QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315 DUAL STEP-DOWN 1.5A LED DRIVER LT3475-1 DESCRIPTION Demonstration circuit 1315 features the addition of a PWM MOSFET and the resistor- LT 3475-1 dual step-down 1.5A LED driver in a short connection from LED- to GND removed. 16-lead TSSOP thermally enhanced package. The efficiency of DC1315A with 20V input and 4 The demonstration circuit is designed to drive white LEDs on each 1.5A channel is 91.5%. two 1.5A LED strings (one on each channel) NOTE: The input voltage is limited to 30V during from a high input voltage. The high switching startup. During normal (non-startup) operation, frequency scheme permits the use of a small the input voltage can be as high as 36V. inductor and ceramic capacitors to save space DC1315 allows LED string voltages up to 23V. and cost. The current mode PWM control However, some combinations of VIN, VLED, scheme provides fast transient response and and ILED can result in the LT3475-1 getting hot. cycle-by-cycle current limiting. DC1315 is Please consult the datasheet for thermal calcu- specifically designed for high power LED lations and details. The voltages, the LED cur- applications such as automotive and avionic rent, and the switching frequency can be ad- lighting, architectural detail lighting and display justed for optimal thermal performance. backlighting. See the LT3475-1 datasheet for more detailed information on LED brightness Design files for this circuit board are avail- and dimming control using both the PWM pin able. Call the LTC factory. function and the V pin function. The PWM ADJ LT is a registered trademark of Linear Technology Corporation terminals can be used to dim the LEDs with the QUICK START PROCEDURE DC1315 is easily set up for evaluating the diode or use another boost method. Please LT3475-1 dual step-down 1.5A LED driver see the datasheet for details. circuit. Follow the test procedure outlined below 4. Turn on the input power supply and set and see Figure 1. the voltage above the LED string voltage 1. Connect a bench supply with power OFF and below 30V. to the V and GND terminals. IN 5. Observe the LEDs working properly at 2. Connect a 1.5A LED string with forward 1.5A without looking straight at the LEDs. voltage >13V to the LED1+ and LED1- The brightness is harmful to the eye. terminals. 3. Connect a 1.5A LED string with forward voltage >13V to the LED2+ and LED2- terminals. NOTE: The boost circuitry on DC1315 is de- signed to work best with VLED >13V since there is a 9V zener diode limiting the boost voltage. If lower voltage LED strings are re- quired, please lower the voltage of the zener 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315 DUAL STEP-DOWN 1.5A LED DRIVER to be added to the board. The LED- to GND resis- tor-short connections (R2, R3) must be removed from the board to enable PWM dimming when the PWM dimming MOSFETs are stuffed with their gate pull-down resistors (R6, R7). EXTENDED PWM DIMMING The PWM dimming range can be extended by adding the components Q3, R13, and C7 to the circuit as shown in the datasheet. 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 ex- tended dimming range components. When these components are added to the circuit, the PWM1 signal is directly connected to the extended dim- ming range components. If both channel 1 and channel 2 are being used in PWM dimming mode Figure 1. Proper Measurement Equipment and the extended dimming range circuit is used, Setup the PWM2 signal must be synchronized with the PWM1 signal. The rising edges must occur at the INPUT VOLTAGE RANGE same time although their duty cycles may vary. Failure to do this may result in significant jitter on The operating input voltage range for the LT3475- the second channel or limited PWM dimming 1 IC is 4V to 36V with an absolute maximum rat- range. Please note that a C-R-C compensation ing of 40V. Under some conditions, it is not rec- network may also help provide the highest pwm ommended to run the IC up to 36V non-transient dimming range as shown in the datasheet. DC voltage. Please see the datasheet for input voltage range calculations based on switching fre- LED STRINGS quency, LED output voltage, and compensation network topology during startup. As assembled, DC1315 is assembled with optimized components the DC1315 should not be started up above 30V for driving four white LEDs at 1.5A on each of its DC input without first assembling the C-R-C com- two channels from an input voltage of 20V. The pensation network as shown in the datasheet or LEDs are not assembled on the PCB and should using a bigger inductor with up to 3.2A capability be attached to the LED terminals as shown in Fig- in order to maintain control of the switch current. ure 1. If the LEDs are not attached, DC1315 will protect itself and survive an open LED condition. PWM DIMMING If a different number of LEDs is attached in series DC1315 is assembled to show a combination of to either output, the inductor size, switching fre- the most basic operation of the IC and the small quency, and/or input voltage range may need to solution size. The optional PWM dimming MOS- be adjusted for proper operation. Consult the FETs (Q1, Q2) are not stuffed on DC1315, but datasheet for proper sizing of the inductor for there are two SOT23 spaces designated for these given input and output voltage conditions. The 2