DESIGN IDEAS L Low Power Boost Regulator with Dual Half-Bridge in 3mm × 2mm DFN Drives MEMS and Piezo Actuators by Jesus Rosales Introduction Advances in manufacturing technology have made it possible for actuators, sensors, RF relays, and other moveable parts to be manufactured at a very small scale. These devices, referred to as MEMS (micro-electro-mechanical systems) or micro-machines, are finding their way into daily life in applications unheard of just a few years ago. MEMS are used in automotive, military, medical and consumer product applications. Many types of MEMS devices consume very little power to operate and generally require the use of two support circuits, a step-up converter and a dual half-bridge driver. These support circuits must be very small and highly efficient to keep pace with ever-shrinking MEMS applications. To this end, the LT8415 integrates the step-up converter power switch and diode and the dual half-bridge driver in a 12-pin, 3mm × 2mm DFN package. Its novel switching architecture consumes very little power throughout the load range, VIN 2.6V to 5V BOOST CONVERTER Figure 2 shows a MEMS driver that takes a 2.6V–5V input and produces a 34V output. This circuit draws very little source current when the dual half-bridge is disabled. The input current is only 320µA at 2.6VIN and 128µA at 5VIN. A logic level signal at IN1 and IN2 activates the dual halfbridge switches. Figure 3 shows the turn-on delay and rise time for OUT1 and OUT2 with both half-bridges activated. Figure 4 shows the turn-off delay and fall time with the 200pF and 1nF capacitive loads shown in Figure 2. See the data sheet details for measuring delay time. L1 100µH C1 2.2µF 22nF SW CAP VCC VOUT LT8415 SHDN VIN ENABLE 2.6V–5V Input to 34V Output MEMS Driver making it an ideal match for driving low current MEMS. The LT8415 generates output voltages up to 40V from sources ranging from 2.5V to 16V. The output is then available for the integrated complementary half-bridge drivers and is available via OUT1 and OUT2 (see Figure 1). Each half-bridge is made up of an N-channel MOSFET and a P-channel MOSFET, which are synchronously controlled by a single pin and never turn on at the same time. OUT1 and OUT2 are of the same polarity as IN1 and IN2, respectively. When the part is turned off, all MOSFETs are turned off, and the OUT1 and OUT2 nodes revert to a high impedance state with 20MΩ pull-down resistors to ground. LOGIC LEVEL 0.1µF IN 2 OUT 1 OUT 2 VREF GND FBP IN 1 34V/0V 137K VOUT 40V MAX 34V/0V 1nF 200pF 887K L1: COILCRAFT DO2010-104ML VOUT IN1 Figure 2. 2.6V–5V input to 34V dual half-bridge boost converter OUT1 VOUT IN2 OUT2 OUT1 10V/DIV OUT2 10V/DIV OUT1 10V/DIV OUT2 10V/DIV IN1/IN2 1V/DIV IN1/IN2 1V/DIV LT8415 Figure 1. Simplified block diagram of the LT8415 Linear Technology Magazine • September 2009 5µs/DIV 5µs/DIV Figure 3. Turn-on delay and rise time for OUT1 and OUT2 Figure 4. Turn-off delay and fall time for OUT1 and OUT2 31 L DESIGN FEATURES Integrated Resistor Divider L1 100µH VIN 3V to 10V The LT8415 contains an integrated resistor divider such that if the FBP pin is at 1.235V or higher, the output is clamped at 40V. For lower output voltage levels use R1 and R2, calculating their values as instructed by the data sheet. This method of setting the output voltage ensures the voltage divider draws minimal current from the input when the part is turned off. 0.1µF 2.2µF SW CAP VCC VOUT LT8415 SHDN LOGIC LEVEL 1µF IN 2 OUT 1 OUT 2 VREF GND FBP IN 1 16V 1.6mA AT 3VIN 10mA AT 10VIN 16V/0V 16V/0V 1nF 604K 200pF 412K L1: COILCRAFT DO2010-104ML Conclusion Figure 5. 3V–10V input to 16V dual half-bridge plus 16V output boost converter 3V–10V Input to 16V Output MEMS Driver and Bias Supply Figure 5 shows a 3V–10V input to 16V output converter, where the output drives the dual half-bridge and also provides bias current for other LTM4614/15, continued from page 26 to promote good thermal conductivity. Figure 10 shows that thermal dissipation is well-balanced between the two switching regulators. Output Voltage Tracking Tracking can be programmed using the TRACK1 and TRACK2 pins. To implement coincident tracking, at the slave’s TRACK pin, divide the master regulator’s output with a resistor divider that is the same as the slave regulator’s feedback divider. Figure 11 VIN 5V 82µF + circuitry. The converter in Figure 2 can be used in a similar fashion, but the current available at the output is reduced as the output voltage is increased. See the data sheet for details about maximum output current. shows a tracking design and Figure 12 shows the output. VOUT2 tracks VOUT1 in master-slave design with both outputs ramping up coincidently. The smooth start-up time is attributed to the soft-start capacitor. Conclusion The cumbersome designs typical of multivoltage regulation are a thing of the past. The LTM4614 and LTM4615 µModule multiple-output regulators can be easily fit into space-constrained 100µF 4.02k 22µF VIN2 PGOOD2 VOUT2 PGOOD1 VOUT1 FB2 FB1 VIN1 10µF RUN/SS1 10k 22µF LTM4615 4.99k TRACK2 RUN/SS2 LDO_IN VIN 10k LDO_OUT EN3 GND FB3 PGOOD3 3.32k Figure 11. Output voltage tracking design example 32 VOUT2 1.2V 4A 100µF COMP2 COMP1 TRACK1 VOUT2 system boards with far fewer components than discrete solutions. The dual-output LTM4614 µModule regulator and triple-output LTM4615 are small in size, have excellent thermal dissipation and have high efficiency. Independent input and output voltage rails give these µModule regulators unmatched flexibility. They can be used in a variety of input-output combinations, including input and output current sharing, output voltage tracking, and low noise output. L 10µF =2 VIN1 VOUT1 1.8V 4A The LT8415 is an ideal match for driving low power MEMS. It integrates a step-up converter power switch and diode, a complementary dual half-bridge, and a novel switching architecture that minimizes power dissipation. L 4.7µF VOUT1 VOUT3 1V 1.5A VOUT1 (1.8V) 500mV/DIV VOUT2 (1.2V) 500mV/DIV 200µs/DIV Figure 12. Start-up waveforms for the circuit in Figure 11 Linear Technology Magazine • September 2009