1.5A Rail-to-Rail Output Synchronous Step-Down Regulator Adjusts with a Single Resistor Jeff Zhang The LTC3600 features a wide output range with tight regulation over the entire range. In most regulators, the lowest output voltage is limited to the reference voltage. The LTC3600, however, uses a novel regulation scheme with a precision 50µA current source and a voltage follower, creating an adjustable output from “0V” to close to VIN. It also features constant loop gain, independent of the output voltage, giving excellent regulation at any output and allowing multiple regulators to be paralleled for high output currents. LTC3600 VIN VIN 12V RUN 50µA 10µF BOOST + – Figure 1. High efficiency, 12V to 3.3V 1MHz stepdown regulator with programmable reference 2.2µH SW PWM CONTROL AND SWITCH DRIVER ERROR AMP 0.1µF VOUT 3.3V 22µF GND VOUT ISET 0.1µF MODE/ SYNC INTVCC RT RSET 66.5k 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 resistor to program the reference voltage PGFB ITH PGOOD 1µF 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. Figure 3. Efficiency of 12V input to 3.3V output regulator in CCM and DCM mode Figure 2. 0A to 1.5A load step response of the Figure 1 schematic 100 90 80 VOUT 100mV/DIV IOUT 1A/DIV EFFICIENCY (%) VOUT 100mV/DIV IOUT 1A/DIV 70 60 50 40 30 20 2VIN to 3.3VOUT (DCM) 2VIN to 3.3VOUT (CCM) 10 50µs/DIV 12VIN TO 3.3VOUT INTERNAL COMPENSATION ITH TIED TO INTVCC 30 | July 2012 : LT Journal of Analog Innovation 50µs/DIV 12VIN TO 3.3VOUT ITH TIED TO EXTERNAL RC COMPENSATION (R=56kΩ, C=68pF) 0 –0.1 0.1 0.3 0.5 0.7 0.9 ILOAD (A) 1.1 1.3 1.5 design ideas VIN BOOST LTC3600 12V RUN 50µA + 10µF 0.1µF PWM CONTROL AND SWITCH DRIVER ERROR AMP – 2.2µH SW 0.1Ω GND IOUT = 0A TO 1.5A 22µF VOUT 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. ISET RT PGFB ITH PGOOD 1µF RSET 0k TO 3k Figure 4. The LTC3600 as a programmable 0A to 1.5A current source Figure 5. 15V to 2.5V, 3A dual phase regulator VIN 4V TO 15V LTC3600 VIN RUN 100k 50µA 10µF BOOST 0.1µF + PWM CONTROL AND SWITCH DRIVER ERROR AMP – 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. MODE/ SYNC INTVCC 22µF VOUT ISET MODE/ SYNC INTVCC RT GND PGFB VOUT = 2.5V 3A ITH PGOOD 10pF 1µF 27k VIN 4V TO 15V 9 LTC3600 VIN 100k RUN 50µA 5 10µF 150pF BOOST 8 0.1µF + PWM CONTROL AND SWITCH DRIVER ERROR AMP – 2.2µH SW 7 VOUT 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. n 2.2µH SW ISET MODE/ SYNC INTVCC 6 1 10 24.9k 0.1µF 100k RT 3 GND PGFB 13 1µF V+ INTVCC 11 GND SET LTC6908-1* 4 ITH PGOOD 2 22µF 12 10pF OUT1 OUT2 MOD *EXTERNAL CLOCK FOR FREQUENCY SYNCHRONIZATION IS RECOMMENDED July 2012 : LT Journal of Analog Innovation | 31