Tiny, Highly Flexible, Dual Boost/Inverter Tracks Supplies Design Note 357 Jeffrey Huang Introduction Linear Technology’s LT®3471 is a space saving and extremely versatile dual switching regulator available in a tiny 3mm × 3mm DFN package. With a 2.4V to 16V input range, the device can easily implement any combination of boost, inverting and SEPIC topologies to generate two positive and/or negative outputs up to ±40V. It can readily make the two output voltages track each other with minimal additional circuitry, or it can independently regulate the outputs, thus saving space by replacing two separate ICs. Either way, the LT3471 fits into tight spaces and is easily adaptable for a wide range of applications. LT3471 Features The LT3471 combines two independent, in-phase, 1.3A, 42V switches with error amplifiers that can sense to ground, providing boost and inverting capabilities. Both inputs of each error amplifier are available as high impedance feedback pins with a typical bias current of only 60nA. An accurate 1.00V reference is also pin accessible. The LT3471 switches at a fixed 1.2MHz which allows for the use of tiny, low profile inductors and capacitors. Its internally compensated, current mode PWM architecture yields low, predictable output noise that is easy to filter. Additionally, the device provides a programmable soft-start function that requires only a simple external RC circuit to mitigate high inrush current. Easy-to-Implement ±15V Dual Tracking Supplies Figure 1 shows a Li-Ion to ±15V dual tracking supply application, highlighting the versatility of the LT3471 by taking advantage of the accessible error amplifier inputs and reference voltage. The circuit generates well-regulated ±15V outputs that track each other with switchers 1 and 2 in boost and inverting configurations, respectively. The two converters can talk to each other through their shared external feedback resistor network, specifically R1. When regulating, the control loop of switcher 1 servos 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. L1 15µH D1 VIN VIN 2.8V TO 4.2V CONTROL 2 1.8V OV 8 C1 RSS 10µF 10k SW1 VIN FB1N FB1P 9 C1 3.3µF VREF SHDN/SS2 GND C1, C2: X5R OR X7R 6.3V C3, C4: X5R OR X7R 16V C5: X5R OR X7R 25V D1, D2: ON SEMICONDUCTOR MBRM-120 L1: SUMIDA CR43-150 L2, L3: SUMIDA CLS62-100 SW2 11 VIN FB2N 2 3 C2 0.1µF C5 10µF R1 1.58k 4 5 R3 23.7k 6 L2 10µH L3 D2 10µH Figure 1. Li-Ion to ±15V Dual Tracking Supplies 05/16/357 C3 22µF 1 SHDN/SS1 LT3471 FB2P 7 CPL1 2200pF R2 22.1k 10 VOUT1 15V 100mA CPL2 500pF C4 22µF VOUT2 –15V 100mA DN357 F01 FB1N to 1V while that of switcher 2 servos FB2P to ground. Selecting the value of R2 to be (14 • R1) and R3 to be (15 • R1) produces ±15V outputs. In this way, the magnitudes of VOUT1 and VOUT2 track each other; that is, a change in the +15V output causes a similar change in the –15V output, and vice versa. Figure 2 shows the circuit efficiency. 85 VIN = 3.6V 80 and 5 show the effect of soft-start on the output voltages (top two traces) and input current (bottom trace). Figure 4 depicts the start-up waveforms of the circuit without soft-start (CSS removed). The output voltages reach ±15V very quickly but the inrush current sharply peaks above 2A. With soft-start, as shown in Figure 5, the input current peaks slightly to about 500mA, while the output voltages comfortably rise and settle to their steady-state values. In both cases, VOUT1 and VOUT2 track each other. EFFICIENCY (%) 75 VOUT1 10V/DIV 70 65 VOUT2 10V/DIV 60 55 50 IIN 1A/DIV 0 80 60 100 120 140 20 40 LOAD CURRENT FOR EACH OUTPUT (mA) DN357 F02 Figure 2. Efficiency of the Circuit in Figure 1 The interaction between the two converters may compromise the stability of each. To prevent this, phase lead capacitors CPL1 and CPL2 improve the phase margin of both switchers. Figure 3 shows the transient response of the circuit under a constant 50mA load on VOUT1 and 50mA load step on VOUT2. The upper waveform depicts the 15V output tracking the clean, damped response of the –15V output, the middle waveform. The output voltages remain within 0.3% of their nominal values during the step transient. VOUT2 similarly tracks VOUT1 under a load step on switcher 1 and constant load on switcher 2. VOUT1 50mV/DIV AC COUPLED VOUT2 50mV/DIV AC COUPLED SW2 LOAD CURRENT 50mA/DIV 0.5ms/DIV DN357 F03 Figure 3. Transient Response of the Circuit in Figure 1 Both switchers implement a programmable soft-start feature. The RC circuit at the SS1 and SS2 pins in Figure 1 sets the input current ramp rate. Figures 4 Data Sheet Download www.linear.com/LT3471 Linear Technology Corporation 5ms/DIV DN357 F04 Figure 4. Without Soft-Start (CSS Removed in Figure 1), the Output Voltages Quickly Reach Regulation, but the LT3471 Draws Very High Inrush Current VOUT1 10V/DIV VOUT2 10V/DIV IIN 1A/DIV 5ms/DIV DN357 F05 Figure 5. With Soft-Start Enabled (via the RSS CSS Network in Figure 1), the Output Voltages Slowly Reach Regulation, While the Controlled Inrush Current Safely Ramps Up with Little Peaking Conclusion The LT3471 is a tiny, dual switching regulator easily configurable for a broad array of applications, including organic and white LED drivers, TFT-LCD bias supplies and other portable device and medical diagnostic equipment functions. A flexible pinout makes it simple to generate any combination of two positive and/or negative outputs. The LT3471’s small footprint and versatile, 2-switcher capability make it a great fit in a variety of power management solutions. For applications help, call (408) 432-1900 dn357fa LT/TP 0516 REV A 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