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Final Electrical Specifications
LTC3251 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter August 2002 DESCRIPTIO
The LTC(R)3251 is a 2-phase charge pump step-down DC/DC converter that produces a regulated output from a 2.7V to 5.5V input. The part uses switched capacitor fractional conversion to achieve a typical efficiency increase of 50% over that of a linear regulator. No inductors are required. VOUT is resistor programmable from 0.9V to 1.6V with up to 500mA of load current A unique 2-phase spread spectrum architecture provides a very low noise regulated output as well as low noise at the input. The part has four operating modes: Continuous Spread Spectrum, Spread Spectrum with Burst Mode operation, Super BurstTM mode operation and shutdown. Low operating current (35A in Burst Mode operation, 8A in Super Burst mode operation) and low external parts count (five small ceramic capacitors and two resistors) make the LTC3251 ideally suited for space-constrained battery-powered applications. The part is short-circuit and overtemperature protected, and is available in a thermally enhanced 10-pin MSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. Super Burst is a trademark of Linear Technology Corporation.
FEATURES
s s s s s
s s s s
s s s s s
500mA Output Current No Inductors 2.7V to 5.5V Input Voltage Range Typical Efficiency 50% Higher Than LDOs 2-Phase, Spread Spectrum Operation for Low Input and Output Noise Shutdown Disconnects Load from VIN Adjustable Output Voltage Range: 0.9V to 1.6V Super Burst, Burst and Burst Defeat Operation Low Operating Current: IIN = 35A (Burst Mode(R) Operation) Super Burst Operating Current: IIN = 8A Low Shutdown Current: IIN = 0.01A Typ Soft-Start Limits Inrush Current at Turn-On Short-Circuit and Overtemperature Protected Available in a Thermally Enhanced 10-Pin MSOP Package
APPLICATIO S
s s s s
Handheld Devices Cellular Phones Portable Electronic Equipment DSP Power Supplies
TYPICAL APPLICATIO
OFF ON
Spread Spectrum Step-Down Converter
1 9 MD0 MD1 1-CELL Li-Ion OR 3-CELL NiMH LTC3251 7 2 VOUT VIN 8 3 C2+ C1+ 1F 4 6 1F C2- C1- 10 5 GND FB
100 90 80
VOUT = 1.5V 500mA 10F 5pF
EFFICIENCY (%)
70 60 50 40 30 20 LDO
1F
475k
536k
3251 TA01
10 0 3 3.5 4.5 4 INPUT VOLTAGE (V) 5 5.5
3251 TA02
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.
U
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1.5V Efficiency vs Input Voltage (Burst Mode Operation)
IOUT = 250mA LTC3251
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LTC3251
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW MD0 VIN C1 + C1- GND 1 2 3 4 5 10 9 8 7 6 FB MD1 C2+ VOUT C2-
VIN to GND ................................................... -0.3V to 6V MD0, MD1 and FB to GND ............. - 0.3V to (VIN + 0.3V) IOUT (Note 2) ...................................................... 650mA Operating Temperature Range (Note 3) ... -40C to 85C Storage Temperature Range .................. - 65C to 150C Lead Temperature (Soldering, 10 sec)................... 300C
ORDER PART NUMBER LTC3251EMSE MSE PART MARKING LTB4
MSE PACKAGE 10-LEAD PLASTIC MSOP EXPOSED PAD IS GROUND (MUST BE SOLDERED TO PCB) TJMAX = 125C, JA = 40C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER VIN Minimum Operating Voltage VIN Maximum Operating Voltage VIN Continuous Mode Operating Current VIN Burst Mode Operating Current VIN Super Burst Mode Operating Current VIN Shutdown Current VFB Regulation Voltage IOUT Continuous Output Current IOUT Super Burst Output Current Load Regulation (Referred to FB Pin) Line Regulation (Referred to FB Pin) IFB FB Input Current VR Output Ripple Spread Spectrum Frequency Range VIH VIL IIH IIL tON ROL MD0, MD1 Input High Voltage MD0, MD1 Input Low Voltage MD0, MD1 Input High Current MD0, MD1 Input Low Current Turn-On Time Open-Loop Output Impedance
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, VOUT = 1.5V, C1 = C2 = 1F, CIN = 1F, COUT = 10F, all capacitors ceramic, unless otherwise noted.
CONDITIONS (Note 4) IOUT = 0mA, VMD0 = 0, VMD1 = VIN IOUT = 0mA, VMD0 = VIN, VMD1 = 0 IOUT = 0mA, VMD0 = VIN, VMD1 = VIN VMD0 = 0V, VMD1 = 0V IOUT = 0mA, 2.7V VIN 5.5V VMD0 = 0, VMD1 = VIN or VMD0 = VIN, VMD1 = 0 VMD0 = VIN, VMD1 = VIN 0mA IOUT 500mA 0mA IOUT 500mA VFB = 0.85V IOUT = 500mA fMIN Switching Frequency fMAX Switching Frequency 2.7V VIN 5.5V 2.7V VIN 5.5V MD0 = VIN, MD1 = VIN MD0 = 0V, MD1 = 0V ROUT = 3, Burst or Continuous Mode Operation VIN = 3V, IOUT = 200mA (Note 5)
q q q q q q q q q q q q q q q q q
MIN 2.7
TYP
MAX 5.5
UNITS V V mA A A A V mA mA mV/mA %/V
3 35 8 0.01 0.78 0.8
5 60 15 1 0.82 500 40
0.045 0.2 -50 12 0.8 1.0 1.6 0.8 0.4 -1 -1 1 0.45 0.7 0.8 1 1 2 1.2 50
mVP-P MHz MHZ V V A A ms
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Based on long term current density limitations. Note 3: The LTC3251E is guaranteed to meet specified performance from 0C to 70C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 4: Minimum operating voltage required for regulation is: VIN 2 * (VOUT + ROL * IOUT) Note 5: Output not in regulation; VFB = 0.76V, ROL = (VIN/2 - VOUT)/IOUT.
2
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nA
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WW
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LTC3251 TYPICAL PERFOR A CE CHARACTERISTICS
No Load Supply Current vs Supply Voltage (Continuous Mode)
7 6 5
-40C 25C 85C
IIN (mA)
IIN (A)
IIN (A)
4 3 2 1 0 2.7
3.2
3.7
4.2 VIN (V)
4.7
1.5V Output Voltage vs Supply Voltage (Burst Mode Operation/ Continuous Mode)
1.60 1.58 1.56 1.54 IOUT = 0mA TA = 25C 1.300 1.280 1.260 1.240
VOUT (V)
VOUT (V)
1.50 1.48 1.46 1.44 1.42 1.40 3 3.5 4
IOUT = 250mA IOUT = 500mA
1.200 1.180 1.160 1.140 1.120 IOUT = 500mA
VOUT (V)
1.52
4.5 VIN (V)
FB Voltage vs Output Current (Burst Mode Operation/ Continuous Mode)
0.805 VOUT = 1.5V 0.800 EFFICIENCY (%)
VFB (V)
0.795
60 50 40 30
VIN = 4V VIN = 5V
EFFICIENCY (%)
0.790
0.785
0.780
0
100
200
300 400 IOUT (mA)
UW
5.2
3251 G01
No Load Supply Current vs Supply Voltage (Burst Mode Operation)
50 45 85C 40 25C 35 -40C 30 25 20 2.7
20 18 16 14 12 10 8 6 4 2
No Load Supply Current vs Supply Voltage (Super Burst Mode Operation)
85C 25C -40C
3.2
3.7
4.2 VIN (V)
4.7
5.2
3251 G02
0 2.7
3.2
3.7
4.2 VIN (V)
4.7
5.2
3251 G02
1.2V Output Voltage vs Supply Voltage (Burst Mode Operation/ Continuous Mode)
TA = 25C 1.60 1.58 1.56 1.54 IOUT = 250mA IOUT = 0mA 1.52 1.50 1.48 1.46 1.44 1.42 3.2 3.7 4.2 VIN (V) 4.7 5.2
3251 G05
1.5V Output Voltage vs Supply Voltage (Super Burst Mode Operation)
TA = 25C 0mA 10mA 40mA
1.220
5
5.5
3251 G04
1.100 2.7
1.40
3
3.5
4
4.5 VIN (V)
5
5.5
3251 G06
1.5V Output Efficiency vs Output Current (Burst Mode Operation)
100 90 80 70 VIN = 3.3V VIN = 3.6V
100 90 80 70 60 50 40 30 20 10
1.5V Output Efficiency vs Output Current (Super Burst Mode Operation)
VIN = 3.6V VIN = 3.3V
VIN = 4V VIN = 5V
20 10
500 600
0 0.1
MD0 = VIN, MD1 = 0V 1 10 IOUT (mA) 100 1000
3251 G08
0 0.1
MD0 = MD1 = VIN 1 IOUT (mA)
3251 G09
10
100
3251 G07
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LTC3251 TYPICAL PERFOR A CE CHARACTERISTICS
MD0/MD1 Input Threshold Voltage vs Supply Voltage
1.2 1.1
MD0/MD1 THRESHOLD (V)
2.0 1.9 1.8 1.7 FREQUENCY (MHz) -40C MAX 25C MAX
1.0 0.9 0.8 0.7 0.6 0.5 0.4 2.7 3.2 3.7 4.2 VIN (V) 4.7 5.2
3251 G10
Output Transient Response (Continuous Mode)
IOUT 450mA 50mA IOUT 450mA 50mA
VOUT 20mV/DIV (AC)
TA = 25C 10s/DIV COUT = 10F X5R 6.3V VOUT = 1.5V
PI FU CTIO S
MD0 (Pin 1)/MD1 (Pin 9): Mode Input Pins. The Mode input pins are used to set the operating mode of the LTC3251. The modes of operation are:
MD1 0 0 1 1 MD0 0 1 0 1 OPERATING MODE Shutdown Spread Spectrum with Burst Continuous Spread Spectrum Super Burst
MD0 and MD1 are high impedance CMOS inputs and must not be allowed to float.
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UW
Max/Min Oscillator Frequency vs Supply Voltage
-40C 25C 85C
1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 2.7 3.2 3.7 85C MIN 4.2 VIN (V) 4.7 5.2
3251 G11
85C MAX
25C MIN -40C MIN
Output Transient Response (Burst Mode Operation)
VIN 4.5V 3.5V
Supply Transient Response (Continuous Mode)
VOUT 20mV/DIV (AC)
VOUT 20mV/DIV (AC)
3251 G13
10s/DIV TA = 25C COUT = 10F X5R 6.3V VOUT = 1.5V
3251 G14
20s/DIV TA = 25C COUT = 10F X5R 6.3V IOUT = 250mA VOUT = 1.5V
3251 G15
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VIN (Pin 2): Input Supply Voltage. Operating VIN may be between 2.7V and 5.5V. Bypass VIN with a 1F low ESR ceramic capacitor to GND (COUT). C1+ (Pin 3): Flying Capacitor 1 Positive Terminal (C1). C1- (Pin 4): Flying Capacitor 1 Negative Terminal (C1). GND (Pin 5): Ground. Connect to a ground plane for best performance. C2 - (Pin 6): Flying Capacitor 2 Negative Terminal (C2).
LTC3251
PI FU CTIO S
VOUT (Pin 7): Regulated Output Voltage. VOUT is disconnected from VIN during shutdown. Bypass VOUT with a low ESR ceramic capacitor to GND (CIN). See VOUT Capacitor Selection for capacitor size requirements. C2 + (Pin 8): Flying Capacitor 2 Positive Terminal (C2). FB (Pin 10): Feedback Input Pin. An output divider should be connected from VOUT to FB to program the output voltage.
SI PLIFIED BLOCK DIAGRA
OVERTEMP
2
VIN
BURST DETECT CIRCUIT
OPERATIO
(Refer to Block Diagram)
The LTC3251 uses a dual phase switched capacitor charge pump to step down VIN to a regulated output voltage. Regulation is achieved by sensing the output voltage through an external resistor divider and modulating the charge pump output current based on the error signal. A 2-phase nonoverlapping clock activates the two charge
-
+
W
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W
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1 MD0
9 MD1
SWITCH CONTROL AND SOFT-START
SPREAD SPECTRUM OSCILLATOR
CHARGE PUMP 1 C1+
3
C1-
4
VOUT CHARGE PUMP 2 C2+
7
8
C2-
6
FB
10
GND 5
3251 BD
pumps. The two charge pumps work in parallel, but out of phase from each other. On the first phase of the clock, current is transferred from VIN, through the external flying capacitor 1, to VOUT via the switches of Charge Pump 1. Not only is current being delivered to VOUT on the first phase, but the flying capacitor is also being charged. On
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LTC3251
OPERATIO
the second phase of the clock, flying capacitor 1 is connected from VOUT to ground, transferring the charge stored during the first phase of the clock to VOUT via the switches of Charge Pump 1. Charge Pump 2 operates in the same manner, but with the phases of the clock reversed. This dual phase architecture achieves extremely low output and input noise by providing constant charge transfer from VIN to VOUT. Using this method of switching, only half of the output current is delivered from VIN, thus achieving a 50% increase in efficiency over a conventional LDO. A spread spectrum oscillator, which utilizes random switching frequencies between 1MHz and 1.6MHz, sets the rate of charging and discharging of the flying capacitors. The part also has two types of low current Burst Mode operation to improve efficiency even at light loads. In shutdown mode, all circuitry is turned off and the LTC3251 draws only leakage current from the VIN supply. Furthermore, VOUT is disconnected from VIN. The MD0 and MD1 pins are CMOS inputs with threshold voltages of approximately 0.8V to allow regulator control with low voltage logic levels. The LTC3251 is in shutdown when a logic low is applied to both mode pins. Since the mode pins are high impedance CMOS inputs, they should never be allowed to float. Always drive the mode pins with valid logic levels. Short-Circuit/Thermal Protection The LTC3251 has built-in short-circuit current limiting as well as overtemperature protection. During short-circuit conditions, internal circuitry automatically limits the output current to approximately 800mA. At higher temperatures, or in cases where internal power dissipation causes excessive self heating on chip (i.e., output short circuit), the thermal shutdown circuitry will shut down the charge pumps when the junction temperature exceeds approximately 160C. It will reenable the charge pumps once the junction temperature drops back to approximately 150C. The LTC3251 will cycle in and out of thermal shutdown without latch-up or damage until the overstress condition is removed. Long term overstress (IOUT > 650mA and/or TJ > 125C) should be avoided as it can degrade the performance or shorten the life of the part.
6
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(Refer to Block Diagram)
Soft-Start To prevent excessive current flow at VIN during start-up, the LTC3251 has built-in soft-start circuitry. Soft-start is achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500s. Soft-start is enabled whenever the device is brought out of shutdown, and is disabled shortly after regulation is achieved. Spread Spectrum Operation Switching regulators can be particularly troublesome where electromagnetic interference (EMI) is concerned. Switching regulators operate on a cycle-by-cycle basis to transfer power to an output. In most cases the frequency of operation is either fixed or is a constant based on the output load. This method of conversion creates large components of noise at the frequency of operation (fundamental) and multiples of the operating frequency (harmonics). Figure 1a shows a conventional buck switching converter. Figures 1b and 1c are the input and output noise spectrums for the buck converter of Figure 1 with VIN = 3.6V, VOUT = 1.5V and IOUT = 500mA. Unlike conventional buck converters, the LTC3251's internal oscillator is designed to produce a clock pulse whose period is random on a cycle-by-cycle basis, but fixed between 1MHz and 1.6MHz. This has the benefit of spreading the switching noise over a range of frequencies, thus significantly reducing the peak noise. Figures 2b and 2c are the input and output noise spectrums for the LTC3251 of Figure 2a with VIN = 3.6V, VOUT = 1.5V and IOUT = 500mA. Note the significant reduction in peak output noise (>20dBm) with only 1/2 the output capacitance and the virtual elimination of input harmonics with only 1/10 the input capacitance. Spread spectrum operation is used exclusively in "continuous" mode and for output currents greater than about 50mA in Burst Mode operation. Low Current Burst Mode Operation To improve efficiency at low output currents, a Burst Mode function is included in the LTC3251. An output current sense is used to detect when the required output current drops below an internally set threshold (50mA typ). When this occurs, the part shuts down the internal oscillator and
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LTC3251
OPERATIO
VIN 10F IN
COMP GND
Figure 1a. Conventional Buck Switching Converter
-40 -50 NOISE (dBm)
NOISE (dBm)
-60 -70 -80 -90 START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01b
Figure 1b. Conventional Buck Converter Output Noise Spectrum with 22F Output Capacitor (IO = 500mA)
-40 -50
NOISE (dBm)
-60 -70 -80 -90 START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01c
NOISE (dBm)
Figure 1c. Conventional Buck Converter Input Noise Spectrum with 10F Input Capacitor (IO = 500mA)
goes into a low current operating state. The LTC3251 will remain in the low current operating state until the output voltage has dropped enough to require another burst of current. When the output current exceeds 50mA, the LTC3251 will operate in continuous mode. Unlike traditional charge pumps, where the burst current is dependant on many factors (i.e., supply, switch strength, capacitor selection, etc.), the LTC3251's burst current is set by the burst threshold and hysteresis. This means that the VOUT ripple voltage in Burst Mode operation will be fixed and is typically 15mV with a 10F output capacitor. Ultralow Current Super Burst Mode Operation To further optimize the supply current for low output current requirements, a Super Burst mode operaton is
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SW FB
(Refer to Block Diagram)
4.7H 10nH* VOUT 22F 1F VIN 1F IN OUT 10F 1F LTC3251 C1+ 1F C1- FB C2 + C2 - GND 1F 10nH* VOUT
*10nH = 1cm OF PCB TRACE
3251 F01a
*10nH = 1cm OF PCB TRACE
3251 F02a
Figure 2a. LTC3251 Buck Converter
-40 -50 -60 -70 -80 -90 START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F02b
Figure 2b. LTC3251 Output Noise Spectrum with 10F Output Capacitor (IO = 500mA)
-40 -50 -60 -70 -80 -90 START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F02c
Figure 2c. LTC3251 Input Noise Spectrum with 1F Input Capacitor (IO = 500mA)
included in the LTC3251. This mode is very similar to Burst Mode operation, but much of the internal circuitry and switch is shut down to further reduce supply current. In Super Burst mode operation an internal hysteretic comparator connected to the FB pin is used to enable/disable charge transfer. The hysteresis of the comparator and the amount of current deliverable to the output are limited to keep output ripple low. The VOUT ripple voltage in Super Burst mode operation is typically 35mV with a 10F output capacitor. The LTC3251 can deliver 40mA of current in Super Burst mode operation but does not switch to continuous mode. VOUT Capacitor Selection The style and value of capacitors used with the LTC3251
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LTC3251
OPERATIO
determine several important parameters such as regulator control loop stability, output ripple and charge pump strength. The dual phase nature of the LTC3251 minimizes output noise significantly but not completely. What small ripple that does exist is controlled by the value of COUT directly. Increasing the size of COUT will proportionately reduce the output ripple. The ESR (equivalent series resistance) of COUT plays the dominant role in output noise. When the LTC3251 switches between clock phases there is a period where all switches are turned off. This "blanking period" shows up as a spike at the output and is a direct function of the output current times the ESR value. To reduce output noise and ripple, it is suggested that a low ESR (< 0.08) ceramic capacitor be used for COUT. Tantalum and aluminum capacitors are not recommended because of their high ESR. Both the style and value of COUT can significantly affect the stability of the LTC3251. As shown in the Block Diagram, the LTC3251 uses a control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output charge storage capacitor. Thus the charge storage capacitor also serves to form the dominant pole for the control loop. The desired output voltage also affects stability. As the divider ratio (RA/RB) drops, the effective closed-loop gain increases, thus requiring a larger output capacitor for stability. Figure 3 shows the suggested output capacitor for optimal transient response. The value of the output capacitance should not drop below the minimum capacitance line to prevent excessive ring-
16 15 14 13 12 OPTIMUM CAPACITANCE
COUT (F)
11 10 9 8 7 6 5 4 0.9 1.0 1.1 1.2 1.3 VOUT (V) 1.4 1.5 1.6 MINIMUM CAPACITANCE
8
U
(Refer to Block Diagram)
ing or instability. (see Ceramic Capacitor Selection Guidelines section). Likewise excessive ESR on the output capacitor will tend to degrade the loop stability of the LTC3251. The closed loop output impedance of the LTC3251 is approximately: RO = 0.045 * VOUT 0.8 V
For example, with the output programmed to 1.5V, the RO is 0.085, which produces a 40mV output change for a 500mA load current step. For stability and good load transient response, it is important for the output capacitor to have 0.08 or less of ESR. Ceramic capacitors typically have exceptional ESR, and combined with a tight board layout, should yield excellent stability and load transient performance. Further output noise reduction can be achieved by filtering the LTC3251 output through a very small series inductor as shown in Figure 4. A 10nH inductor will reject the fast output transients caused by the blanking period. The 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of 1mm wide PC board trace.
10nH VOUT LTC3251 GND 10F 1F
3251 F04
VOUT
Figure 4. 10nH Inductor Used for Additional Output Noise Reduction
VIN Capacitor Selection The dual phase architecture used by the LTC3251 makes input noise filtering much less demanding than conventional charge pump regulators. The LTC3251 input current should be continuous at about IOUT/2. The blanking period described in the VOUT section also effects the input. For this reason it is recommended that a low ESR, 1F (0.4F min) or greater ceramic capacitor be used for CIN (see Ceramic Capacitor Selection Guidelines section). In cases where the supply impedance is high, heavy output transients can cause significant input transients. These input transients feed back to the output which slows the output transient recovery and increases overshoot. This effect can generally be avoided by using low impedance
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3251 F03
Figure 3
LTC3251
OPERATIO
supplies and short supply connections. If this is not possible, a 4.7F capacitor is recommended for the input capacitor. Aluminum and tantalum capacitors are not recommended because of their high ESR. Further input noise reduction can be achieved by filtering the input through a very small series inductor as shown in Figure 5. A 10nH inductor will reject the fast input transients caused by the blanking period, thereby presenting a nearly constant load to the input supply. For economy, the 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of 1mm wide PC board trace.
VIN SUPPLY 10nH VIN 1F LTC3251 GND
3251 F05
Figure 5. 10nH Inductor Used for Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitors since their voltages can reverse upon start-up of the LTC3251. Ceramic capacitors should always be used for the flying capacitors.
The flying capacitors control the strength of the charge pump. In order to achieve the rated output current, it is necessary for the flying capacitor to have at least 0.4F of capacitance over operating temperature with a 2V bias (see Ceramic Capacitor Selection Guidelines). If only 200mA or less of output current is required for the application, the flying capacitor minimum can be reduced to 0.15F. Ceramic Capacitor Selection Guidelines Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X5R or X7R material will retain most of its capacitance from - 40C to 85C, whereas a Z5U or Y5V style capacitor will lose considerable capacitance over that range (60% to 80% loss typ). Z5U and Y5V capacitors may also have a very strong
U
(Refer to Block Diagram)
voltage coefficient, causing them to lose an additional 60% or more of their capacitance when the rated voltage is applied. Therefore, when comparing different capacitors, it is often more appropriate to compare the amount of achievable capacitance for a given case size rather than discussing the specified capacitance value. For example, over rated voltage and temperature conditions, a 4.7F, 10V, Y5V ceramic capacitor in an 0805 case may not provide any more capacitance than a 1F, 10V, X5R or X7R available in the same 0805 case. In fact, over bias and temperature range, the 1F, 10V, X5R or X7R will provide more capacitance than the 4.7F, 10V, Y5V. The capacitor manufacturer's data sheet should be consulted to determine what value of capacitor is needed to ensure minimum capacitance values are met over operating temperature and bias voltage. Below is a list of ceramic capacitor manufacturers and how to contact them:
AVX Kemet Murata Taiyo Yuden Vishay www.avxcorp.com www.kemet.com www.murata.com www.t-yuden.com www.vishay.com
Layout Considerations Due to the high switching frequency and transient currents produced by the LTC3251, careful board layout is necessary for optimal performance. A true ground plane and short connections to all capacitors will improve performance and ensure proper regulation under all conditions. Figure 6 shows the recommended layout configuration.
RB
VIN CI 1F C1 1F
RA
CA 5pF VOUT
GND
C2 1F
CO 10F
3251 F06
Figure 6. Recommended Layout
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LTC3251
OPERATIO
The flying capacitor pins C1+, C1-, C2+, C2- will have very high edge rate wave forms. The large dv/dt on these pins can couple energy capacitively to adjacent printed circuit board runs. Magnetic fields can also be generated if the flying capacitors are not close to the LTC3251 (i.e., the loop area is large). To decouple capacitive energy transfer, a Faraday shield may be used. This is a grounded PC trace between the sensitive node and the LTC3251 pins. For a high quality AC ground, it should be returned to a solid ground plane that extends all the way to the LTC3251. Keep the FB trace away from or shielded from the flying capacitor traces or degraded performance could result. Thermal Management If the junction temperature increases above approximately 160C, the thermal shutdown circuitry will automatically deactivate the output. To reduce the maximum junction temperature, a good thermal connection to the PC board is recommended. Connecting the 10-pin MSE paddle directly to a ground plane, and maintaining a solid ground plane under the device on one or more layers of the PC board, can reduce the thermal resistance of the package and PC board considerably. Using this method a JA of 40C/W should be achieved. Power Efficiency The power efficiency () of the LTC3251 is approximately 50% higher than a conventional linear regulator. This occurs because the input current for a 2-to-1 step-down charge pump is approximately half the output current. For an ideal 2-to-1 step-down charge pump the power efficiency is given by: POUT VOUT * IOUT 2VOUT = = 1 PIN VIN VIN * IOUT 2
At moderate to high output power the switching losses and quiescent current of the LTC3251 is negligible and the expression above is valid. For example with VIN = 3.6V, IOUT = 200mA and VOUT regulating to 1.5V the measured efficiency is 81% which is in close agreement with the theoretical 83.3% calculation.
10
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(Refer to Block Diagram)
Programming the LTC3251 Output Voltage (FB Pin) The LTC3251 is programmed to an arbitrary output voltage via an external resistive divider. Figure 7 shows the required voltage divider connection. The voltage divider ratio is given by the expression:
RA VOUT = -1 RB 0.8 V
Typical values for total voltage divider resistance can range from several ks up to 1M. The user may want to consider load regulation when setting the desired output voltage. The closed loop output impedance of the LTC3251 is approximately: VOUT 0.8 V For a 1.5V output, RO is 0.085, which produces a 40mV output change for a 500mA load current step. Thus, the user may want to target an unloaded output voltage slightly higher than desired to compensate for the output load conditions. The output may be programmed for regulation voltages of 0.9V to 1.6V. RO = 0.045 * Since the LTC3251 employs a 2-to-1 charge pump architecture, it is not possible to achieve output voltages greater than half the available input voltage. The minimum VIN supply required for regulation can be determined by the following equation: VIN(MIN) 2 * (VOUT(MIN) + IOUT * ROL) The compensation capacitor (CA) is necessary to counteract the pole caused by the large valued resistors RA and RB, and the input capacitance of the FB pin. For best results, CA should be 5pF for all RA or RB greater than 10k and can be omitted if both RA and RB are less than 10k.
VOUT LTC3251 FB RB GND
3251 F07
VOUT CA RA COUT
R 0.8V 1 + A RB
()
Figure 7. Programming the LTC3251
3251i
LTC3251
TYPICAL APPLICATIO
0.9V Output Continuous/Burst Mode Operation with Shutdown
OFF ON 1 9 MD0 MD1 1-CELL Li-Ion OR 3-CELL NiMH LTC3251 2 7 VOUT VIN 3 8 C2+ C1+ 1F 4 6 1F C2- C1- 5 10 GND FB VOUT = 0.9V 500mA 10F 5pF 73.2k 4.7F
PACKAGE DESCRIPTIO
2.794 0.102 (.110 .004)
5.23 (.206) MIN
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
0.254 (.010) GAUGE PLANE
0.18 (.007)
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
3251i
U
U
1F 536k
3251 TA05
MSE Package 10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1663)
BOTTOM VIEW OF EXPOSED PAD OPTION 0.889 0.127 (.035 .005) 1 2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004)
2.083 0.102 3.2 - 3.45 (.082 .004) (.126 - .136)
10 3.00 0.102 (.118 .004) (NOTE 3) 10 9 8 7 6
0.497 0.076 (.0196 .003) REF
4.88 0.10 (.192 .004) DETAIL "A" 0 - 6 TYP 12345 0.53 0.01 (.021 .006) DETAIL "A" SEATING PLANE 1.10 (.043) MAX
3.00 0.102 (.118 .004) NOTE 4
0.86 (.034) REF
0.17 - 0.27 (.007 - .011)
0.50 (.0197) TYP
0.13 0.05 (.005 .002)
MSOP (MSE) 1001
11
LTC3251
TYPICAL APPLICATIO S
3.3V to 1.5V Conversion, Continuous Spread Spectrum Operation with Shutdown
OFF ON 1 9 MD0 MD1 VIN 3.3V LTC3251 7 VIN VOUT 8 + + C2 C1 1F 4 6 1F C1- C2- 10 5 GND FB 2 3 VOUT = 1.5V IOUT 275mA 10F 4.75k
1F
RELATED PARTS
PART NUMBER LTC1514 LTC1515 LT1776 LTC1911-1.5 LTC1911-1.8 LTC3250-1.5 LTC3404 DESCRIPTION 50mA, 650kHz, Step-Up/Down Charge Pump with Low Battery Comparator 50mA, 650kHz, Step-Up/Down Charge Pump with Power-On Reset 500mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 250mA, 1.5MHz, High Efficiency Step-Down Charge Pump 250mA, 1.5MHz, High Efficiency Step-Down Charge Pump 250mA, 1.5MHz, High Efficiency Step-Down Charge Pump 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter COMMENTS VIN = 2.7V to 10V, VOUT = 3V or 5V, Regulated Output, IQ = 60A, ISHDN = 10A, S8 VIN = 2.7V to 10V, VOUT = 3.3V or 5V, Regulated Output, IQ = 60A, ISHDN = <1A, S8 90% Efficiency, VIN = 7.4V to 40V, VOUT Min = 1.24V, IQ = 3.2mA, ISHDN = 30A, N8, S8 75% Efficiency, VIN = 2.7V to 5.5V, VOUT = 1.5V, Regulated Output, IQ = 180A, ISHDN = 10A, MS8 75% Efficiency, VIN = 2.7V to 5.5V, VOUT = 1.8V, Regulated Output, IQ = 180A, ISHDN = 10A, MS8 85% Efficiency, VIN = 3.1V to 5.5V, VOUT = 1.5V, Regulated Output, IQ = 35A, ISHDN = <1A, ThinSOT 95% Efficiency, VIN = 2.7V to 6V, VOUT Min = 0.8V, IQ = 10A, ISHDN = <1A, MS8 95% Efficiency, VIN = 2.7V to 6V, VOUT Min = 0.8V, IQ = 20A, ISHDN = <1A, ThinSOT 95% Efficiency, VIN = 2.5V to 5.5V, VOUT Min = 0.6V, IQ = 20A, ISHDN = <1A, ThinSOT 95% Efficiency, VIN = 2.5V to 5.5V, VOUT Min = 0.8V, IQ = 60A, ISHDN = <1A, MS10 95% Efficiency, VIN = 2.5V to 5.5V, VOUT Min = 0.8V, IQ = 60A, ISHDN = <1A, TSSOP-16E 95% Efficiency, VIN = 2.5V to 5.5V, VOUT Min = 2.5V, IQ = <25A, ISHDN = 1A, MS10
3251i LT/TP 0802 1.5K * PRINTED IN USA
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter LTC3411 LTC3412 LTC3440 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
U
1.2V Output with Processor Control of Operating Modes
P 1 9 MD0 MD1 LTC3251 2 7 VOUT VIN 3 8 + + C1 C2 1F 4 6 1F - - C2 C1 5 10 GND FB
VOUT = 1.2V IOUT UP TO 250mA, VIN 2.7V IOUT UP TO 500mA, VIN 3.0V 10F 5pF 287k 2.2F
1-CELL Li-Ion OR 3-CELL NiMH
1F
5.36k
3251 TA03
549k
3251 TA04
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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