DESIGN IDEAS Quad Output Switching Regulator by Dongyan Zhou Powers Large TFT Displays Introduction The LT1943 is a highly integrated, four -output switching regulator designed to power large TFT Liquid Crystal Displays (LCDs). The first step-down switching regulator provides a logic voltage with up to 2A of current. The other three regulators are a high-power boost regulator, a lowpower boost regulator and an inverting regulator, which provide the three bias voltages, AVDD, VON and VOFF, required by LCDs. Switching regulators are chosen over linear regulators to accommodate a wide input voltage range (providing both step up and step down functions) and to minimize power dissipation. The LT1943’s wide input range, 4.5V to 22V, allows it to accept a variety of power sources, including the commonly used 5V, 12V and 19V rails. The low-profile 28-pin TSSOP package has an exposed metal pad on the backside to maximize thermal performance. All of the regulators are synchronized to a 1.2MHz internal clock, allowing the use of small, low cost inductors and ceramic capacitors. Since different types of panels may require different bias voltages, all output voltages are adjustable for maximum flexibility. Programmable soft-start capability is included in each of the regulators to limit inrush current. Operation Figure 1 shows a 4-output TFT LCD power supply with 8V to 20V input voltage range. The first output provides a 3.3V, up to 2A, logic supply using a buck regulator. The second output employs a SEPIC converter to generate a 13V, 500mA AVDD bias supply. Another boost converter and an inverter generate VON and VOFF. When power is first applied to the input, the RUN-SS capacitor starts charging. When its voltage reaches 0.7V, switcher one is enabled. The 30 2.2µF 10V 0.47µF 16V 80.6k L2 10µH VIN, 8V TO 20V ZHCS400 VOFF –10V 50mA 10µF 25V VIN SW3 10pF L5 10µH 0.22µF 10V L1,4.7µH B230A PGOOD VON 30V 30mA VON E3 232k SW1 FB3 VC1 22µF 6.3V 18k 100pF 2.2nF 0.047µF PGOOD FB1 10.0k 0.015µF CT BOOST 10µF 16V 10.0k SS-234 BIAS VLOGIC 3.3V 2A 95.3k 0.015µF RUN-SS LT1943 FB4 16.2k L3 10µH SW2 FB2 NFB4 CMDSH-3 1µF 25V L4 33µH AVDD 13V 500mA ZHCS400 SW4 10.0k B240A 100pF VC2 6.8k 2.2nF VC3 10.0k GND SGND VC4 27k 100pF 680pF 2.2µF 35V 13k 100pF 2.2nF 0.47µF 35V L1: TDK RLF7030T-4R7M3R4 L2, L3: TDK SLF6028T-100M1R3 L4: TDK RLF5018T-330MR51 L5: TDK RLF5018T-100MR94 Figure 1. Quad output TFT-LCD power supply with 8V-20V input voltage range capacitor at RUN-SS pin controls the ramp-rate for the Switcher 1 output, VLOGIC and inrush current in L1. Switchers 2, 3 and 4 are controlled by the BIAS pin, which is usually connected to VLOGIC. When the BIAS pin is higher than 2.8V, the capacitor at the SS-234 pin begins charging to enable switchers 2, 3 and 4. When AVDD reaches 90% of its programmed voltage, the PGOOD pin is pulled low. RUN-SS 2V/DIV VLOGIC 5V/DIV AVDD 10V/DIV VOFF 10V/DIV VE3 20V/DIV VON 50V/DIV IIN(AVG) 1A/DIV 5ms/DIV Figure 2. Startup waveforms of the power supply in Figure 1. When AVDD, VOFF and E3 all reach 90% or their programmed voltages, the CT timer is enabled and a 20µA current source begins to charge CT. When the CT pin reaches 1.1V, the output PNP turns on, connecting E3 to VON. Figure 2 shows the start-up sequence of the circuit in Figure 1. If one of the regulated voltages, VLOGIC, AVDD, VOFF or E3 dips more than 10%, the internal PNP turns off to shut down VON . This action protects the panels, as VON must be present to turn on the TFT display. Each regulator has a frequency foldback oscillator, which reduces the switching frequency to 250kHz when its FB pin is at 0V. This frequency foldback feature reduces the average inductor current during start-up and overload conditions, minimizing the power dissipation in the power switches and external components. It also helps the short-circuit protection for the Buck Linear Technology Magazine • February 2004 DESIGN IDEAS 100 topology and lower input voltage, the overall circuit efficiency increases to 90%. The PGOOD pin can drive an optional PMOS device at the output of the boost regulator to disconnect the load from the input during shutdown. EFFICIENCY (%) 90 VIN = 12V 80 VIN = 19V Layout Considerations 70 60 0 500 1000 1500 LOAD AT VLOGIC (mA) Careful PC board layout is important for proper operation. Paths that carry high switching current should be short and wide to minimize parasitic inductance. In a buck regulator, this loop includes the input capacitor, internal power switch and Schottky diode. In a boost regulator, this loop includes the output capacitor, internal power switch and Schottky diode. In a SEPIC converter, this loop includes the internal power switch, flying capacitor, Schottky diode and the output capacitor. Keep all the loop compensation components and feedback resistors away from the high switching current paths. The LT1943 pinout was designed to facilitate PCB 2000 Figure 3. Total Circuit Efficiency of the power supply in Figure 1 (load at AVDD: 500mA; load at VON: 30mA; load at VOFF: 40mA). and SEPIC regulators. The overall efficiency is shown in Figure 3. The converter uses all ceramic capacitors, with X5R and X7R types recommended, as these materials maintain capacitance over a wide temperature range. If the input voltage is 5V, a boost regulator can be use in place of the SEPIC to generate the AVDD supply. With the higher efficiency of the boost layout. Use a separate ground trace to connect the ground return of the compensation components and bottom feedback resistors to the signal ground (SGND pin). Connect the SGND to the power ground on the backside of the IC. Keep the traces from the center of the feedback resistors to the corresponding FB pins as short as possible. LT1943 has an exposed ground pad on the backside of the IC to reduce thermal resistance. A ground plane with multiple vias into ground layers should be placed underneath and near the part to conduct heat away from the IC. Conclusion The LT1943 provides compact power supply solutions for TFT-LCD panels. All four outputs come from switching regulators for wide input voltage range and minimum power dissipation. All four circuits use only ceramic capacitors to minimize ripple, size and cost. LTC2904, LTC2905, LTC2906 and LTC2907 continued from page 23 5V DC/DC CONVERTER SYSTEM LOGIC 3.3V V1 SUPPLY CONTROLLER Conclusion V2 LTC2905 VIN 0.1µF THREE-STATE S1 TMR S2 GND TOL RST 0.1µF 22nF Figure 10. 5V, 3.3V dual supply monitor with voltage margining application for automated onboard testing introduced here complement such testing in two ways. First, a test-module can provide more supply headroom by lowering the reset trip thresholds for automated onboard testing. Using the 3-state programming input pin TOL, the global supply tolerance can be set to 5%, 7.5% or 10%. Figure 10 shows an LTC2905 used in such an application. Second, the S1 and S2 pins can be driven by 3-state outputs for a more aggressive lowering of the trip threshLinear Technology Magazine • February 2004 part is again configured to monitor 5V and 3.3V. old. For example, in Figure 10 the S1 and S2 pins are tied to V1 to configure the part to monitor 5V and 3.3V. If TRI-STATE buffers are added to drive the S1 and S2 pins, these pins can be set to GND during testing. Connecting S1 and S2 to GND configures the LTC2905 to monitor 2.5V and 1.8V, a significant change in trip threshold from the 5V and 3.3V thresholds respectively. During normal operation, the TRI-STATE buffers can set S1 and S2 back to V1 (High) so that the The LTC2904, LTC2905, LTC2906 and LTC2907 micropower dual supervisors provide the space saving, simplicity, versatility, accuracy and reliability required in a wide variety multi-voltage monitoring applications. Input supply combinations are programmable without external components. The LTC2906 and LTC2907 also include a low voltage positive adjustable threshold. The comparators are 1.5% accurate over temperature and feature built-in noise rejection. Reset state is correct for internal VCC down to 1V. Reset time-out periods in the LTC2905 and LTC2907 are user adjustable with external capacitors. Power supply margining features include real-time supply tolerance and voltage threshold selections. 31