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LTC3452 Synchronous Buck-Boost MAIN/CAMERA White LED Driver
FEATURES

DESCRIPTIO
High Efficiency: 85% Over Entire Li-Ion Battery Range Wide VIN Range: 2.7V to 5.5V Independent MAIN/CAMERA Current Control Up to 425mA Continuous Output Current Internal Soft-Start Open/Shorted LED Protection PWM Brightness Control LED Current Matching Typically <2.5% Constant Frequency 1MHz Operation Low Shutdown Current: 6.5A Overtemperature Protection Small Thermally Enhanced 20-Lead (4mm x 4mm) QFN Package
The LTC(R)3452 is a synchronous buck-boost DC/DC converter optimized for driving two banks of white LEDs from a single Li-Ion battery input. Five parallel LEDs can be driven at up to 25mA each in the low power LED bank, while two LEDs can be driven at up to 150mA each (or a single LED at 300mA) in the high power LED bank. The regulator operates in either synchronous buck, synchronous boost or buck-boost mode, depending on input voltage and LED maximum forward voltage. Optimum efficiency is achieved by sensing which LED requires the largest forward voltage drop at its programmed current, and regulating the common output rail for lowest dropout. Efficiency of 85% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). Maximum LED current for each LED display is programmable with a single external resistor. Dual enable pins allow for PWM brightness control in the low power bank and independent on/off control for the high current bank (optimal for LED camera flash). In shutdown, the supply current is only 6.5A. A high constant operating frequency of 1MHz allows the use of a small external inductor. The LTC3452 is offered in a low profile (0.75mm) thermally enhanced 20-lead (4mm x 4mm) QFN package.
APPLICATIO S

Cell Phones Digital Cameras PDAs Portable Devices
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
L 4.7H VIN SINGLE Li-Ion CELL 2.7V TO 4.2V
5 x 20mA White LED Display + 200mA Camera Light Driver
+
ENH ISETH 6.19k LEDL1 VC 0.1F ENL ISETL 10.2k GND GND PGND EXPOSED PAD MAIN DISPLAY LED BACKLIGHT D1: AOT 2015 D2 TO D6: NICHIA NSCW100 L: COILCRAFT DO3314-472 LTC3452 1MHz BUCK/BOOST LEDL2 LEDL3 LEDL4 LEDL5
2.2F VIN PVIN SW1 SW2 VOUT
D1 CAM LEDH1 LEDH2 D2 0mA TO 20mA D3 0mA TO 20mA D4 0mA TO 20mA D5 0mA TO 20mA D6 0mA TO 20mA 200mA
4.7F
EFFICIENCY (%)
3452 TA01a
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Torch and Flash Mode Efficiency
95 93 91 89 87 85 83 81 79 77 75 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 FLASH MODE AT 200mA TA = 25C (V - VLEDx) * ILEDx EFFICIENCY = OUT VIN * IIN TORCH MODE AT 100mA
3452 TA01b
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LTC3452
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
UF PACKAGE 20-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 40C/W EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC3452EUF
UF PART MARKING 3452
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.
ELECTRICAL CHARACTERISTICS
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2)
PARAMETER Input Supply Voltage (VIN) Input DC Supply Current Normal Operation Shutdown UVLO Undervoltage Lockout Threshold ENL,H DC Threshold for Normal Operation (VIH) ENL,H DC Threshold for Shutdown (ILEDx = 0) (VIL) ENL,H Input Current (IIH, IIL) ENL PWM Frequency ISETL,H Servo Voltage LEDHx Output Current Ratio (ILEDHx/ISETH) LEDHx Output Current Matching LEDHx Pin Voltage 2.7V VIN 5.5V, RISETL = RISETH = 51.1k, ILEDx = 0 (Note 4) 2.7V VIN 5.5V, VENL = VENH = 0V VIN < UVLO Threshold VIN Rising VIN Falling 2.7V VIN 5.5V, VENL,H Rising 2.7V VIN 5.5V, VENL,H Falling 2.7V VIN 5.5V 2.7V VIN 5.5V (Note 5) RISETL = RISETH = 20k

CONDITIONS
MIN 2.7
LEDH1
LEDL3
LEDL4
LEDL5
GND
VIN, PVIN, SW1, SW2, VOUT Voltage ........... - 0.3V to 6V LEDL1 to LEDL5 Voltage ... - 0.3V to (VOUT + 0.3V) or 6V LEDH1, LEDH2 Voltage ..... - 0.3V to (VOUT + 0.3V) or 6V VC, ENL, ENH, ISETL, ISETH Voltage ............ - 0.3V to (VIN + 0.3V) or 6V LEDL1 to LEDL5 Current ....................................... 50mA LEDH1, LEDH2 Current ....................................... 250mA Operating Temperature Range (Note 2) .. - 40C to 85C Junction Temperature (Note 3) ............................ 125C Storage Temperature Range ................ - 65C to 125C
PGND
20 19 18 17 16 VIN 1 ENL 2 ISETL 3 LEDL1 4 LEDL2 5 6 7 8 9 10 21 15 VC 14 ENH 13 ISETH 12 LEDH2 11 GND
VOUT
SW1
SW2
PVIN
TYP
MAX 5.5
UNITS V mA A A V V V V
0.6 6.5 3 1.6

1 18 5 2.3 1.2
2.0 1.87 0.54 0.52
0.2 -1 10 788 780 730 714
1 800 800 768 768 1 250 812 812 806 806 6
ILEDHx = 100mA, VLEDHx = 300mV
mA/mA mA/mA % mV
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(Max - Min)/[(Max + Min)/2] * 100%, ILEDHx = 100mA, VLEDHx = 300mV, 2.7V VIN 5.5V ILEDHx = 100mA
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A kHz mV mV
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LTC3452
ELECTRICAL CHARACTERISTICS
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2)
PARAMETER LEDLx Output Current Ratio (ILEDLx/ISETL) (Note 6) CONDITIONS ILEDLx|MAX = 20mA, VLEDLx = 300mV PWM Duty Cycle = 6%
MIN 1.8 1.75 3.66 3.56 7.32 7.12 14.72 14.32 29.44 28.64 58.88 57.92 117.12 114.56 234.24 229.12
TYP 2 2 4 4 8 8 16 16 32 32 64 64 128 128 256 245 2.5 130
MAX 2.16 2.21 4.28 4.38 8.56 8.76 17.04 17.44 33.92 34.56 67.2 68.16 134.4 137.6 268.8 272.64 8
UNITS mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA mA/mA % mV
PWM Duty Cycle = 19%
PWM Duty Cycle = 31%
PWM Duty Cycle = 44%
PWM Duty Cycle = 56%
PWM Duty Cycle = 69%
PWM Duty Cycle = 81%
PWM Duty Cycle = 94%
LEDLx Output Current Matching LEDLx Pin Voltage Regulated Maximum VOUT PMOS Switch RON NMOS Switch RON Forward Current Limit Reverse Current Limit PMOS Switch Leakage NMOS Switch Leakage Oscillator Frequency Soft-Start Time
(Max - Min)/[(Max + Min)/2] * 100%, ILEDLx = 20mA, VLEDLx = 300mV ILEDLx = 20mA VLEDLx = VLEDHy = 0V Switches A and D at 100mA Switches B and C at 100mA Switch A Switch D Switches A and D Switches B and C -1 -1 0.9 1000
4.35
4.5 210 205 1600 200
4.75
V m m
2400 1 1
mA mA A A MHz s
1 650
1.1
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 LTC3452E is guaranteed to meet specifications 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 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * JAC/W). Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: Do not exceed 50kHz PWM frequency in the application. Note 6: This parameter is tested in a setup which forces conditions equivalent to those programmed by the indicated duty cycle.
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LTC3452 TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Current vs Temperature
20 18
SHUTDOWN CURRENT (A)
SHUTDOWN CURRENT (A)
12 10 8 6 4 2 0 2.7
16 14 12
10 8
6 4 2 VIN = 2.7V
VIN = 3.6V
VIN = 5.5V VIN = 4.2V
UVLO THRESHOLD (V)
0 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
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Enable Thresholds vs Temperature
1200 1100 VIN = 3.6V 900 1000
ENABLE THRESHOLDS (mV)
ENABLE THRESHOLDS (mV)
1000 900 800 700 600 500 400 300 200 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3452 G04
700 600 500 400 300 200 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 VIH VIL
VISETL,H (mV)
VIH VIL
ISETL,H Servo Voltage vs VIN
812 808 804 TA = 25C
4.60 4.58 4.56 4.54
VISETL,H (mV)
VOUT (V)
800 796 792 788
784 780 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5
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Shutdown Current vs VIN
TA = 25C 2.5
Undervoltage Lockout Threshold vs Temperature
2.3
2.1
VIN RISING
1.9
VIN FALLING
1.7
3.1
3.5
3.9 4.3 VIN (V)
4.7
5.1
5.5
1.5 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
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Enable Thresholds vs VIN
TA = 25C
ISETL,H Servo Voltage vs Temperature
812 808 804 800 796 792 788 784 780 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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VIN = 3.6V RISETL = 10.2k RISETH = 4.99k
800
3452 G05
Maximum Regulated VOUT vs Temperature
VIN = 3.6V
4.52 4.50 4.48 4.46 4.44 4.42 4.40 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3452 G08
3452 G07
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LTC3452 TYPICAL PERFOR A CE CHARACTERISTICS
PMOS RDS(ON) vs Temperature
325 300 VIN = 2.7V 275 275 250 225 200 175 150 5 25 45 65 85 105 125 TEMPERATURE (C)
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250 225 VIN = 5.5V 200 175 150 125 -55 -35 -15 VIN = 4.2V
FREQUENCY (kHz)
RDS(ON) (m)
RDS(ON) (m)
VIN = 3.6V
Output Voltage Ripple (Front Page Application)
VIN = 3V VOUT = 3.1V ILED = 100mA
UW
NMOS RDS(ON) vs Temperature
325 300
Oscillator Frequency vs Temperature
1050 1040 1030 VOUT = 3V
VIN = 2.7V VIN = 3.6V
1020 1010 1000 990 980 970 960 VIN = 4.2V
VIN = 5.5V
VIN = 3.6V
VIN = 5.5V VIN = 4.2V
VIN = 2.7V
125 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
3452 G11
950 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
3452 G12
Start-Up Transient
CH1, VOUT 1V/DIV CH2, ILED 300mA FINAL VALUE CH3, ENH 1V/DIV
3452 G13
VIN = 3.6V ILEDH = 300mA
3452 G14
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LTC3452
PI FU CTIO S
VIN (Pin 1): Signal Voltage Input Supply Pin (2.7V VIN 5.5V). Recommended bypass capacitor to GND is 2.2F ceramic or larger. Connect to PVIN (Pin 20). ENL (Pin 2): Enable Input Pin and PWM Brightness Control for Low Power LED Bank. Active high. For constant IMAXL operation, connect the ENL pin to VIN (or any voltage >1.2V). For ENL voltage <0.2V, all low power bank LED current source outputs are Hi-Z (if both ENL and ENH are <0.2V, the part is in shutdown and the input supply current drops to ~6A). For brightness control between zero current and IMAXL, drive the ENL pin with a PWM waveform of frequency 10kHz. The low power LED bank currents will be equal to a percentage of IMAXL as given in Table 1. The ENL pin is high impedance and should not be floated. ISETL (Pin 3): Low Power LED Bank Current Programming Pin. A resistor to ground programs each low power bank current source output maximum to ILEDLx|MAX = 256 * (0.8V/RISETL). Enabled by ENL (Pin 2). PWM brightness control also via ENL. LEDL1 to LEDL5 (Pins 4 to 8): Individual Low Dropout Current Source Outputs for Low Power LED Bank Current Biasing. Connect each low power LED between VOUT and an individual LEDLx pin. Unused LEDLx outputs should be connected to VOUT. GND (Pins 9 and 11): Signal Ground Pins. Connect together and to PGND (Pin 18) and Exposed Pad ground (Pin 21). LEDH1, LEDH2 (Pins 10, 12): Individual Low Dropout Current Source Outputs for High Power LED Bank Current Biasing. Connect each high power LED between VOUT and an individual LEDHx pin. Unused LEDHx outputs should be connected to VOUT. ISETH (Pin 13): High Power LED Bank Current Programming Pin. A resistor to ground programs each high power bank current source output to ILEDHx = 768(0.8V/RISETH). Enabled by ENH (Pin 14). ENH (Pin 14): Enable Input Pin for High Power LED Bank. Active high. The ENH pin is high impedance and should not be floated. VC (Pin 15): Compensation Point for the Internal Error Amplifier Output. Recommended compensation capacitor to GND is 0.1F ceramic or larger. VOUT (Pin 16): Buck-Boost Output Pin. Recommended bypass capacitor to GND is 4.7F ceramic. SW2 (Pin 17): Switching Node Pin. Connected to internal power switches C and D. External inductor connects between SW1 and SW2. Recommended value is 4.7H. PGND (Pin 18): Power Ground Pin. Connect to GND (Pins 9 and 11). SW1 (Pin 19): Switching Node Pin. Connected to internal power switches A and B. External inductor connects between SW1 and SW2. Recommended value is 4.7H. PVIN (Pin 20): Power Voltage Input Supply Pin. Connect to VIN (Pin 1). Exposed Pad (Pin 21): Heat Sink Ground. Connect to GND (Pins 9 and 11) and solder to PCB ground for electrical contact and rated thermal performance.
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LTC3452
BLOCK DIAGRA
VIN 2.7V TO 5.5V VIN 1
UNDERVOLTAGE LOCKOUT
OVERTEMPERATURE PROTECTION
BANDGAP REFERENCE
VC
15 VBIAS VFB
800mV ISETL RISETL ENL 2 ENH 14
+ -
LOW POWER LED CURRENT SETTING AMP
3
SDL
800mV ISETH
+ -
HIGH POWER LED CURRENT SETTING AMP IMAXH 768
13
RISETH
SDH 9 11 18 21
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VOUT PVIN 20 SW1 19 SW2 17 16 4 UV SWITCH A SWITCH B GATE DRIVERS AND ANTI-CROSSFORWARD CONDUCTION CURRENT LIMIT SWITCH D SWITCH C LED DETECT REVERSE CURRENT LIMIT 6 LED DETECT 5 LEDL2 LOW POWER LED BANK LEDL1 VOUT OT 1.23V
+
1600mA
+ -
200mA
LED DETECT 7
LEDL3
-
AB PWM COMPARATOR LOGIC CD PWM COMPARATOR
+ -
UV 1MHz OSCILLATOR OT
+ -
LED DETECT 8 LED DETECT
LEDL4
LEDL5
- +
MAIN ERROR AMP
SAFETY ERROR AMP
1.23V
- +
VOUT 327k 123k
1.23V
SOFT-START CLAMP
IMAXL 256
EXPONENTIAL BRIGHTNESS CONTROL
8 LEVELS 10 SDL LED DETECT SHUTDOWN 12 SDH LED DETECT LEDH2 LEDH1 HIGH POWER LED BANK
SHUTDOWN CIRCUIT
GND
GND
PGND
EXPOSED PAD
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LTC3452
OPERATIO
Buck-Boost DC-DC Converter The LTC3452 employs an LTC proprietary buck-boost DC/DC converter to generate the output voltage required to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below or equal to the output voltage by properly phasing four internal power switches. The error amp output voltage on the VC pin determines the duty cycle of the switches. Since the VC pin is a filtered signal, it provides rejection of frequencies well below the factory trimmed switching frequency of 1MHz. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across synchronous rectifier switch B and synchronous rectifier switch D are not required, but if used, do provide a lower voltage drop during the break-before-make time (typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads. Figure 1 shows a simplified diagram of how the four internal power switches are connected to the inductor, VIN = PVIN, VOUT and GND. Figure 2 shows the regions of operation of the buck-boost as a function of the control voltage VC. The output switches are properly phased so transitions between regions of operation are continuous, filtered and transparent to the user. When VIN approaches VOUT, the buck-boost region is reached where the conduction time of the four switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation encountered as VC increases will now be described.
PVIN 20 PMOS A SW1 19 NMOS B SW2 17
Figure 1. Simplified Diagram of Internal Power Switches
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Buck Mode (VIN > VOUT) In buck mode, switch D is always on and switch C is always off. Referring to Figure 2, when the control voltage VC is above voltage V1, switch A begins to turn on each cycle. During the off time of switch A, synchronous rectifier switch B turns on for the remainder of the cycle. Switches A and B will alternate conducting similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DCBUCK|max given by: DCBUCK|max = 100% - DC4SW where DC4SW equals the duty cycle in % of the "four switch" range. DC4SW = (150ns * f) * 100% where f is the operating frequency in Hz. Beyond this point the "four switch" or buck-boost region is reached. Buck-Boost or Four-Switch Mode (VIN VOUT) Referring to Figure 2, when the control voltage VC is above voltage V2, switch pair AD continue to operate for duty cycle DCBUCK|max, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When the VC voltage reaches the edge of the buck-boost range at voltage V3, switch pair AC completely phases out switch pair BD and the boost region begins at
75% DMAX BOOST
VOUT 16 PMOS D
V4 (2.1V) A ON, B OFF BOOST REGION PWM CD SWITCHES
DMIN BOOST DMAX BUCK FOUR SWITCH PWM BUCK/BOOST REGION
V3 (1.65V) V2 (1.55V) D ON, C OFF PWM AB SWITCHES BUCK REGION
NMOS C
0% DUTY CYCLE
V1 (0.9V) CONTROL VOLTAGE, VC
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Figure 2. Switch Control vs Control Voltage, VC
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LTC3452
OPERATIO
duty cycle DC4SW. The input voltage VIN where the four switch region begins is given by:
VIN =
VOUT 1 - (150ns * f)
and the input voltage VIN where the four switch region ends is given by:
VIN = VOUT * 1 - (150ns * f)
Boost Mode (VIN < VOUT)
[
In boost mode, switch A is always on and switch B is always off. Referring to Figure 2, when the control voltage VC is above voltage V3, switches C and D will alternate conducting similar to a typical synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VC is above V4. Forward Current Limit If the current delivered from VIN through PMOS switch A exceeds 1600mA (typical), switch A is shut off immediately. Switches B and D are turned on for the remainder of the cycle in order to safely discharge the forward inductor current at the maximum rate possible. Reverse Current Limit If the current delivered from VOUT backwards through PMOS switch D exceeds 200mA (typical), switch D is shut off immediately. Switches A and C are turned on for the remainder of the cycle in order to safely discharge the reverse inductor current at the maximum rate possible. Undervoltage Lockout To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3452. When the input supply voltage drops below approximately 1.9V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws only a few microamperes.
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Overtemperature Protection If the junction temperature of the LTC3452 exceeds 130C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11C. Soft-Start The LTC3452 includes an internally fixed soft-start which is active when powering up or coming out of shutdown. The soft-start works by clamping the voltage on the VC node and gradually releasing it such that it requires 650s to linearly slew from 0.9V to 2.1V. This has the effect of limiting the rate of duty cycle change as VC transitions from the buck region through the buck-boost region into the boost region. Once the soft-start times out, it can only be reset by entering shutdown, or by an undervoltage or overtemperature condition. Main Error Amp The main error amplifier is a transconductance amplifier with source and sink capability. The output of the main error amplifier drives a capacitor to GND at the VC pin. This capacitor sets the dominant pole for the regulation loop. (See the Applications Information section for selecting the capacitor value.) The error amp gets its feedback signal from a proprietary circuit which monitors all 7 LED current sources to determine which LED to close the regulation loop on. Safety Error Amp The safety error amplifier is a transconductance amplifier with sink only capability. In normal operation, it has no effect on the loop regulation. However, if any of the LED pins open-circuits, the output voltage will keep rising, and safety error amp will eventually take over control of the regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 4.5V.
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LTC3452
OPERATIO
LED Current Setting Amps The maximum forward current per LED for all LEDs in a given bank is programmed by a single external resistor to ground at the corresponding ISETL,H pin according to the following formulas:
0.8 0.8 IMAXL = 256 , IMAXH = 768 RISETH RISETL
For operation at currents below IMAXL in the low power bank, refer to the Exponential Brightness Control section and also to external circuit options given in the Applications Section. For operation at currents below IMAXH in the high power bank, refer only to the external circuit options given in the Applications Section. Shutdown Circuit The shutdown circuit monitors the voltages at the ENL,H pins. Logic high on either/both inputs enables the part and logic low on both puts the part in shutdown. Since the ENL pin doubles as a PWM input for LED brightness control, an output filter in the shutdown circuit is employed to prevent the part from toggling in and out of shutdown for normal PWMing of the ENL input when ENH is low. If ENH is low, the LTC3452 is enabled immediately after a rising edge at the ENL pin, but waits 200s (typical) after a falling edge to enter shutdown. Consequently, a minimum PWM frequency is required for smooth brightness control at currents below IMAXL. The recommended PWM frequency is 10kHz to 50kHz. Exponential Brightness Control (Low Power LED Bank Only) The LTC3452 implements an exponential brightness control function for the low power LED bank only in which the LEDLx current is a function of the PWM duty cycle at the ENL pin. The LED current will be equal to a fraction of IMAXL as given in Table 1. As the duty cycle (that the PWM waveform is logic high) increases linearly, the LED current will increase exponentially from 1/128th IMAXL to 128/128ths IMAXL in seven binary steps. The function
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implemented results in "smoother" brightness and dimming control as perceived by the human eye, which is logarithmic in nature.
Table 1. Low Power Bank Brightness Control
ENL DUTY CYCLE (% LOGIC HIGH) 0% (Logic Low) 0% < Duty Cycle < 12.5% 12.5% < Duty Cycle < 25% 25% < Duty Cycle < 37.5% 37.5% < Duty Cycle < 50% 50% < Duty Cycle < 62.5% 62.5% < Duty Cycle < 75% 75% < Duty Cycle < 87.5% 87.5% < Duty Cycle 100% LEDLx CURRENT 0 (Shutdown) 1/128 * IMAXL 1/64 * IMAXL 1/32 * IMAXL 1/16 * IMAXL 1/8 * IMAXL 1/4 * IMAXL 1/2 * IMAXL IMAXL
LED Current Sources Each LED pin is driven by a current source specifically designed for low dropout. The LTC3452 employs a proprietary architecture that determines which of the seven LEDs requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage based on this one for closing the buck-boost regulation loop. This results in the lowest output voltage required for regulating all of the LEDs and thus the highest LED power efficiency. The voltage present at the LED pin of the "controlling LED" will be typically 130mV at 20mA (low power bank) or 250mA at 100mA (high power bank) of current. LED Detect Circuit If fewer than five LED outputs in the low power bank and/ or fewer than two LED outputs in the high power bank are required, unused outputs should be connected to VOUT. Each LED pin has an internal LED detect circuit that disables the output current source to save power if an output is not needed. A small current is employed to detect the presence of an LED at startup. This current is typically 10A for the low power bank and 30A for the high power bank.
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LTC3452
APPLICATIO S I FOR ATIO
COMPONENT SELECTION Inductor Selection
The high frequency operation of the LTC3452 allows the use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum average inductor current. For a given ripple the inductance term in boost mode is: L> VIN(MIN)2 *
( VOUT - VIN(MIN) ) * 100%
f * IOUT(MAX ) * %Ripple * VOUT 2
and in buck mode is:
L>
where:
VOUT * VIN(MAX ) - VOUT * 100% f * IOUT(MAX ) * %Ripple * VIN(MAX )
(
)
f = operating frequency, Hz %Ripple = allowable inductor current ripple, % VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V IOUT(MAX) = maximum output load current For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. For the white LED application, a 4.7H inductor value is recommended. See Table 2 for a list of component suppliers.
Table 2. Inductor Vendor Information
SUPPLIER Coilcraft Cooper/Coiltronics Murata Sumida Vishay-Dale WEB SITE www.coilcraft.com www.cooperet.com www.murata.com www.japanlink.com/sumida www.vishay.com
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Input Capacitor Selection Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2F, low ESR bypass capacitor to ground. See Table 3 for a list of component suppliers.
Table 3. Capacitor Vendor Information
SUPPLIER AVX Sanyo Taiyo Yuden TDK WEB SITE www.avxcorp.com www.sanyovideo.com www.t-yuden.com www.component.tdk.com
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Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by:
IOUT(MAX ) * VOUT - VIN(MIN) * 100 COUT * VOUT 2 * f
%Ripple _ Boost =
(
)
%
%Ripple _ Buck =
8 * VIN(MAX ) * f 2 * L * COUT
( VIN(MAX) - VOUT ) * 100 %
where COUT = output filter capacitor, F The output capacitance is usually many times larger in order to handle the transient response of the converter. For a rule of thumb, the ratio of the operating frequency to the unity-gain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain the desired transient response. The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple. For surface mount applications, Taiyo Yuden, TDK, AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white LED application, a 4.7F capacitor value is recommended. See Table 3 for a list of component suppliers.
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LTC3452
APPLICATIO S I FOR ATIO
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and D are not required, but provide a lower drop during the break-before-make time (typically 20ns) of the NMOS to PMOS transition, improving efficiency. Use a Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. Closing the Feedback Loop The LTC3452 incorporates voltage mode PWM control. The control to output gain varies with operation region (Buck, Boost, Buck/Boost), but is usually no greater than 15. The output filter exhibits a double pole response given by:
fFILTER _ POLE = 1 Hz 2 * * L * COUT
where COUT is the output filter capacitor. The output filter zero is given by:
fFILTER _ ZERO = 1 2 * * RESR * COUT Hz
where RESR is the capacitor equivalent series resistance. A troublesome feature in Boost mode is the right-half plane zero (RHP), and is given by:
fRHPZ
VIN = Hz 2 * * IOUT * L * VOUT
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The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin, the loop is required to be crossed over a decade before the LC double pole.
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The unity-gain frequency of the error amplifier with the Type I compensation is given by:
fUG = gm 2 * * CVC
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where gm is the error amp transconductance (typically 1/5.2k) and CVC is the external capacitor to GND at the VC pin. For the white LED application, a 0.1F or greater capacitor value is recommended. Paralleling LED Outputs for Higher Current Two or more LED output pins can be connected together in parallel to achieve higher output current in fewer than 7 LEDs. For a very high power LED such as a LumiLED, all 7 outputs can be connected in parallel for maximum total output current, as shown in the back page application of this data sheet. Maximum LED Current As described in the Operation section, the maximum output LED currents are equal to:
0.8 V IMAXL = 256 RISETL and 0.8 V IMAXH = 768 RISETH
Since the maximum LED current for the low power bank is 25mA, this sets a minimum limit on RISETL of:
0.8 V RMINL = 256 = 8192 25mA
Similarly, for the high power bank:
0.8 V = 4096 RMINH = 768 150mA
In addition, since the maximum continuous output current for the buck-boost is limited to 425mA, this may impose higher resistor value minimums if all outputs are used.
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LTC3452
APPLICATIO S I FOR ATIO
Although the LTC3452 can safely provide this current continuously, the external LED(s) may not be rated for this high a level of continuous current. Higher current levels in a single LED are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by programming a high current with one or both of the RISET resistors and pulsing the appropriate enable pin or pins as shown in the back page application.
VIN
VOUT
ENL ISETL RSET RMINL VOLTAGE DAC VDAC
LEDL1 LTC3452 LEDL5 0.8V - VDAC ILED = 256 RSET CURRENT DAC
(3a)
VIN VOUT VIN
ENL ISETL RMINL RPOT
LEDL1 LTC3452 LEDL5 0.8V ILED = 256 RMINL + RPOT RSET 100
(3c)
Figure 3. Additional Brightness Control Methods: (3a) Using Voltage DAC, (3b) Using Current DAC, (3c) Using Potentiometer, (3d) Using PWM Input
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Varying LED Brightness Linearly Continuously variable LED brightness control can be achieved by interfacing directly to one or both of the ISET pins. Figure 3 shows four such methods employing a voltage DAC, a current DAC, a simple potentiometer or a PWM input applied to the ISETL pin for controlling the low power bank LED currents. These four techniques can be similarly applied to the ISETH pin for controlling the high power bank LED currents.
VIN VOUT ENL ISETL IDAC 0.8V RMINL LEDL1 LTC3452 LEDL5 ILED = 256 * IDAC
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(3b)
VOUT
ENL ISETL RSET RMINL VPWM 1F
LEDL1 LTC3452 LEDL5 ILED = 256 = 256 DVCC fPWM 10kHz 0.8V - VPWM RSET 0.8V - (DC% * VDVCC) RSET
(3d)
3452 F03
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LTC3452
APPLICATIO S I FOR ATIO
Unused Outputs
If fewer than 7 LED pins are to be used, unused LEDx pins should be connected to VOUT. The LTC3452 senses which current source outputs are not being used and shuts off the corresponding output currents to save power. A small trickle current (10A: low power bank, 30A: high power bank) is still applied to unused outputs to detect if a white LED is later switched in and also to distinguish unused outputs from used outputs during start-up. LED Failure Modes If an individual LED fails as a short circuit, the current source biasing it is shut off to save power. This is the same operation as described previously (if the output were
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initially designated unused at power-up by connecting its LEDx pin to VOUT). Efficiency is not materially affected. If an individual LED fails as an open circuit, the control loop will initially attempt to regulate off of its current source feedback signal, since it will appear to be the one requiring the largest forward voltage drop to run at its programmed current. This will drive VOUT higher. As the open circuited LED will never accept its programmed current, VOUT must be voltage-limited by means of a secondary control loop. The LTC3452 limits VOUT to 4.5V in this failure mode. The other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will decrease.
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LTC3452
PACKAGE DESCRIPTIO
4.50 0.05 3.10 0.05 2.45 0.05 (4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW--EXPOSED PAD 4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 2.45 0.10 (4-SIDES) 0.75 0.05 R = 0.115 TYP 19 20 0.38 0.10 1 2 PIN 1 NOTCH R = 0.30 TYP
NOTE: 1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-1)--TO BE APPROVED 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
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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UF Package 20-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1710)
0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC
(UF20) QFN 10-04
0.200 REF 0.00 - 0.05
0.25 0.05 0.50 BSC
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LTC3452
TYPICAL APPLICATIO
VIN 3V TO 5.5V 2.2F VIN ENH ENH ISETH 4.02k LEDH2 LEDL1, 20mA VC 0.1F ENL ENL ISETL 10.2k GND GND PGND EXPOSED PAD
3452 TA02a
4 x 20mA White LED Display + 2 x 150mA Camera Light Driver
L 4.7H 150mA PVIN SW1 SW2 VOUT D1 150mA D2 4.7F
RELATED PARTS
PART NUMBER LT1618 DESCRIPTION Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD = <1A, MS10 Package/EDD Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD = <1A, ThinSOT Package VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD = <1A, ThinSOT Package VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, ThinSOT Package/SC70 Package VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50A, ISD = <1A, QFN-24 Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD = <2.5A, DFN Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD = <2.5A, DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25A/50A, ISD = <1A, MS-10 Package/DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28A, ISD = <1A, DFN Package VIN: 2.7V to 5.5V, VOUT(MAX) = 4.5V, IQ = 600A, ISD = 6A, QFN-16 Package VIN: 2.7V to 5.5V, VOUT(MAX) = 5.15V, IQ = 825A, ISD = 0A, DFN Package
LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter LT1932 LT1937 LTC3205 LTC3215 LTC3216 LTC3440/ LTC3441 LTC3443 LTC3453 LTC3454 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator High Efficiency, Multi-Display LED Controller 700mA Low Noise High Current LED Charge Pump 1A Low Noise High Current LED Charge Pump with Independent Flash/Torch Current 600mA/1.2A IOUT, 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter 600mA/1.2A IOUT, 600kHz, Synchronous Buck-Boost DC/DC Converter 500mA Synchronous Buck-Boost High Power White LED Driver 1A Synchronous Buck-Boost High Power White LED Driver
LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, Boost Regulator with Integrated Schottky Diode ThinSOT Package LT3466 LT3479 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = <16A, DFN Package VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD = <1A, DFN Package/TSOPP Package
3452f LT 0406 * PRINTED IN THE USA
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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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LEDH1 D3 D4 D5 D6 D1, D2: AOT 2015 D3-D6: NICHIA NSCW100 L: COILCRAFT D03314-472 LOW POWER LED BANK 1MHz BUCK/BOOST LEDL2, 20mA LEDL3, 20mA LTC3452 LEDL4, 20mA LEDL5, UNUSED
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

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