advertisement Dual DC/DC Converter with Integrated Schottkys Generates ±40V Outputs and Consumes Only 40µA Quiescent Current Design Note 359 David Kim Introduction As portable devices become more sophisticated and require higher display resolution, there is an increased demand for accurate, high voltage bias supply solutions with wide input and output voltage ranges. The traditional methods of using arrays of capacitors to implement a charge pump or a bulky and expensive transformer no longer meet the accuracy and size requirements of today’s portable devices. The LT®3463 is both accurate and compact. It fits both a positive output converter plus a negative output converter into a tiny (3mm × 3mm)DFN package, including Schottky diodes and switches capable of 250mA (400mA on the negative channel of the LT3463A). The LT3463 works in a wide range of applications due to its 2.3V to 15V input voltage range and output capability to ±40V. Each converter is designed to operate with a quiescent current of only 20μA, which drops to less than 1μA in shutdown, making the LT3463 solution ideal for battery-powered portable applications. L1 10μH C1 1μF 3 VIN 9 2 SHDN1 FB1 SHDN2 GND 11 L2 10μH 1 SW1 VOUT1 LT3463 8 R2 1M VREF FB2 SW2 D2 4 5 C4 0.1μF 10 R1 66.5k L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VOUT1 20V 9mA 80 20V OUTPUT C2 0.47μF 7 6 This design can produce ±20V at 9mA from a 2.7V input and up to 20mA from a 5V input. The efficiency shown in Figure 2 exceeds 70% over a wide load current range reaching 75% at 20mA. The LT3463’s constant off-time architecture allows 20μA quiescent current operation for each output, making the LCD bias circuit efficient even at 100μA load current. This circuit can accommodate different load voltages as the LCD bias voltage varies for different manufacturers (typically 9V ~ 25V). EFFICIENCY (%) VIN 2.7V TO 5V Dual Output ±20V Converter Figure 1 shows a ±20V LCD bias voltage supply using the LT3463. This circuit generates both positive and negative 20V outputs from a Li-Ion battery. Low profile inductors and capacitors keep the circuit under 9mm × 9.5mm × 1.2mm, making this circuit ideal for small wireless devices such as cellular phones or DSCs. R3 61.9k –20V OUTPUT 70 60 R4 1M D1 D1: DIODES, INC. B0540W L1, L2: MURATA LQH32CN100K53 C3 0.47μF VOUT2 –20V 9mA 50 0.1 1 10 LOAD CURRENT (mA) 100 DN359 F01 DN359 F02 Figure 1. Dual Output ±20V Converter 03/05/359_conv Figure 2. Efficiency of Circuit in Figure 1 at VIN = 3.6V Dual Output (±40V) Converter The circuit in Figure 3 demonstrates the impressive input and output voltage range of the LT3463. As shown, the 42V internal switches allow up to ±40V output without a transformer or an array of diodes and capacitors. The output voltages can be easily changed by adjusting the values of R1 and R3. The circuit is designed to operate from a Li-Ion battery or two alkaline cells (down to 2.4V). L1 10μH VIN 9 2 SHDN1 FB1 VREF SHDN2 GND 11 R2 1M 1 SW1 VOUT1 LT3463 8 C1 4.7μF 3 VIN 9 FB2 SW2 8 SHDN1 FB1 SHDN2 GND 11 L2 10μH 1 VREF FB2 SW2 4 10 7 6 R3 154k D2 5 C4 1μF R4 1M D1 R1 32.4k C5 10pF VOUT2 –8V 50mA C3 4.7μF D1: DIODES, INC. B0540W L1, L2: MURATA LQH32CN100K53 C2 0.22μF C2 2.2μF R1 90.9k DN359 F04 Figure 4. CCD Sensor Bias Supply 7 R3 31.6k 6 80 R4 1M D2 5 VIN = 3.6V 240 15V EFFICIENCY 200 75 D1 L2 10μH D1: DIODES, INC. B0540W L1, L2: MURATA LQH32CN100K53 VOUT2 –40V C3 0.22μF DN359 F03 Figure 3. 2.7V to ±40V Dual Output Converter 70 160 –8V EFFICIENCY 120 65 80 60 POWER LOSS (mW) 4 C4 0.1μF 10 2 VOUT1 15V 10mA R2 1M SW1 VOUT1 LT3463A VOUT1 +40V 3 C1 4.7μF L1 10μH EFFICIENCY (%) VIN 2.7V TO 5V VIN 2.7V TO 5V 15V LOSS 40 55 –8V LOSS CCD Sensor Bias Supply The circuit in Figure 4 shows a CCD sensor bias supply for a cellular camera phone application. The two outputs, 15V and –8V, are generated from a Li-Ion battery input. With a minimum input voltage of 3.3V, the circuit is designed to output 15V at 10mA and –8V at 40mA to accommodate the maximum current consumption of the CCD sensor. The low power consumption of the LT3463 and its small circuit size also make this solution ideal as a general-purpose TFT display bias supply for portable devices. Figure 5 shows the efficiency and power loss data for the circuit. Data Sheet Download www.linear.com Linear Technology Corporation 50 0.1 1 10 LOAD CURRENT (mA) 0 100 DN359 F05 Figure 5. Efficiency and Power Loss for the Circuit in Figure 4 Conclusion The LT3463/LT3463A are ideal solutions for high resolution portable display applications requiring multiple (positive and negative) high output voltages, wide input voltage range, low quiescent current, small circuit size and accurate output regulation. For applications help, call (408) 432-1900 dn359f_conv LT/TP 0305 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005