QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER LT3475 DESCRIPTION Demonstration circuit 923 features the LT®3475 dual step-down 1.5A LED driver in a 16-lead TSSOP thermally enhanced package. The demonstration circuit is designed to drive two 1.5A LEDs on separate channels (one on each channel) from a wide input voltage range. The high switching frequency scheme permits the use of a small inductor and ceramic capacitors to save space and cost. The current mode PWM control scheme provides fast transient response and cycle-by-cycle current limiting. Frequency foldback and thermal shutdown provide additional protection. DC923A is specifically designed for high power LED applications such as automotive and avionic lighting, architectural detail lighting and display backlighting. The LT3475 has a few LED-dimming circuit options. See the datasheet for more detailed information on LED brightness and dimming control using both the PWM pin function and the VADJ pin function. The PWM terminals can be used to dim the LEDs with the addition of PWM MOSFETS and the resistor-short connections from LED- to GND removed. NOTE: The input voltage is limited to 4-30V during startup. During normal (non-startup) operation, the input voltage can be as high as 36V. Design files for this circuit board are available. Call the LTC factory. LT is a registered trademark of Linear Technology Corporation QUICK START PROCEDURE DC923A is easily set up for evaluating the LT3475 dual step-down 1.5A LED driver circuit. Follow the test procedure outlined below and see Figure2. 1. Connect a bench supply with power OFF to the VIN and GND terminals. 2. Connect a single white 1.5A LED to the LED1+ and LED1- terminals. 3. Connect a single white 1.5A LED to the LED2+ and LED2- terminals. 84 83 VLED1,2 = 3.8V 82 ILED1,2 = 1.5A FSW = 600KHZ 81 EFFICIENCY (%) 80 79 78 4. Turn on the input power supply and set the voltage above 6V and below 30V. 77 5. Observe the LEDs working properly at 1.5A without looking straight at the LEDs. The extreme brightness may be harmful to the eye. 74 76 75 5 10 15 20 25 30 35 40 INPUT VOLTAGE (V) Figure 1. DC923A Efficiency 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER the PWM dimming MOSFETs are stuffed with their gate pull-down resistors (R6, R7). For best results, the C-R-C compensation network shown in Figure 3 should be stuffed for each channel when PWM dimming is used. The C-R-C network provides the most optimal LED current transient waveform coming out of PWM dimming off-time. The C-R-C network also helps maintain switch current control during startup with input voltages above 30V. EXTENDED PWM DIMMING Figure 2. Proper Measurement Equipment Setup INPUT VOLTAGE RANGE The operating input voltage range for the LT3475 IC is 4V to 36V with an absolute maximum rating of 40V. Under some conditions, it is not recommended to run the IC up to 36V non-transient DC voltage. Please see the datasheet for input voltage range calculations based on switching frequency, LED output voltage, and compensation network topology during startup. As assembled, the DC923A should not be started up above 30V DC input without first assembling the C-R-C compensation network shown in Figure 3 or using a bigger inductor with up to 3.2A capability in order to maintain control of the switch current. PWM DIMMING DC923A is assembled to show a combination of the most basic operation of the IC and the small solution size. The optional PWM dimming MOSFETs (Q1, Q2) are not stuffed on DC923A, but there are two SOT23 spaces designated for these to be added to the board. The LEDto GND resistor-short connections (R2, R3) must be removed from the board to enable PWM dimming when The PWM dimming range can be extended by adding the components Q3, R13, and C7 to the circuit as shown in Figure 4. Q3 is located on the back of the PCB near the PWM terminal. The datasheet describes the calculation of maximum PWM dimming range with and without the extended dimming range components. When these components are added to the circuit, the PWM1 signal is directly connected to the extended dimming range components. If both channel 1 and channel 2 are being used in PWM dimming mode and the extended dimming range circuit is used, the PWM2 signal must be synchronized with the PWM1 signal. The rising edges must occur at the same time although their duty cycles may vary. Failure to do this may result in significant jitter on the second channel or limited PWM dimming range. LED STRINGS DC923A is assembled with optimized components for driving a single white LED at 1.5A on each of its two channels. The LEDs are not assembled on the PCB and should be attached to the LED terminals as shown in Figure 2. If the LEDs are not attached, the DC923A will protect itself and survive an open LED condition. If more than one LED is attached in series to either output, the inductor size, switching frequency, and/or input voltage range may need to be adjusted for proper operation. Consult the datasheet for proper sizing of the inductor for given input and output voltage conditions. The maximum LED output voltage is 13.5V as defined by the clamp voltage in the datasheet. This limits the number of LEDs in series on the output. 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER If a single LED such as a red or amber LED with forward voltage below 2.7V is attached to the output, the boost circuit must be altered to supply a boost voltage above 3V from either the input, the other output channel, or from a separate supply. There is a space on each channel on DC923A to accommodate an alternate boost supply or boost diode as shown in Figure 5. If the alternate boost supply is not used in this case, the internal power switch will not run in low VCE saturation and the part may heat up and run inefficiently. VIN 5V-36V CIN 4.7uF 50V 20k PWM1 VIN VIN SHDN SHDN PWM2 LT3475EFE WHITE 1.5A 10uH COUT1 2.2uF PWM1 100Hz 800:1 1N4448W LED1 LED2 OUT1 OUT2 BOOST1 BOOST2 0.22uF SW1 Sumida 8D28 B240A REF Si2312BDS VADJ1 C5 0.1uF 2.2uF VADJ2 R11 VC1 C3 3300pF 600kHz RT GND GND PWM2 Q2 R10 10k WHITE COUT2 Si2312BDS REF 10k PWM Dimming MOSFET and Gate Resistor Sumida 8D28 REF B240A 1.5A 10uH SW2 Q1 R6 1N4448W 0.22uF 10k 10k VC2 C4 3300pF R7 100Hz 800:1 C6 0.1uF RT 24.3k PWM Dimming C-RC Compensation Figure 3. PWM Dimming MOSFETs, Gate Pull-down Resistors, and Optimal CRC Compensation 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER VIN 5V-36V CIN 4.7uF 50V 20k PWM1 VIN VIN SHDN SHDN PWM2 LT3475EFE WHITE 1.5A 10uH COUT1 100Hz 1200:1 LED2 OUT1 OUT2 BOOST1 BOOST2 0.22uF 0.22uF B240A Sumida 8D28 B240A REF VADJ1 10k 0.1uF R13 VC1 GND GND 3300pF VC2 COUT2 2.2uF PWM2 10k 10k 3300pF 100Hz 1200:1 0.1uF 1.0M Q3 C7 VADJ2 RT WHITE Si2312BDS REF 10k 1.5A 10uH REF Si2312BDS 2N7002 1N4448W SW2 SW1 Sumida 8D28 2.2uF PWM1 1N4448W LED1 220pF 600kHz RT 24.3k Extended Dimming Range Figure 4. Extended Dimming Range Components Q3, R13, and C7 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER 4V-36V* 3k CIN 4.7uF 2222A 50V CMHZ5231 5.1V 4V Boost from VIN Over Wide VIN Range VIN 20k VIN VIN PW M1 OUT2 BOOST1 RED 10uH COUT1 2.2uF 0.22uF SW 1 MMSD4148 SW 2 B240A REF RED 10uH Sum ida 8D 28 REF B240A 1.5A BOOST2 0.22uF Sum ida 8D 28 CMHZ5231 5.1V LED 2 OUT1 MMSD4148 2222A SH DN PW M 2 LT3475EFE LED1 1.5A 3k SHDN COUT2 2.2uF REF VADJ1 Red Vf from 2.2V to 3.2V VC1 VADJ2 GNDGND RT 0.1uF VC 2 0.1uF 600kHz RT 24.3k Figure 5. Single Red LED Driver With Alternate Boost Supply From VIN 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER VIN 8V-36V Higher String Voltage Must Remain Under 13.5V CIN 4.7uF 50V 20k PWM1 VIN VIN SHDN PWM2 LT3475EFE LED1 1.5A SHDN OUT1 WHITE 15uH WHITE COUT1 2.2uF 1N4448W LED2 BOOST1 RED BOOST2 0.22uF 1N4448W 0.22uF SW1 Sumida 104R 1.5A OUT2 15uH SW2 Sumida 104R REF B240A B240A REF RED COUT2 2.2uF REF VADJ1 VADJ2 Bigger L for Higher LED String Voltage VC1 RT GND GND 0.1uF VC2 0.1uF 600kHz RT 24.3k Figure 6. LED strings up to 13.5V require larger inductors to avoid subharmonic oscillations at high duty cycles. 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923 DUAL STEP-DOWN 1.5A LED DRIVER 7