DC484A

GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 484 SINGLE CELL WHITE LED DRIVER LT1937 DESCRIPTION Demonstration circuit 484 features the LT1937 constant- white LED backlight applications using 24 LEDs, mak- current white LED driver in a small SOT23 ThinSOT ing LT1937-SOT23 an ideal solution for battery powered package. The demo board is designed to demonstrate portable applications such as cellular phones, digital the high efficiency performance and the small size of the cameras, MP3 players, GPS receivers, and handheld LT1937. The LT1937 features a 95mV feedback voltage, computers (PDAs). LT1937 offers numerous LED dim- 1.2MHz switching frequency and a special internal com- ming circuit options, see the datasheet for details. pensation network, which allows small (0603) input and Design files for this circuit board are available from output capacitors. DC484 is specifically designed for the LTC factory. QUICK START PROCEDURE Demonstration circuit 484 is easy to set up to evaluate For bench supply input, place the jumper (JP1) in the the performance of the LT1937. Refer to Figure 1 for Vin position and connect a 3.3V, 500mA Bench supply proper measurement equipment setup and follow the to the Vin and GND terminals. procedure below: 2. Turn on the input power supply or connect the single NOTE: When measuring the input or output voltage rip- cell Li-Ion battery. Turn on the LT1937 LED driver cir- ple, care must be taken to avoid a long ground lead on cuit by pushing down SW1. Observe all four white the oscilloscope probe. Measure the input or output LEDs light up. The output voltage should measure voltage ripple by touching the probe tip directly across about 13V across the Vout Test terminal to GND ter- the Vin or Vout and GND terminals. See Figure 2 for minal. proper scope probe technique. 3. The brightness of the LEDs can be controlled by ap- 1. The demo board can be powered from a 3V Lithium plying a PWM signal to the SHDN terminal. The typi- battery installed on the demo board (B1) or other in- cal frequency range recommended for PWM dimming put source (bench top power supply) by selecting in- is 1KHz10KHz with the signal magnitude of the PWM put source jumper (JP1). signal higher than 1.5V. See the LT1937 data sheet for more information about PWM dimming control. For battery (B1) input operation, place the jumper (JP1) to the BATTERY position. OPTIONAL SUPPLY TO ON-BOARD BATTERY OPTIONAL DIMMING CONTROL VIA + 1KHz-10KHz + PWM SIGNAL (>1.5V) + SEE LT1937 DATASHEET FOR DETAILS Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Ripple 1 +QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 484 SINGLE CELL WHITE LED DRIVER Vin 2.7V - 5 V E1 JP1 D1 L1 Vou t Test VIN 2 BATTERY 22uH E3 CMDSH-3 LED1 SW1 LED2 ON/OFF 12mA 5 1 LED3 U1 Vin SW LT1937E S5 LED4 C2 + C1 0. 22uF B1 WHITE 1uF R2 16V BATTERY 4 3 SHDN FB 6.3V 100 K GND 2 R1 E5 7. 5 1% SHDN GND GND E2 E4 Figure 3. Schematic 2 1 3