, a load current of 2.5A and an ambient temperature of 70C. From the typical performance graph of switch resistance, the RDS(ON) of the Pchannel switch at 70C is approximately 97m. Therefore, power dissipated by the part is: PD = (ILOAD2)(RDS(ON)) = (2.5A)2(97m) = 0.61W For the TSSOP package, the JA is 37.6C/W. Thus the junction temperature of the regulator is: TJ = 70C + (0.61W)(37.6C/W) = 93C which is below the maximum junction temperature of 125C. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ILOAD(ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH pin external components and output capacitor shown in Figure 1 will provide adequate compensation for most applications. Design Example As a design example, consider using the LTC3412 in an application with the following specifications: VIN = 2.7V to 4.2V, VOUT = 2.5V, IOUT(MAX) = 2.5A, IOUT(MIN) = 10mA, f = 1MHz. Because efficiency is important at both high and low load current, Burst Mode operation will be utilized.
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First, calculate the timing resistor:
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ROSC =
3.23 * 1011 - 10k = 313k 1* 106
Use a standard value of 309k. Next, calculate the inductor value for about 40% ripple current at maximum VIN:
2.5V 2.5V L= = 1.01H 1- (1MHz)(1A) 4.2V
Using a 1H inductor, results in a maximum ripple current of:
2.5V 2.5V IL = = 1.01A 1- (1MHz)(1H) 4.2V
COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement and the bulk capacitance needed for loop stability. In this application, two tantalum capacitors will be used to provide the bulk capacitance and a ceramic capacitor in parallel to lower the total effective ESR. For this design, two 100F tantalum capacitors in parallel with a 10F ceramic capacitor will be used. CIN should be sized for a maximum current rating of:
2.5V 4.2V - 1 = 1.23ARMS IRMS = (2.5A ) 4.2V 2.5V
Decoupling the PVIN and SVIN pins with a 22F ceramic capacitor and a 220F tantalum capacitor is adequate for most applications. The burst clamp and output voltage can now be programmed by choosing the values of R1, R2 and R3. The voltage on the MODE pin will be set to 0.32V by the resistor divider consisting of R2 and R3. A burst clamp voltage of 0.32V will set the minimum inductor current, IBURST, as follows: 3.75V IBURST = (0.32V - 0.2V ) = 563mA 0.8V
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LTC3412
APPLICATIO S I FOR ATIO
If we set the sum of R2 and R3 to 185k, then the following equations can be solved:
R2 + R3 = 185k R2 0.8V 1+ = R3 0.32V
The last two equations shown result in the following values for R2 and R3: R2 = 110k , R3 = 75k. The value of R1 can now be determined by solving the equation shown below:
1+ R1 2.5V = 185k 0.8V R1 = 393k
A value of 392k will be selected for R1. Figure 4 shows the complete schematic for this design example. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3412. Check the following in your layout.
Top Side Figure 3. LTC3412 Layout Diagram
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1. A ground plane is recommended. If a ground plane layer is not used, the signal and power grounds should be segregated with all small-signal components returning to the SGND pin at one point which is then connected to the PGND pin close to the LTC3412. The exposed pad should be connected to SGND. 2. Connect the (+) terminal of the input capacitor(s), CIN, as close as possible to the PVIN pin. This capacitor provides the AC current into the internal power MOSFETs. 3. Keep the switching node, SW, away from all sensitive small-signal nodes. 4. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. You can connect the copper areas to any DC net (PVIN, SVIN, VOUT, PGND, SGND, or any other DC rail in your system). 5. Connect the VFB pin directly to the feedback resistors. The resistor divider must be connected between VOUT and SGND.
Bottom Side
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LTC3412
APPLICATIO S I FOR ATIO U
CFB 22pF X5R R1 392k CIN1 220F VIN 2.7V TO 4.2V 1 RPG 100k PGOOD CITH 680pF X7R RITH 7.15k CC 100pF 4 R3 75k R2 110k ROSC 309k 5 SVIN PGOOD ITH LTC3412 PGND VFB PGND RT SW SW PVIN 11 10 9 CIN2 22F X5R 6.3V *TOKO D62CB A920CY-1ROM **SANYO POSCAP 4TPB100M TAIYO YUDEN LMK325BJ106MN SANYO POSCAP 2R5TPC220M
3412 F04
RSS 4.7M
Figure 4. Single Lithium-Ion to 2.5V, 2.5A Regulator at 1MHz, Burst Mode Operation Using POSCAPs
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PVIN SW SW
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2 3
15 14 13 12 COUT2 10F L1* 1H
VOUT 2.5V 2.5A
6 SYNC/MODE 7 RUN SGND
+
CSS 470pF X7R 8
COUT1** 100F x2
GND
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LTC3412
TYPICAL APPLICATIO S
2.5V, 2.5A Regulator Using All Ceramic Capacitors
CIN3** 100F VIN 2.7V TO 5.5V
PGOOD
CITH 1000pF X7R RITH 15k CC 100pF
R3 75k RSS 4.7M
*TOKO D62CB A920CY-1ROM **TDK C4532X5R0J107M
1.8V, 2.5A Step-Down Regulator at 1MHz, Burst Mode Operation
VIN 3.3V R1 232k CIN1** 22F
PGOOD
CITH 560pF X7R RITH 10k C2 47pF
R3 75k
RSS 4.7M
*SUMIDA CR431R0 **AVX 12066D226MAT
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C1 22pF X5R R1 392k CIN1 22F X5R 6.3V
1 RPG 100k
SVIN PGOOD ITH LTC3412
PVIN SW SW PGND
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2 3
15 14 13 12 11 10 9 CIN2 22F X5R 6.3V
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4 R2 110k ROSC 309k 5 6 7 CSS 470pF X7R 8
VFB PGND RT SW SYNC/MODE RUN SGND SW PVIN
L1* 1H
VOUT 2.5V 2.5A
COUT** 100F
GND
C1 22pF X5R
1 RPG 100k
SVIN PGOOD ITH LTC3412
PVIN SW SW PGND
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2 3
15 14 13 12 11 10 9 CIN2 22F**
3412 TA05
4 R2 110k ROSC 309k 5 6 7 CSS 470pF X7R 8
VFB PGND RT SW SYNC/MODE RUN SGND SW PVIN
L1 1H*
VOUT 1.8V 2A COUT** 22F x2
GND
LTC3412
TYPICAL APPLICATIO S
2.5V, 2.5A Low Output Noise Regulator at 2MHz
VIN 3.3V
CIN3 0.1F X5R
PGOOD
CITH 1000pF X7R RITH 22.1k C1 56pF
RSS 4.7M
*VISHAY DALE IHLP-2525CZ-01 0.47 **TDK C4532X5R0J107M
EFFICIENCY (%)
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RIN 5
CFF 22pF X7R R1 392k CIN1** 100F
1 RPG 100k
SVIN PGOOD ITH LTC3412
PVIN SW SW PGND
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2 3
15 14 13 12 11 10 9 CIN2 100F**
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4 R2 182k ROSC 137k 5 6 7 CSS 470pF X7R 8
VFB PGND RT SW SYNC/MODE RUN SGND SW PVIN
L1 0.47H*
VOUT 2.5V 2.5A COUT** 100F x2
GND
Efficiency vs Load Current 2MHz, Low Noise
100 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) 10
3412 TA07
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LTC3412
TYPICAL APPLICATIO S
3.3V, 2.5A Step-Down Regulator at 1MHz, Forced Continuous Mode Operation
CIN3** 100F VIN 5V
PGOOD
CITH 1000pF X7R RITH 15k CC 100pF R2 200k ROSC 309k
*PULSE P1166.162T **TDK C4532X5R0J107M
Lithium-Ion to 3.3V, Single Inductor Buck-Boost Converter
CIN3** 100F x2 R1 576k CIN1 22F X5R 6.3V GND VIN 2.7V TO 4.2V
PGOOD
CITH 1000pF X7R RITH 15k C2 100pF
*TOKO D63CB **TDK C4532X5R0J107M
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C1 22pF X5R R1 634k CIN1 22F X5R 6.3V
1 RPG 100k
SVIN PGOOD ITH LTC3412
PVIN SW SW PGND
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2 3 4
15 14 13 12 11 10 9 CIN2 22F X5R 6.3V
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VFB 5 6 7 PGND RT SW SYNC/MODE RUN SGND SW PVIN
L1* 1H
VOUT 3.3V 2.5A
COUT** 100F
RSS 4.7M
CSS 470pF X7R 8
GND
C1 22pF
1 RPG 100k
SVIN PGOOD ITH LTC3412
PVIN SW SW PGND
16
2 3
15 14 13 12 11 10 9 CIN2 22F X5R 6.3V
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4 R3 75k RSS 4.7M R2 110k ROSC 309k 5 6 7 CSS 470pF X7R 8
VFB PGND RT SW SYNC/MODE RUN SGND SW PVIN
L1* 2H
D1 DIODES, INC. B320A VOUT 3.3V M1 SILICONIX Si2302DS COUT** 100F
GND VIN 2.7V 3V 3.5V 4.2V MAXIMUM IOUT 800mA 900mA 1.05A 1.2A
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LTC3412
PACKAGE DESCRIPTIO U
FE Package 16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BA
2.74 (.108) 4.90 - 5.10* (.193 - .201) 2.74 (.108) 16 1514 13 12 1110 9
6.60 0.10 4.50 0.10
SEE NOTE 4
2.74 (.108) 0.45 0.05 1.05 0.10 0.65 BSC 2.74 6.40 (.108) (.252) BSC
RECOMMENDED SOLDER PAD LAYOUT
12345678 1.10 (.0433) MAX
0 - 8
4.30 - 4.50* (.169 - .177)
0.25 REF
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
0.65 (.0256) BSC
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE16 (BA) TSSOP 0204
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
UF Package 16-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1692)
BOTTOM VIEW--EXPOSED PAD 4.00 0.10 (4 SIDES) 0.72 0.05 PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER
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16 0.55 0.20 1
2.15 0.05 (4 SIDES) 4.35 0.05 2.90 0.05
2.15 0.10 (4-SIDES)
2
PACKAGE OUTLINE 0.30 0.05 0.65 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 0.200 REF 0.00 - 0.05
(UF16) QFN 1004
0.30 0.05 0.65 BSC
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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LTC3412
TYPICAL APPLICATIO
2.5V, 2.5A Step-Down Regulator Synchronized to 1.25MHz
VIN 2.7V TO 5.5V
PGOOD
CITH 1000pF X7R RITH 15k CC 100pF
RSS 4.7M
*TOKO D62CB A920CY-1ROM **TDK C4532X5R0J107M
RELATED PARTS
PART NUMBER LTC1701/LTC1701B LTC1772/LTC1772B LTC1773 LTC1875 LTC1877 LTC1878 LTC1879 LTC3404 LTC3405A LTC3406/LTC3406B LTC3411 DESCRIPTION 700mA (IOUT), 1MHz Step-Down Converter Constant 550kHz Current Mode Step-Down DC/DC Controller Constant Frequency 550kHz Step-Down DC/DC Controller 1.5A (IOUT), 500kHz Synchronous Step-Down Converter 600mA (IOUT), 500kHz Synchronous Step-Down Converter 600mA (IOUT), 550kHz Synchronous Step-Down Converter 1.2A (IOUT), 550kHz Synchronous Step-Down Converter 600mA (IOUT), 1.4MHz Synchronous Step-Down Converter 300mA (IOUT), 1.5MHz Synchronous Step-Down Converter 600mA (IOUT), 1.5MHz Synchronous Step-Down Converter 1.25A (IOUT), 4MHz Synchronous Step-Down Converter COMMENTS VIN = 2.5V to 5V, B Version: Burst Mode Defeat, ThinSOTTM VIN = 2.5V to 9.8V, 94% Efficiency, 100% Duty Cycle, ThinSOT VIN = 2.65V to 8.5V, 95% Efficiency, VOUT from 0.8V to VIN, MSOP-10 VIN = 2.65V to 6V, 95% Efficiency, PLL, SSOP-16 VIN = 2.65V to 10V, 95% Efficiency, MSOP-8 VIN = 2.65V to 6V, 95% Efficiency, MSOP-8 VIN = 2.65V to 10V, 95% Efficiency, SSOP-16 VIN = 2.65V to 6V, 95% Efficiency, MSOP-8 VIN = 2.65V to 6V, 96% Efficiency, ThinSOT Package VIN = 2.5V to 5.5V, 95% Efficiency, ThinSOT, B Version: Burst Mode Defeat VIN = 2.5V to 5.5V, 95% Efficiency, MSOP-10
ThinSOT is a trademark of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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C1 22pF X5R R1 392k CIN1 22F X5R 6.3V CIN3** 100F 1 RPG 100k SVIN PGOOD ITH LTC3412 4 R2 182k 5 RT SW SW PVIN 11 10 9 CIN2 22F X5R 6.3V
3412 TA02
PVIN SW SW PGND
16
2 3
15 14 13 12 L1* 1H
VFB PGND
VOUT 2.5V 2.5A
ROSC 309k 1.25MHz 6 SYNC/MODE EXT CLOCK 7 RUN CSS 470pF X7R 8 SGND
COUT1** 100F
GND
3412fb LT 0707 REV B * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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