QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 688 QUAD OUTPUT SWITCHING REGULATORS FOR TFT SUPPLY AND LED DRIVER LT1942 DESCRIPTION Demonstration Circuit DC688 is a quad output power dimming control. A voltage between 0.25V to 2.5V can supply intended for use in small to medium size TFT be applied to CTRL4 pin to control the dimming level. panels. The circuit features the LT1942 quad output An internal high side PNP provides delayed turn on of switching regulator and generates an LED driver along the V signal for panel protection. A P-channel MOSFET ON with the triple output supply required for the TFT panels. driven by the output of PGOOD pin is added at the out- With an input voltage range of 3V to 4.2V, two step-up put of AV to provide output disconnect in shutdown. DD regulators and one inverting regulator provide AV , V DD ON All four switchers are synchronized to the internal 1MHz and V bias voltages for the LCD panels. Another high OFF clock, allowing the use of low profile inductors and ce- power boost is used to drive 12 LEDs (2 strings of 6 ramic capacitors. Both the AV boost converter and the each). For 3V minimum input voltage, up to 20mA cur- DD LED driver have soft-start to limit inrush current. rent is available for each LED string. A built-in LED bal- last circuit is included to precisely match the LED cur- Design files for this circuit board are available. Call rents in the two strings. The LED driver has a precise the LTC factory. Table 1. Performance Summary PARAMETER CONDITION VALUE Output Voltage AV V = 3V to 4.5V, I = 0mA to 40mA 5V 4% DD IN OUT Maximum Output Current at AV V = 3V to 4.5V (Note 1) 40mA DD IN Current at LED1 or LED2 V = 3V to 4.5V, R7=4.99W 20mA 6% IN Typical Output Ripple AV V = 4V, load at AV = 40mA (20MHz BW) 8mV DD IN DD PP Typical Switching Frequency 1MHz Logic Low Voltage-Off, T = 25C 0.4V MAX A SHDN Control Logic High Voltage-On, T = 25C 1.0V MIN A Note 1: The AVDD output can supply more current with a larger inductor. For example, it can deliver 50mA load current with Sumida CLS4D14-330 (33mmmmH). QUICK START PROCEDURE Demonstration circuit 688 is easy to set up to evaluate 1. With power off, connect the input power supply to Vin the performance of the LT1942. Refer to Figure 1 for and GND. proper measurement equipment setup and follow the 2. Place JP1 in RUN position JP2 in ON position. procedure below: 3. Set the input voltage between 3V to 4.5V. Turn on the When measuring the input or output voltage ripple, power at the input. care must be taken to avoid a long ground lead on the 4. Check for the proper output voltages. If any output oscilloscope probe. Measure the input or output volt- voltage is lower than the set value, temporarily dis- age ripple by touching the probe tip directly across the connect the load to make sure that the load is not set input or one of the outputs and GND terminals. See too high. Figure 2 for proper scope probe technique. 1 GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 688 QUAD OUTPUT SWITCHING REGULATORS FOR TFT SUPPLY AND LED DRIVER 5. Once the proper output voltages are established, ad- 20mA each string) can be easily achieved by applying just the loads within the operating range and observe a voltage between 0.1V to 2.5V at CTRL4 pin. Refer to the output voltage regulation, ripple voltage, efficiency datasheet for more information. and other parameters. 6. The LED driver can be turned off independently by putting JP2 in off position. LED dimming (2.5mA to Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2