1.5A Rail-to-Rail Output Synchronous Step-Down Regulator Adjusts with a Single Resistor Design Note 506 Jeff Zhang Introduction A new regulator architecture the LTC3600 (first introduced with the LT3080 linear regulator) has wider output range and better regulation than traditional regulators. Using a precision 50µA current source and a voltage follower, the output is adjustable from “0V” to close to VIN. Normally, the lowest output voltage is limited to the reference voltage. However, this new regulator has a constant loop gain independent of the output voltage giving excellent regulation at any output and allowing multiple regulators to be paralleled for higher output currents. Operation The LTC3600 is a current mode monolithic step-down buck regulator with excellent line and load transient responses. The 200kHz to 4MHz operating frequency can be set by a resistor or synchronized to an external clock. The LTC3600 internally generates an accurate 50μA current source, allowing the use of a single external LTC3600 VIN VIN 12V resistor to program the reference voltage from 0V to 0.5V below VIN. As shown in Figure 1, the output feeds directly back to the error amplifier with unity gain. The output equals the reference voltage at the ISET pin. A capacitor can be paralleled with RSET for soft start or to improve noise while an external voltage applied to the ISET pin is tracked by the output. Internal loop compensation stabilizes the output voltage in most applications, though the design can be customized with external RC components. The device also features a power good output, adjustable soft-start or voltage tracking and selectable continuous/discontinuous mode operation. These features, combined with less than 1μA supply current in shutdown, VIN overvoltage protection and output overcurrent protection, make this regulator suitable for a wide range of power applications. L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 9 BOOST 0.1µF 8 RUN 50µA 5 10µF + – 7 ISET 0.1µF MODE/ SYNC INTVCC RT 6 RSET 66.5k VOUT 3.3V 22µF GND 13 VOUT 1 2.2µH SW PWM CONTROL AND SWITCH DRIVER ERROR AMP 10 3 PGFB 4 11 ITH PGOOD 2 12 DN4JZ F01 1µF Figure 1. High Efficiency, 12V to 3.3V 1MHz Step-Down Regulator with Programmable Reference 09/12/506 Applications Figure 1 shows the complete LTC3600 schematic in a typical application that generates a 3.3V output voltage from 12V input. Figure 2 shows the load step transient response using internal compensation and with external compensation. Figure 3 shows the efficiency in CCM and DCM modes. Furthermore, the LTC3600 can be easily configured to be a current source, as shown in Figure 4. By changing the RSET resistance from 0Ω to 3kΩ, the output current can be programmed from 0A to 1.5A. VOUT 100mV/DIV 100 90 80 EFFICIENCY (%) 70 IOUT 1A/DIV 60 50 40 30 20 50µs/DIV 12VIN TO 3.3VOUT, INTERNAL COMPENSATION, ITH TIED TO INTVCC 12VIN to 3.3VOUT (CCM) 12VIN to 3.3VOUT (DCM) 10 0 –0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 LOAD CURRENT (A) Figure 3. Efficiency of 12V Input to 3.3V Output Regulator in CCM and DCM Mode VOUT 100mV/DIV Conclusion The LTC3600 uses an accurate internal current source to generate a programmable reference, expanding the range of output voltages. This unique feature gives the LTC3600 great flexibility, making it possible to dynamically change the output voltage, generate current sources, and parallel regulators for applications that would be difficult to implement using a standard DC/DC regulator configuration. IOUT 1A/DIV 50µs/DIV 12VIN TO 3.3VOUT, ITH TIED TO EXTERNAL RC COMPENSATION (R=56kΩ, C=68pF) Figure 2. 0A to 1.5A Load Step Response of the Figure 1 Schematic VIN 12V 9 LTC3600 BOOST 8 RUN 50µA 5 + 10µF 0.1µF PWM CONTROL AND SWITCH DRIVER ERROR AMP – 2.2µH SW 7 GND 13 VOUT MODE/ SYNC INTVCC ISET 1 RSET 0k TO 3k 6 10 0.1Ω IOUT = 0A TO 1.5A 22µF 11 RT PGFB ITH PGOOD 3 4 2 12 1µF DN4JZ F04 Figure 4. The LTC3600 as a Programmable 0A to 1.5A Current Source Data Sheet Download www.linear.com Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear.com For applications help, call (408) 432-1900, Ext. 3845 dn506 AP/LT 0912 196k • PRINTED IN THE USA LINEAR TECHNOLOGY CORPORATION 2012