QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 620 HIGH CURRENT QUAD OUTPUT SWITCHING REGULATORS FOR TFT-LCD PANELS LT1943 DESCRIPTION Demonstration circuit 620 is a quad output power All four switchers are synchronized to the internal supply intended for use in large TFT-LCD panels. The 1.2MHz clock, allowing the use of low profile circuit features the LT 1943 high current quad-output inductors and ceramic capacitors. They all have soft- switching regulator and generates a 3.3V logic supply start to limit start-up inrush current. along with the triple output supply required for the Long wires run from input sources (such as wall TFT-LCD panels. With an input voltage range of 8V to adaptors) can cause large voltage spikes during initial 20V, a step down regulator provides 3.3V VLOGIC with plug-in. C17 is installed on DC620 to damp the up to 2A current a high power SEPIC converter, a possible voltage spikes. C17 is not required for lower power boost converter and an inverting applications when input source is close to the converter provide three independent output voltages regulator. Please refer to Application Note 88 for AV , VON and VOFF required by the LCD panels. A high DD details. side PNP provides delayed turn on of the VON signal Design files for this circuit board are available. and can source up to 30mA at 30V. Call the LTC factory. LT is a registered trademark of Linear Technology Corporation Table 1. Performance Summary (T = 25C unless otherwise noted) A PARAMETER CONDITION VALUE Minimum Input Voltage -40 C to 85 C 8V Maximum Input Voltage -40C to 85 C 20V Output Voltage V V = 8V to 20V, I = 0A to 2A 3.3V 3% LOGIC IN OUT Maximum Output Current at V V = 8V to 20V (Note 1) 2A LOGIC IN Typical Output Ripple V V = 12V, load at V = 2A (20MHz BW) 23mV LOGIC IN LOGIC PP Typical Output Ripple AV V = 12V, load at AV = 500mA (20MHz BW) 64mV DD IN DD PP Typical Switching Frequency 1.2MHz Logic Low Voltage-Off, -40C to 85 C 0.4V MAX On/Off Control Logic High Voltage-On 2.4V MIN Note 1: 2A maximum output current is guaranteed when V is unloaded. Since V is supplied from V , the load at V reduces the available cur- OFF OFF LOGIC OFF rent at V . More output current is available at higher input voltage. Please refer to the LT1943 data sheet for output current vs. input voltage LOGIC curve. QUICK START PROCEDURE Demonstration circuit 620 is easy to set up to NOTE: When measuring the input or output evaluate the performance of the LT1943. Refer to voltage ripple, care must be taken to avoid a long Figure 1 for proper measurement equipment setup ground lead on the oscilloscope probe. Measure and follow the procedure below: the input or output voltage ripple by touching the 1 GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 620 HIGH CURRENT QUAD OUTPUT SWITCHING REGULATORS FOR TFT-LCD PANELS probe tip directly across the V or V and 3. Check for the proper output voltages. If there is no IN OUT GND terminals. See Figure 2 for proper scope output, temporarily disconnect the load to make probe technique. sure that the load is not set too high. 1. Make sure the input voltage does not exceed 20V. 4. Once the proper output voltages are established, With power off, connect the input power supply to adjust the loads within the operating range and ob- V and GND. serve the output voltage regulation, ripple voltage, IN efficiency and other parameters. 2. Turn on the power at the input. Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2