DESIGN FEATURES Inverting Switching Regulator Delivers High Current/Low Noise Outputs in a SOT-23 by Albert Wu Introduction The LT1931 is the industry’s highest power SOT-23 inverting switching regulator. The on-chip 1A switch allows high output currents to be generated. In a typical 5V to –5V application, the part can deliver 350mA; a 5V to –12V application can provide 150mA. The LT1931 switches at a fixed frequency of 1.2MHz, allowing the use of tiny, low cost capacitors and inductors of 2mm or less in height. The constant switching frequency keeps the output voltage noise low and predictable, making designs with output voltage ripple less than 5mVP-P easily achievable. An integrated shutdown feature limits the supply current to less then 1µA when the part is disabled, extending battery life. The high power and low noise LT1931 comes in a tiny 5-lead SOT-23 package, saving valuable board space. The part is pin-for-pin compatible with the LT1611, providing a simple upgrade path for users of the older parts who need more power. VOUT 10mV/DIV AC COUPLED VSW 10V/DIV 0.2µs/DIV Figure 2. Output voltage ripple for Figure 1’s circuit VOUT 20mV/DIV AC COUPLED IL1A + IL1B 0.5A/DIV AC COUPLED ILOAD 200mA 100mA 100µs/DIV Figure 3. Transient response for Figure 1’s circuit C2 1µF L1A 10µH VIN 5V L1B 10µH 100 D1 90 SW LT1931 SHDN NFB GND R1 29.4k VOUT –5V 350mA 220pF R2 10k C3 22µF 85 EFFICIENCY (%) VIN C1 4.7µF VIN = 5V VOUT = –5V 95 80 75 70 65 60 C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R LMK212BJ105MG C3: TAIYO YUDEN X5R JMK212BJ226MM D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CLS62-100 (408) 573-4150 55 1613 • TA01 (602) 244-6600 (847) 956-0667 Figure 1. 5V to –5V/350mA inverting converter 8 50 0 50 100 150 200 250 LOAD CURRENT (mA) 300 350 Figure 4. Efficiency of Figure 1’s circuit Linear Technology Magazine • September 2000 DESIGN FEATURES C2 1µF L1A 10µH VIN 5V L1B 10µH 100 VIN = 5V VOUT = –12V 95 D1 90 C1 4.7µF SW LT1931 SHDN R1 84.5k NFB GND 85 VOUT –12V 150mA R2 10k EFFICIENCY (%) VIN C3 10µF 80 75 70 65 60 C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R TMK316BJ105ML C3: TAIYO YUDEN X5R EMK325BJ106MM D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CLS62-100 55 (408) 573-4150 50 0 1613 • TA01 Figure 5. 5V to –12V/150mA inverting DC/DC converter C2 1µF L1 10µH VIN 5V 25 (602) 244-6600 (847) 956-0667 50 75 100 LOAD CURRENT (mA) VIN L2 10µH C2 1µF L1A 10µH VIN 5V L1B 10µH D1 SW LT1931 SHDN NFB GND R1 29.4k 150 Figure 6. Efficiency of Figure 5’s circuit D1 C1 4.7µF 125 VIN VOUT –5V 300mA 220pF VSS C1 4.7µF RSS 15k SW LT1931 SHDN NFB GND R2 10k C3 22µF R1 29.4k VOUT –5V 350mA 220pF R2 10k C3 22µF D2 1N4148 C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R LMK212BJ105MG C3: TAIYO YUDEN X5R JMK212BJ226MM D1: ON SEMICONDUCTOR MBR0520 L1, L2: MURATA LQH3C100K04 CSS 68nF (408) 573-4150 1613 • TA01 (602) 244-6600 (814) 237-1431 VOUT C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R LMK212BJ105MG C3: TAIYO YUDEN X5R JMK212BJ226MM D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CLS62-100 Figure 7. 5V to –5V converter fits in 0.1in2 1613 • TA01 (602) 244-6600 (847) 956-0667 Figure 8. RSS and CSS at the SHDN pin provide soft start to LT1931 inverting converter 5V to –5V Local Supply 5V to –12V Local Supply Figure 1 shows a 5V to –5V application that can deliver up to 350mA of output current. The output voltage ripple for the circuit is less than 3mVP-P, as shown in Figure 2. Figure 3 is an oscillograph of the transient response as the load current steps from 100mA to 200mA and back again. The output voltage and inductor current produce a nicely damped response and recover within 100µs. The voltage overshoot and undershoot is approximately 30mV. Figure 4 shows the efficiency of the circuit. The efficiency stays above 75% over a wide load range of 60mA to the full load current, reaching as high as 79% at 150mA. This solution can fit in a board area as small as 0.19in2 (0.5in • 0.38in). Figure 5 shows a 5V to –12V converter that can provide 150mA of output current. The output voltage ripple is less than 10mVP-P. The efficiency, shown in Figure 6, stays above 80% from a load current of 50mA to the full load current. This solution can fit within a 0.19in2 area. Linear Technology Magazine • September 2000 (408) 573-4150 Tiny 5V to –5V Converter In order for the previous applications to achieve high current outputs, slightly larger coupled inductors were used. Although the total solution still remained small, further reductions in size can be made if a lower maximum output current can be tolerated. Figure 7 shows such a circuit. The circuit uses tiny uncoupled induc- tors, enabling the solution to fit within a 0.1in2 (0.33in • 0.3in) area. This circuit can provide 300mA of output current and has an output voltage ripple of only 6mVP-P. Soft Start In applications where soft start is required, the simple circuit shown driving the shutdown pin in Figure 8 can be used. This circuit, which consists of only a resistor, a diode and a capacitor, effectively limits how quickly VOUT can be charged. Figure 9 shows the soft-start circuit at work. After VSS is driven high, the input current slowly ramps up to a peak value of 350mA. The output voltage takes 1.6ms to reach its final value of 9 DESIGN FEATURES –5V. The values of RSS and CSS can be adjusted to obtain the desired startup performance. 0V VOUT 2V/DIV Conclusion The LT1931 is the highest power SOT23 inverting DC/DC converter in the industry. The circuits presented here demonstrate the high output currents and low output noise achievable with the LT1931. The high switching frequency of the part allows the total solution to fit in an area as small as 1/10th of a square inch. –5V IIN 0.5A/DIV 0A VSS 5V 0V 500µs/DIV Figure 9. Soft-start waveforms of Figure 8’s circuit; RSS = 15k, CSS = 68nF LT1616, continued from page 7 D4 pin to the shorted input. The circuit in Figure 9 addresses both problems. The diode at the input prevents a short from drawing large currents, and the part automatically shuts down when the input supply is absent so that no current will flow through the SW pin. 5 VIN 100k VIN BOOST LT1616 4 SHDN GND 100k 1 2 SW 6 VOUT FB 3 BACKUP D4: ON SEMICONDUCTOR MBR0520L (602) 244-6600 Conclusion The LT1616’s high switching frequency and tiny SOT-23 package result in a very small step-down switching regulator. With its current mode architecture and internal loop compensation, it provides the benefits of an all-ceramic design: low noise, small size and no concerns with tantalum reliability and availability. The 3.6V to 25V input handles power sources ranging from 5V logic supplies to unruly wall transformers Figure 9. This application circuit protects the LT1616 and its load from reversed input voltage. The LT1616 will enter shutdown mode when the input supply is disconnected. and automobile cigarette lighters. The LT1616 is an ideal replacement for bulky (and potentially hot) TO-220 linear regulators. 1 Among the evils of oversized wall transformers: (a) the polarized blades and the arrangement of your outlet inevitably conspire such that plugging in the transformer covers the second outlet, locally reducing the utility of your AC power system by 50%; and (b) they fall out of the outlet under the force of gravity. I keep my Kitchen-Aid mixer pushed up against an AC adapter to hold it (the adapter) in place. I don’t know whether to blame the designer of a radio that needs such a large wall wart or the contractor who installed the outlet a quarter inch too deep in the wall. KitchenAid makes a fine product: with its heavy-duty construction, high torque motor and no-slip rubber feet, the model K5SS mixer can hold even the heaviest wall transformer in place. Authors can be contacted at (408) 432-1900 http://www.linear-tech.com/ezone/zone.html Articles, Design Ideas, Tips from the Lab… 10 Linear Technology Magazine • September 2000