DESIGN IDEAS Compact Power Supply Drives TFT-LCD and LED Backlight by Dongyan Zhou Introduction Li-Ion to 4-Inch or The LT1942 is a highly integrated, 5-Inch TFT-LCD L1 22µH VIN 3V TO 4.2V R5 442k C3 0.22µF 16V C2 0.22µF 16V R6 63.4k R4 10k R3 665k VIN D3 FB3 SW3 0.1µF 16V L5 47µH L2 47µH SHUTDOWN C6 SW1 FB1 NFB2 SW4 LT1942 D4 D2 LED1 SW2 LED2 SHDN CTRL4 AGND LED CONTROL PGND14 PGND23 SS1 SS4 C7 0.1µF C8 0.1µF 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 5 R2 100k L4 33µH 20 10 15 LED2 CURRENT (mA) Figure 2. Typical current matching between LED1 and LED2 4.7pF R8 1M AVDD 5V 40mA C1 4.7µF 6.3V PGOOD PGND23 0.8 0 R1 301k VCC 0.9 M1 PMOS PGND14 VOUT3 1.0 D1 L3 22µH C5 2.2µF VON 10V 2mA VOFF –10V 2mA Figure 1 shows a complete power supply for three TFT bias voltages (AVDD, VON, and VOFF) and a white LED driver. A typical application of this design is a 4- or 5- inch amorphous silicon TFT-LCD panel powered by a single cell Li-ion input. Two boost converters are used to supply AVDD and VON, while the negative output converter generates VOFF. The LT1942 has built-in power sequencing to properly power up the TFT panel. When the shutdown pin is driven above 1V, the AVDD switcher is enabled first. After its output reaches 97% of the set value, the PGOOD pin is driven low, which enables both the VOFF and VON switchers. A built-in PNP separates the VON bias supply from its boost regulator output. The PNP is not turned on until the programmable delay set by the CT pin has elapsed. The panel is not activated and stays in a low current state until VON is present. This delay gives the column drivers and the digital part of the LCD panel time to get ready before the panel is turned on. LED1 CURRENT MATCHING ERROR (%) 4-output switching regulator designed to power small to medium size TFT panels. Three of the switching regulators provide the TFT bias voltages. The fourth regulator is designed to drive backlight LEDs. The TFT supply includes two boost converters and one negative output DC/DC converter. Since different types of panels may require different bias voltages, all three output voltages are adjustable for maximum flexibility. The LED driver is a boost converter that has built-in precise dimming control. The user can choose to drive a single string or two strings of LEDs. A built-in ballast circuit helps to match the LED currents precisely if two strings are used. All four regulators are synchronized to a 1MHz internal clock, allowing the use of small, low cost inductors and ceramic capacitors. Programmable soft-start capability is available for both the primary TFT supply and LED driver to control the inrush current. The LT1942 is available in a tiny 4mm × 4mm QFN package. The fourth switcher in the LT1942 is a boost regulator designed to drive up to 20 LEDs (in two strings) to power the backlight. Built-in current ballast circuitry keeps the current into LED1 and LED2 actively matched, regardless of the difference in the LED voltage drops. Figure 2 demonstrates the current matching between the two LED strings. The LED regulator has a control pin (CTRL4), which provides both shutdown and dimming functions. If any LED fails open, the output of the LED regulator (D4) is clamped VIN 20mA 20mA C4 4.7µF 25V FB4 CT C9 0.1µF R7 4.99 C1 TO C9: X5R OR X7R D1: ZHCS400 ZETEX SEMICONDUCTOR L1: 22µH MURATA LQH32CN220K53 L3: 22µH TAIYO YUDEN LB2012T220M L2, L5: 47µH TAIYO YUDEN LB2012T470M L4: 33µH SUMIDA CDPH4D19-330MC M1: Si2301BDS SILICONIX Figure 1. TFT bias voltages and LED backlight power supply from single Lithium-Ion battery input Linear Technology Magazine • November 2004 31 DESIGN IDEAS at around 42V to protect the internal power devices. Proper layout is important to achieve the best performance. Paths that carry high switching current should be kept short and wide to minimize the parasitic inductance. In the boost regulator, the switching loop includes the internal power switch, the Schottky diode (internal or external), and the output capacitor. In the negative output regulator, the switching loop includes the internal power switch, the flying capacitor between the SW2 and D2 pins, and the internal Schottky diode. Connect the output capacitors of the AVDD and LED switchers directly to the PGND14 pin before returning to the ground plane. Connect the output capacitor of the VON switcher to the PGND23 pin before returning to the ground plane. Also connect the bottom feedback resistors to the AGND pin. Connect the PGND14, PGND23 and AGND pins to the top layer ground pad underneath the exposed copper ground on the backside of the IC. The exposed copper helps to reduce thermal resistance. Multiple vias into ground layers can be placed on the ground pad directly underneath the part to conduct the heat away from the part. LTC3426, continued from page 22 Component Selection current should be greater than 1A. A low forward voltage Schottky diode reduces power loss in the converter circuit. Layout Considerations least 750mA from a VIN as low as 3V. When fully charged to 4.2V, over 1A can be supplied. The photograph of a demonstration board in Figure 5 shows just how small the board area is for this application, 10mm × 12mm. Tiny ceramic bypass capacitors and surface mount inductors keep the design small. Figure 6 shows efficiency exceeding 90% and remaining greater than 85% over a load range from 10mA to 900mA with a fully charged battery. LTC3426 SHDN FB R1 95.3k 1% R2 30.9k 1% VOUT 5V 750mA AT 3V C2 22µF 80 VIN = 3V 75 70 65 60 50 Figure 4. Compact application circuit for VOUT at 5V further eases the burden of heavy capacitive loads by providing strong pull-up currents during rising edges to reduce the rise time. Thanks to these two features, the LTC4302 enables the implementation of much larger 2-wire bus systems than are possible with a simple unbuffered multiplexer. VIN = 4.2V 85 55 C1: TDK C1608X5R0J475M C2: TAIYO YUDEN JMK316BJ226ML D1: ON SEMICONDUCTOR MBR120VLSFT1 L1: SUMIDA CDRH4D28-2R2 2 LTC4302, continued from page 26 EFFICIENCY (%) VOUT GND 32 100 90 SW OFF ON The addition of the LTC3426 to Linear Technology’s high performance boost converter family allows the designer to deliver high current levels with minimal board space. An on chip switch and internal loop compensation reduces component count to provide an inexpensive solution for spot regulation applications. D1 VIN C1 10µF Conclusion 95 L1 2.2µH VIN 3V TO 4.2V The LTC3426 requires just a few external components to accomodate various VIN and VOUT combinations. Selecting the proper inductor is important to optimize converter performance and efficiency. An inductor with low DCR increases efficiency and reduces selfheating. Since the inductor conducts the DC output current plus half the peak-to-peak switching current, select an inductor with a minimum DC rating of 2A. To minimize VOUT ripple, use low ESR X5R ceramic capacitors. The average Schottky diode forward current is equal to the DC output current therefore the diode average Figure 5. Photograph of demo board of circuit in Figure 4—board area is 10mm × 12mm For further information on any of the devices mentioned in this issue of Linear Technology, use the reader service card or call the LTC literature service number: 1-800-4-LINEAR Ask for the pertinent data sheets and Application Notes. 1 10 100 LOAD CURRENT (mA) 1000 Figure 6. Up to 92% efficiency in Lithium-Ion battery to 5V output applications Impedance Analyzer, continued from page 30 assume that either the inductance is well damped, or it is shunted by large value capacitances. Notes 1. This subject is treated in some detail in the LTC1647 data sheet, Figures 9, 10, and 11 inclusive. 2. An hp 5210A Frequency Meter or any common counter gives adequate accuracy for most measurements. Linear Technology Magazine • November 2004