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FEATURES

LT3497 Dual Full Function White LED Driver with Integrated Schottky Diodes DESCRIPTION
The LT(R)3497 is a dual full function step-up DC/DC converter specifically designed to drive up to 12 white LEDs (6 white LEDs in series per converter) from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors and expensive factory calibration. The two independent converters are capable of driving asymmetric LED strings. Accurate LED dimming and shutdown of the two LED strings can also be controlled independently. The LT3497 features a unique high side LED current sense that enables the part to function as a "one wire current source;" one side of the LED string can be returned to ground anywhere, allowing a simpler 1-wire LED connection. Traditional LED drivers use a grounded resistor to sense LED current, requiring a 2-wire connection to the LED string. The 2.3MHz switching frequency allows the use of tiny inductors and capacitors. Few external components are needed for the dual white LED Driver: open-LED protection and the Schottky diodes are all contained inside the 3mm x 2mm DFN package. With such a high level of integration, the LT3497 provides a high efficiency dual white LED driver solution in the smallest of spaces.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Drives Up to 12 White LEDs (6 in Series per Converter) from a 3V Supply Two Independent Boost Converters Capable of Driving Asymmetric LED Strings Independent Dimming and Shutdown Control of the Two LED Strings High Side Sense Allows "One Wire Current Source" per Converter Internal Schottky Diodes Open LED Protection (32V) 2.3MHz Switching Frequency 5% Reference Accuracy VIN Range: 2.5V to 10V Dual Wide 250:1 True Color PWMTM Dimming Requires Only 1F Output Capacitor per Converter Available in a 3mm x 2mm 10-Pin DFN Package
APPLICATIONS

Cellular Phones PDAs, Handheld Computers Digital Cameras MP3 Players GPS Receivers
TYPICAL APPLICATION
Li-Ion Power Driver for 4/4 White LEDs
VIN 3V TO 5V 1F
EFFICIENCY (%) 80 75 VIN = 3.6V 4/4LEDs
Efficiency
15H SW1 CAP1 10 1F LT3497 VIN
15H SW2 CAP2 10 1F
70 65 60 55
LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
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50 0 5 10 15 LED CURRENT (mA) 20
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LT3497 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW LED1 1 CTRL1 2 GND 3 CTRL2 4 LED2 5 11 10 CAP1 9 8 7 6 SW1 VIN SW2 CAP2
Input Voltage (VIN) ...................................................10V SW1, SW2 Voltages ..................................................35V CAP1, CAP2 Voltages ................................................35V CTRL1, CTRL2 Voltages ............................................10V LED1, LED2 Voltages ................................................35V Operating Temperature Range ................. -40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range................... -65C to 125C
DDB PACKAGE 10-LEAD (3mm x 2mm) PLASTIC DFN TJMAX = 125C, JA = 76C/W, JC = 13.5C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER LT3497EDDB
DDB PART MARKING LCGT
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
PARAMETER Minimum Operating Voltage LED Current Sense Voltage (VCAP1 - VLED1) LED Current Sense Voltage (VCAP2 - VLED2) Offset Voltage (VOS) Between (VCAP1 - VLED1) - (VCAP2 - VLED2) Voltages CAP1, LED1 Pin Bias Current CAP2, LED2 Pin Bias Current VCAP1, VLED1 Common Mode Minimum Voltage VCAP2, VLED2 Common Mode Minimum Voltage Supply Current
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3V, VCTRL1 = VCTRL2 = 3V.
CONDITIONS VCAP1 = 16V VCAP2 = 16V VOS = |(VCAP1 - VLED1) - (VCAP2 - VLED2)| VCAP1 = 16V, VLED1 = 16V VCAP2 = 16V, VLED2 = 16V

MIN 2.5 190 190 0
TYP 200 200 2 20 20
MAX 210 210 8 40 40 2.5 2.5
UNITS V mV mV mV A A V V mA A MHz % mA mA mV mV
VCAP1 = VCAP2 = 16V, VLED1 = VLED2 = 15V, VCTRL1 = VCTRL2 = 3V VCTRL1 = VCTRL2 = 0V 1.8 88

6 12 2.3 92 400 400 200 200 0.1 0.1
8.5 18 2.8
Switching Frequency Maximum Duty Cycle Converter 1 Switch Current Limit SW1 Converter 2 Switch Current Limit SW2 Converter 1 VCESAT Converter 2 VCESAT Switch 1 Leakage Current Switch 2 Leakage Current ISW1 = 200mA ISW2 = 200mA VSW1 = 16V VSW2 = 16V
300 300
5 5
A A
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LT3497 ELECTRICAL CHARACTERISTICS
PARAMETER VCTRL1 Voltage for Full LED Current VCTRL2 Voltage for Full LED Current VCTRL1 or VCTRL2 Voltage to Turn On the IC VCTRL1 and VCTRL2 Voltages to Shut Down the IC CTRL1, CTRL2 Pin Bias Current CAP1 Pin Overvoltage Protection CAP2 Pin Overvoltage Protection Schottky 1 Forward Drop Schottky 2 Forward Drop Schottky 1 Reverse Leakage Current Schottky 2 Reverse Leakage Current ISCHOTTKY1 = 100mA ISCHOTTKY2 = 100mA VR1 = 25V VR2 = 25V

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3V, VCTRL1 = VCTRL2 = 3V.
CONDITIONS VCAP1 = 16V VCAP2 = 16V

MIN 1.5 1.5 100
TYP
MAX
UNITS V V mV
50 100 30 30 32 32 0.8 0.8 4 4 34 34
mV nA V V V V A A
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 LT3497E 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.
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LT3497 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage (VCESAT)
450 SWITCH SATURATION VOLTAGE (mV) 400 350 -50C 300 125C 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 SWITCH CURRENT (mA)
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(TA = 25C unless otherwise specified) Shutdown Current (VCTRL1 = VCTRL2 = 0V)
15 -50C SHUTDOWN CURRENT (A) 12 25C 9 125C
Schottky Forward Voltage Drop
400 SCHOTTKY FORWARD CURRENT (mA) 350 300 250 125C 200 25C 150 -50C 100 50 0 0 0 800 600 SCOTTKY FORWARD DROP (mV) 200 400 1000
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25C
6
3
0
2
4 VIN (V)
6
8
10
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Sense Voltage (VCAP - VLED) vs VCTRL
240 25C OUTPUT CLAMP VOLTAGE (V) 200 SENSE VOLTAGE (mV) 160 -50C 120 80 40 0 0 500 1000 VCTRL (mV) 1500 2000
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Open-Circuit Output Clamp Voltage
34 30 25 INPUT CURRENT (mA) 33
Input Current in Output Open Circuit
150C 20 15 25C 10 -50C 5
125C
-50C 32 25C 31 125C
30
0
2
4 VIN (V)
6
8
10
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0 2 4 6 VIN (V) 8 10
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Switching Waveform
VSW 10V/DIV VCAP 50mV/DIV VCAP 5V/DIV VCTRL 5V/DIV
Transient Response
IL 100mA/DIV 200ms/DIV VIN = 3.6V FRONT PAGE APPLICATION CIRCUIT
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IL 200mA/DIV 1ms/DIV VIN = 3.6V FRONT PAGE APPLICATION CIRCUIT
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LT3497 TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
7 125C 6 QUIESCENT CURRENT (mA) 5 4 3 2 1 0 0 2 4 VIN (V)
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(TA = 25C unless otherwise specified) Schottky Leakage Current vs Temperature (-50C to 125C)
3 SCHOTTKY LEAKAGE CURRENT (A)
Current Limit vs Temperature
500 25C -50C CURRENT LIMIT (mA) 450
2
400
24V 1
350
16V 0 -50
6
8
10
300 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
-25
75 0 25 50 TEMPERATURE (C)
100
125
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Open-Circuit Output Clamp Voltage vs Temperature (-50C to 125C)
36 30 25 INPUT CURRENT (mA) 34 20 15 10 5 28 -50
Input Current in Output Open Circuit vs Temperature (-50C to 125C)
VIN = 3V SWITCHING FREQUENCY (MHz) 2.60 2.50 2.40 2.30 2.20 2.10 2.00 1.90 50 25 75 0 TEMPERATURE (C) 100 125
Switching Frequency vs Temperature
VIN = 3.6V
OUTPUT CLAMP VOLTAGE (V)
32
30
-25
75 0 25 50 TEMPERATURE (C)
100
125
0 -50 -25
1.80 -50 -25
0
50 75 25 TEMPERATURE (C)
100
125
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Sense Voltage (VCAP - VLED) vs VCAP
208 206
Sense Voltage vs Temperature
204 SENSE VOLTAGE (mV) SENSE VOLTAGE (mV) 25 30
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202
200
125C 25C -50C
198
196
194
192
188
5
10
20 15 VCAP (V)
190 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
3497 G17
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LT3497 PIN FUNCTIONS
LED1 (Pin 1): Connection point for the anode of the first LED of the first set of LEDs and the sense resistor (RSENSE1). The LED current can be programmed by: ILED1 = 200mV RSENSE1 CAP2 (Pin 6): Output of Converter 2. This pin is connected to the cathode of internal Schottky diode 2. Connect the output capacitor to this pin and the sense resistor (RSENSE2) from this pin to LED2 pin. SW2 (Pin 7): Switch Pin. Minimize trace area at this pin to minimize EMI. Connect the inductor at this pin. VIN (Pin 8): Input Supply Pin. This pin must be locally bypassed. SW1 (Pin 9): Switch Pin. Minimize trace area at this pin to minimize EMI. Connect the inductor at this pin. CAP1 (Pin 10): Output of Converter 1. This pin is connected to the cathode of internal Schottky diode 1. Connect the output capacitor to this pin and the sense resistor (RSENSE1) from this pin to LED1 pin. Exposed Pad (Pin 11): Ground. Must be soldered to PCB.
CTRL1 (Pin 2): Dimming and Shutdown Pin. Connect CTRL1 below 50mV to disable converter 1. As the pin voltage is ramped from 0V to 1.5V, the LED current ramps from 0 to (ILED1 = 200mV/RSENSE1). The CTRL1 pin must not be left floating. GND (Pin 3): Connect the GND pin to the PCB system ground plane. CTRL2 (Pin 4): Dimming and Shutdown Pin. Connect CTRL2 below 50mV to disable converter 2. As the pin voltage is ramped from 0V to 1.5V, the LED current ramps from 0 to (ILED2 = 200mV/RSENSE2). The CTRL2 pin must not be left floating. LED2 (Pin 5): Connection point for the anode of the first LED of the second set of LEDs and the sense resistor (RSENSE2). The LED current can be programmed by: ILED2 = 200mV RSENSE2
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L1 15H CIN 1F 9 SW1 VIN SW2 8 7
BLOCK DIAGRAM
L2 15H
OVERVOLTAGE PROTECT
OVERVOLTAGE PROTECT
Q1 A2 A2
R QR S R RQ S
RSENSE1 10
+
A3 RAMP GENERATOR A3
R
COUT1 1F
1 2.3MHz OSCILLATOR
LED1
-
CONVERTER 1 gm AMP A = 6.25 1.25V RC CC START-UP CTRL1 2 CTRL2 GND 3 4 RC CC gm AMP A1 CONVERTER 2
+ -
A1
- + +
Figure 1. LT3497 Block Diagram
+ +
R
- - +
A = 6.25 1.25V
- + - + + -
START-UP
10
CAP1 DRIVER DRIVER
CAP2 Q2
6
RSENSE2 10
LED2
5
COUT2 1F
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LT3497
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LT3497 OPERATION
Main Control Loop The LT3497 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It incorporates two identical, but fully independent PWM converters. Operation can be best understood by referring to the Block Diagram in Figure 1. The oscillator, start-up bias and the band gap reference are shared between the two converters. The control circuitry, power switch, Schottky diode etc., are identical for both the converters. At power up, the capacitors at CAP1 and CAP2 pins are charged up to VIN (input supply voltage) via their respective inductor and the internal Schottky diode. If either CTRL1 and CTRL2 or both are pulled higher than 100mV, the bandgap reference, the start-up bias and the oscillator are turned on. The main control loop can be understood by following the operation of converter 1. At the start of each oscillator cycle, the power switch, Q1, is turned on. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A2. When this voltage exceeds the level at the negative input of A2, the PWM logic turns off the power switch. The level at the negative input of A2 is set by the error amplifier, A1, and is simply an amplified version of the difference between the VCAP1 and VLED1 voltage and the bandgap reference. In this manner the error amplifier, A1, sets the correct peak current level in inductor L1 to keep the output in regulation. The CTRL1 pin is used to adjust the LED current. If only one of the converters is turned on, the other converter will stay off and its output will remain charged up to VIN (input supply voltage). The LT3497 enters into shutdown when both CTRL1 and CTRL2 pins are pulled lower than 50mV. The CTRL1 and CTRL2 pins perform independent dimming and shutdown control for the two converters. Minimum Output Current The LT3497 can drive a 4-LED string at 2mA LED current without pulse skipping. As current is further reduced, the device may begin skipping pulses. This will result in some low frequency ripple, although the average LED current remains regulated down to zero. The photo in Figure 2 details circuit operation driving 4 white LEDs at 2mA. Peak inductor current is less than 50mA and the regulator operates in discontinuous mode, meaning the inductor current reaches zero during the discharge phase. After the inductor current reaches zero, the SW pin exhibits ringing due to the LC tank circuit formed by the inductor in combination with the switch and the diode capacitance. This ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions.
IL 50mA/DIV
VSW 10V/DIV
VIN = 4.2V ILED = 2mA 4 LEDs
200ns/DIV
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Figure 2. Switching Waveforms
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LT3497 APPLICATIONS INFORMATION
DUTY CYCLE The duty cycle for a step-up converter is given by: D= where: VOUT = Output voltage VD = Schottky forward voltage drop VCESAT = Saturation voltage of the switch VIN = Input voltage The maximum duty cycle achievable for LT3497 is 88% when running at 2.3MHz switching frequency. Always ensure that the converter is not duty-cycle limited when powering the LEDs at a given frequency. INDUCTOR SELECTION A 15H inductor is recommended for most LT3497 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 2.3MHz and low DCR (copper wire resistance). Some inductors in this category with small size are listed in Table 1. The efficiency comparison of different inductors is shown in Figure 3.
Table 1: Recommended Inductors
L (H) 15 15 10 10 15 15 15 MAX DCR () 0.58 1.6 0.3 1.2 0.654 0.80 0.739 CURRENT RATING (mA) 300 200 450 225 440 360 410
15H MURATA LQH32CN150K53 15H MURATA LQH2MCN150K02 15H COOPER SD3112-150 15H TOKO 1001AS-150M TYPE D312C 15H SUMIDA CDRH2D11/HP 80 75 70 EFFICIENCY (%) 65 60 55 50 45 0 5 10 LED CURRENT (mA) 15 20
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VOUT + VD - VIN VOUT + VD - VCESAT
Figure 3. Efficiency Comparison of Different Inductors
and a 1F output capacitor are sufficient for most applications. Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information on their entire selection of ceramic parts.
Table 2: Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden AVX Murata (800) 368-2496 www.t-yuden.com (803) 448-9411 www.avxcorp.com (714) 852-2001 www.murata.com
PART LQH32CN150K53 LQH2MCN150K02 LQH32CN100K53 LQH2MCN100K02 SD3112-150 1001AS-150M (TYPE D312C) CDRH2D11/HP
VENDOR Murata www.murata.com
OVERVOLTAGE PROTECTION The LT3497 has an internal open-circuit protection circuit for both converters. In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail open circuit, the converter VCAP voltage is clamped at 32V (typ). Figure 4a shows the transient response of the front page application step-up converter with LED1 disconnected. With LED1 disconnected, the converter starts switching at the peak inductor current limit. The converter output starts ramping up and finally gets clamped at 32V (typ). The converter will then switch at low inductor current to regulate the converter output at the clamp voltage. The VCAP and input current during output open circuit are shown in the Typical Performance Characteristics.
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Cooper www.cooperet.com Toko www.toko.com Sumida www.sumida.com
CAPACITOR SELECTION The small size of ceramic capacitors make them ideal for LT3497 applications. Use only X5R and X7R types because they retain their capacitance over wider temperature ranges than other types such as Y5V or Z5U. A 1F input capacitor
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LT3497 APPLICATIONS INFORMATION
VCAP 10V/DIV
For low DCR inductors, which are usually the case for this application, the peak inrush current can be simplified as follows: = r 2 *L 1 r2 - L * C 4 * L2 VIN - 0.6 * exp - * 2 L *
ISW 200mA/DIV
=
VIN = 3.6V FRONT PAGE APPLICATION CIRCUIT 500s/DIV LEDs DISCONNECTED AT THIS INSTANT
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IPK =
Figure 4a. Transient Response of Switcher 1 with LED1 Disconnected from the Output
IL1 50mA/DIV VSW1 20V/DIV IL2 50mA/DIV VSW2 20V/DIV VIN = 3.6V 4 LEDs LED 2 DISCONNECTED 200ms/DIV
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where L is the inductance, r is the DCR of the inductor and C is the output capacitance. Table 3 gives inrush peak currents for some component selections.
Table 3: Inrush Peak Currents
VIN (V) 4.2 4.2 4.2 4.2 r () 0.58 1.6 0.8 0.739 L (H) 15 15 15 15 COUT (F) 1 1 1 1 IP (A) 0.828 0.682 0.794 0.803
Figure 4b. Switching Waveforms with Output 1 Open Circuit
In the event one of the converters has an output open circuit, its output voltage will be clamped at 32V. However, the other converter will continue functioning properly. The photo in Figure 4b shows circuit operation with converter 2 output open circuit and converter 1 driving 4 LEDs at 20mA. Converter 2 starts switching at a lower peak inductor current and begins skipping pulses, thereby reducing its input current. INRUSH CURRENT The LT3497 has built-in Schottky diodes. When supply voltage is applied to the VIN pin, an inrush current flows through the inductor and the Schottky diode and charges up the CAP voltage. Both the Schottky diodes in the LT3497 can sustain a maximum current of 1A. The selection of inductor and capacitor value should ensure the peak of the inrush current to be below 1A.
PROGRAMMING LED CURRENT The LED current of each LED string can be set independently by the choice of resistors RSENSE1 and RSENSE2, respectively. For each LED string, the feedback resistor (RSENSE) and the sense voltage (VCAP - VLED) control the LED current. For each independent LED string, the CTRL pin controls the sense reference voltage as shown in the Typical Performance Characteristics. For CTRL higher than 1.5V, the sense reference is 200mV, which results in full LED current. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and Table 4 for RSENSE selection are shown below. RSENSE = 200mV ILED
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LT3497 APPLICATIONS INFORMATION
Table 4: RSENSE Value Selection for 200mV Sense
ILED (mA) 5 10 15 20 RSENSE () 40 20 13.3 10
PWM 10kHz TYP R1 100k C1 0.1F LT3497 CTRL1,2
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Figure 5. Dimming Control Using a Filtered PWM Signal
DIMMING CONTROL There are three different types of dimming control circuits. The LED current can be set by modulating the CTRL pin with a DC voltage, a filtered PWM signal or directly with a PWM signal. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL pin voltage can be modulated to set the dimming of the LED string. As the voltage on the CTRL pin increases from 0V to 1.5V, the LED current increases from 0 to ILED. As the CTRL pin voltage increases beyond 1.5V, it has no effect on the LED current. The LED current can be set by: ILED ILED 200mV when VCTRL > 1.5V RSENSE VCTRL when VCTRL < 1.25V 6.25 * RSENSE
Direct PWM Dimming Changing the forward current flowing in the LEDs not only changes the intensity of the LEDs, it also changes the color. The chromaticity of the LEDs changes with the change in forward current. Many applications cannot tolerate any shift in the color of the LEDs. Controlling the intensity of the LEDs with a direct PWM signal allows dimming of the LEDs without changing the color. In addition, direct PWM dimming offers a wider dimming range to the user. Dimming the LEDs via a PWM signal essentially involves turning the LEDs on and off at the PWM frequency. The typical human eye has a limit of ~60 frames per second. By increasing the PWM frequency to ~80Hz or higher, the eye will interpret that the pulsed light source is continuously on. Additionally, by modulating the duty cycle (amount of "on time") the intensity of the LEDs can be controlled. The color of the LEDs remains unchanged in this scheme since the LED current value is either zero or a constant value. Figure 6 shows a Li-ion powered 4/4 white LED driver. Direct PWM dimming method requires an external NMOS tied between the cathode of the lowest LED in the string and ground as shown in Figure 6. Si2318DS MOSFETs can be used since its sources are connected to ground. The PWM signal is applied to the (CTRL1 and CTRL2) control pins of the LT3497 and the gate of the MOSFET. The PWM signal should traverse between 0V to 5V to ensure proper turn on and off of the converters and the NMOS transistors (Q1 and Q2). When the PWM signal goes high, LEDs are connected to ground and a current of ILED = (200mV/RSENSE) flows through the LEDs. When the PWM signal goes low, the LEDs are disconnected and turn off. The low PWM input applied to the LT3497 ensures that the respective
Feedback voltage variation versus control voltage is given in the Typical Performance Characteristics. Using a Filtered PWM Signal A filtered PWM can be used to control the brightness of the LED string. The PWM signal is filtered (Figure 5) by a RC network and fed to the CTRL1, CTRL2 pins. The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be much smaller than the internal impedance in the CTRL pins which is 10M (typ).
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LT3497 APPLICATIONS INFORMATION
converter turns off. The MOSFETs ensure that the LEDs quickly turn off without discharging the output capacitors which in turn allows the LEDs to turn on faster. Figures 7 and 8 show the PWM dimming waveforms and efficiency for the Figure 6 circuit. The time it takes for the LEDs current to reach its programmed value sets the achievable dimming range for a given PWM frequency. For example, the settling time of the LEDs current in Figure 7 is approximately 40s for a 3V input voltage. The achievable dimming range for this application and 100Hz PWM frequency can be determined using the following method. Example: = 100Hz, tSETTLE = 40s tPERIOD = 1/ = 1/100 = 0.01s Dim Range = tPERIOD/tSETTLE = 0.01s/40s = 250:1 Min Duty Cycle = tSETTLE/tPERIOD * 100 = 40s/0.01s = 0.4% Duty Cycle Range = 100%0.4% at 100Hz The calculations show that for a 100Hz signal the dimming range is 250 to 1. In addition, the minimum PWM duty cycle of 0.4% ensures that the LEDs current has enough
3V TO 5V 1F L1 15H SW1 CAP1 1F RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 Q1 Si2318DS 100k 5V 0V PWM FREQ PWM FREQ 5V 0V
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L2 15H VIN SW2 CAP2 RSENSE2 10 1F
Q2 Si2318DS 100k
Figure 6. Li-Ion to 4/4 White LEDs with Direct PWM Dimming
80 ILED 20mA/DIV 78 EFFICIENCY (%)
VIN = 3.6V 4/4 LEDs
IL 200mA/DIV PWM 5V/DIV VIN = 3.6V 4 LEDs 2ms/DIV
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76
74
72
Figure 7. Direct PWM Dimming Waveforms
70
0
5
10 LED CURRENT (mA)
15
20
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Figure 8. Efficiency
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LT3497 APPLICATIONS INFORMATION
time to settle to its final value. Figure 9 shows the available dimming range for different PWM frequencies with a settling time of 40s.
10000 3V TO 5V 1F L1 15H SW1 CAP1 PWM DIMMING RANGE 1000 PULSING MAY BE VISIBLE 1F RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 100 5V 10 Q1 Si2318DS 1 10 100 1000 PWM FREQUENCY (Hz) 10000
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L2 15H VIN SW2 CAP2 RSENSE2 10 1F
5V PWM FREQ 0V PWM FREQ Q2 Si2318DS
0V
100k
100k
Figure 9. Dimming Ratio vs Frequency
Figure 10. Li-Ion to 4/4 White LEDs with Both PWM Dimming and Analog Dimming
The dimming range can be further extended by changing the amplitude of the PWM signal. The height of the PWM signal sets the commanded sense voltage across the sense resistor through the CTRL pin. In this manner both analog dimming and direct PWM dimming extend the dimming range for a given application. The color of the LEDs no longer remains constant because the forward current of the LED changes with the height of the CTRL signal. For the 4-LED application described above, the LEDs can be dimmed first, modulating the duty cycle of the PWM signal. Once the minimum duty cycle is reached, the height of the PWM signal can be decreased below 1.5V down to 100mV. The use of both techniques together allows the average LED current for the 4-LED application to be varied from 20mA down to less than 20A. Figure 10 shows the application for dimming using both analog dimming and PWM dimming. A potentiometer must be added to ensure that the gate of the NMOS receives a logic-level signal, while the CTRL signal can be adjusted to lower amplitudes. LOW INPUT VOLTAGE APPLICATIONS The LT3497 can be used in low input voltage applications. The input supply voltage to the LT3497 must be 2.5V or higher. However, the inductors can be run off a
lower battery voltage. This technique allows the LEDs to be powered off two alkaline cells. Most portable devices have a 3.3V supply voltage which can be used to power the LT3497. The LEDs can be driven straight from the battery, resulting in higher efficiency. Figure 11 shows 3/3 LEDs powered by two AA cells. The battery is connected to the inductors and the chip is powered off a 3.3V logic supply voltage.
3.3V 2 AA CELLS 2V TO 3.2V C2 1F L1 15H SW1 CAP1 RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
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C1 1F
L2 15H VIN SW2 CAP2 RSENSE2 10
C3 1F
C4 1F
C1, C2: TAIYO YUDEN LMK212BJ105MG C3, C4: TAIYO YUDEN GMK212BJ105KG L1, L2: MURATA LQH32CN150K53
Figure 11. 2 AA Cells to 3/3 White LEDs
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LT3497 APPLICATIONS INFORMATION
BOARD LAYOUT CONSIDERATIONS As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To prevent electromagnetic interference (EMI) problems, proper layout of high frequency switching paths is essential. Minimize the length and area of all traces connected to the switching node pins (SW1 and SW2). Keep the sense voltage pins (CAP1, CAP2, LED1 and LED2) away from
VIA TO GROUND PLANE
the switching node. Place the output capacitors (COUT1 and COUT2) next to the output pins (CAP1 and CAP2). The placement of a bypass capacitor on VIN needs to be in close proximity to the IC to filter EMI noise from SW1 and SW2. Always use a ground plane under the switching regulator to minimize interplane coupling. Recommended component placement is shown in Figure 12.
COUT2 SW2 L2 CAP2 10 VIA TO GROUND PLANE CIN VIN L1 CAP1 9 8 7 6 LED2 CTRL2 5 4 3 2 1 CTRL1 SW1 COUT1
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GND
LED1
VIAS TO GROUND PLANE
Figure 12. Recommended Component Placement
TYPICAL APPLICATIONS
Li-Ion to 1/2 White LEDs
VIN 3V TO 5V 70 C1 1F C2 1F C3 1F
Conversion Efficiency
VIN = 3.6V 65 1/2LEDs 60 EFFICIENCY (%)
3497 TA02a
L1 10H SW1 CAP1 VIN LT3497
L2 10H SW2 CAP2
55 50 45 40 35
RSENSE1 10
RSENSE2 10
LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
30
0
5
10 LED CURRENT (mA)
15
20
3497 TA02b
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN100K53
3497f
14
LT3497 TYPICAL APPLICATIONS
Li-Ion to 2/2 White LEDs
VIN 3V TO 5V 70 C3 1F 65 60 55 50 45 OFF ON SHUTDOWN AND DIMMING CONTROL 2 40 0 5 10 15 LED CURRENT (mA) 20
3497 TA12b
Conversion Efficiency
VIN = 3.6V 2/2 LEDs
C1 1F
SW1 CAP1 RSENSE1 10
VIN LT3497
SW2 CAP2 RSENSE2 10
3497 TA12a
LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN100K53
Li-Ion to 2/2 White LEDs Conversion Efficiency
3V TO 5V C3 1F L1 10H SW1 CAP1 C1 1F RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
3497TA13a
EFFICIENCY (%) EFFICIENCY (%) C2 1F
L1 10H
L2 10H
C2 1F
L2 10H VIN SW2 CAP2 RSENSE2 10
80 V IN = 3.6V 2/2LEDs 75 70 65 60 55 50 45 40 0 5 10 LED CURRENT (mA)
3497 TA13b
15
20
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN100K53
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15
LT3497 TYPICAL APPLICATIONS
Li-Ion to 2/4 White LEDs Conversion Efficiency
VIN 3V TO 5V C3 1F EFFICIENCY (%) L1 10H SW1 CAP1 RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
3497 TA03a
80 75 70 65 60 55 50 45 0
VIN = 3.6V 2/4LEDs
C1 1F
L2 15H VIN SW2 CAP2 RSENSE2 10 C2 1F
5
10 LED CURRENT (mA)
15
20
3497 TA03b
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1: MURATA LQH32CN100K53 L2: MURATA LQH32CN150K53
Li-Ion to 3/3 White LEDs Conversion Efficiency
VIN 3V TO 5V C3 1F L1 15H SW1 C1 1F CAP1 RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2 45 0 5 10 LED CURRENT (mA)
3497 TA04b
80 75 70 EFFICIENCY (%) 65 60 55
3497 TA04a
VIN = 3.6V 3/3LEDs
L2 15H VIN SW2 CAP2 RSENSE2 10 C2 1F
50
15
20
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN150K53
3497f
16
LT3497 TYPICAL APPLICATIONS
Li-Ion to 4/6 White LEDs
VIN 3V TO 5V 80 C3 1F L1 15H SW1 C1 1F CAP1 RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2 50
3497 TA05a
Conversion Efficiency
VIN = 3.6V 4/6LEDs
L2 15H VIN SW2 CAP2 RSENSE2 10 C2 1F EFFICIENCY (%)
75 70 65 60 55
0
5
10 15 LED CURRENT (mA)
20
3497 TA05b
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN150K53
Li-Ion to 5/5 White LEDs
VIN 3V TO 5V 80 C3 1F L1 15H SW1 C1 1F CAP1 RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
3497 TA06a
Conversion Efficiency
VIN = 3.6V 5/5LEDs
L2 15H VIN SW2 CAP2 RSENSE2 10 C2 1F EFFICIENCY (%)
75 70 65 60 55 50 0 5 10 15 LED CURRENT (mA) 20
3497 TA06b
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN150K53
3497f
17
LT3497 TYPICAL APPLICATIONS
Li-Ion to 6/6 White LEDs
VIN 3V TO 5V C3 1F L1 15H SW1 CAP1 C1 1F RSENSE1 10 LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
3497 TA07a
Conversion Efficiency
80 75 70 65 60 55 50 0 5 10 15 LED CURRENT (mA) 20
3497 TA07b
L2 15H EFFICIENCY (%) VIN SW2 CAP2 RSENSE2 10 C2 1F
VIN = 3.6V 6/6LEDs
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN150K53
2-Cell Li-Ion Movie and Flash Mode/6 White LEDs Control
VIN 6V TO 9V C3 1F RSENSE1 1 C1 4.7F D1 L1 15H CAP1 VIN LED1 LT3497 SW1 LED2 CTRL1 GND CTRL2 FLASH VCTRL1 680mV MOVIE 1.5V OFF ON SHUTDOWN AND DIMMING CONTROL 2 ILED 100mA 200mA C1: TAIYO YUDEN LMK212BJ475KD C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG D1: AOT-2015 HPW1751B L1, L2: MURATA LQH32CN150K53
3497 TA08a
Conversion Efficiency
85 1-100mA LED/6 LEDs
L2 15H 80 CAP2 RSENSE2 10 C2 1F EFFICIENCY (%) SW2
75
70
65 6 6.5 7 7.5 VIN (V) 8 8.5 9
MODE MOVIE FLASH
3497 TA08b
3497f
18
LT3497 PACKAGE DESCRIPTION
DDB Package 10-Lead Plastic DFN (3mm x 2mm)
(Reference LTC DWG # 05-08-1722 Rev O)
0.64 0.05 (2 SIDES) 0.70 0.05 2.55 0.05 1.15 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.39 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 6
3.00 0.10 (2 SIDES)
R = 0.05 TYP
0.40 0.10 10
PIN 1 BAR TOP MARK (SEE NOTE 6)
2.00 0.10 (2 SIDES) 0.64 0.05 (2 SIDES) 5 0.25 0.05 2.39 0.05 (2 SIDES) BOTTOM VIEW--EXPOSED PAD
1
PIN 1 R = 0.20 OR 0.25 x 45 CHAMFER
(DDB10) DFN 0905 REV O
0.200 REF
0.75 0.05
0.50 BSC
0 - 0.05
NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 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
3497f
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.
19
LT3497 TYPICAL APPLICATION
2 Li-Ion to 8/8 White LEDs
VIN 6V TO 9V 85 C3 1F L1 15H SW1 CAP1 RSENSE1 10 C1 1F LT3497 LED1 LED2 CTRL1 GND CTRL2 OFF ON SHUTDOWN AND DIMMING CONTROL 1 OFF ON SHUTDOWN AND DIMMING CONTROL 2
3497 TA11a
Conversion Efficiency
VIN = 7.2V 8/8LEDs
80 75 EFFICIENCY (%) C2 1F 70 65 60 55 50 0
L2 15H VIN SW2 CAP2 RSENSE2 10
5
10 LED CURRENT (mA)
15
20
3497 TA11b
C1, C2: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN LMK212BJ105MG L1, L2: MURATA LQH32CN150K53
RELATED PARTS
PART NUMBER LT1937 LTC3200-5 LTC3201 LTC3202 LTC3205 LT3465/LT3465A LT3466/LT3466-1 LT3486 LT3491 DESCRIPTION Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Low Noise, 2MHz Regulated Charge Pump White LED Driver Low Noise, 1.7MHz Regulated Charge Pump White LED Driver Low Noise, 1.5MHz Regulated Charge Pump White LED Driver High Efficiency, Multidisplay LED Controller Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode COMMENTS Up to 4 White LEDs, VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOTTM/SC70 Packages Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1A, ThinSOT Package Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 6.5mA, ISD < 1A, MS Package Up to 8 White LEDs, VIN: 2.7V to 4.5V, IQ = 5mA, ISD < 1A, MS Package Up to 4 (Main), 2 (Sub) and RGB, VIN: 2.8V to 4.5V, IQ = 50A, ISD < 1A, 24-Lead QFN Package Up to 6 White LEDs, VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT Package
Dual Full Function, 2MHz Diodes White LED Step-Up Converter Up to 20 White LEDs, VIN: 2.7V to 24V, VOUT(MAX) = 39V, with Built-In Schottkys DFN, TSSOP-16 Packages Dual 1.3A White LED Converter with 1000:1 True Color PWM Dimming White LED Driver in SC70 with Integrated Schottky Drives Up to 16 100mA White LEDs. VIN: 2.5V to 24V, VOUT(MAX) = 36V, DFN, TSSOP Packages Drives Up to 6 20mA White LEDs, VIN: 2.5V to 12V, VOUT(MAX) = 27V, 8-Lead SC70 Package
ThinSOT is a trademark of Linear Technology Corporation.
3497f
20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1206 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

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