Compact Triple Step-Down Regulator Offers LDO Driver and Output Tracking and Sequencing – Design Note 441 Tiger Zhou Introduction Typical industrial and automotive applications require multiple high current, low voltage power supplies to drive everything from disk drives to microprocessors. The LT®3507 triple step-down converter fits easily into these applications. It is simple and compact compared to multi-chip solutions. The LT3507 is a single IC current mode triple stepdown regulator with internal power switches and a low dropout linear regulator driver. The switching converters are capable of generating one 2.4A output and two 1.5A outputs. All three converters are synchronized to a single oscillator, with the 2.4A output running antiphase to the other two converters, thereby reducing input ripple current. Each converter has independent shutdown and soft-start circuits and generates a power good signal when its output is in regulation, simplifying both supply sequencing and the interface with microcontrollers and DSPs. Separate input pins for each regulator offer additional flexibility; regulators can be cascaded to reduce circuit size, or each regulator can draw power from a different input source. The switching frequency is set with a single resistor between 250kHz to 2.5MHz. High switching frequency allows the use of small inductors and capacitors resulting in a very compact triple output supply. The constant switching frequency, combined with low impedance ceramic capacitors, results in low output ripple. With its wide input voltage range of 4V to 36V, the LT3507 regulates a broad array of power sources including 5V logic rails, unregulated wall transformers, lead acid batteries and distributed power supplies. 6V to 36V Input to Four Outputs—1.8V, 3.3V, 5V and 2.5V—One IC The triple converter accommodates a 6V to 36V input voltage range and is capable of supplying up to 2.4A, 1.5A and 1.5A, respectively. The 20mA LDO driver output can drive an NPN transistor to provide a fourth low 05/08/441 VIN 6V TO 36V 22μF CMDSH-3 VOUT1 1.8V 2.4A 4.7μH VIN1 VIN2 VIN3 PGOOD1 PGOOD2 PGOOD3 BOOST2 BOOST1 100k CMDSH-3 0.22μF 8.2μH 0.22μF B340A SW1 SW2 FB1 FB2 B240A 12.4k 47μF 10k 31.6k 680pF 20k 680pF LT3507 VC1 20k VOUT2 3.3V 1.3A 22μF 10k VC2 CMDSH-3 BOOST3 VOUT3 5V 1.5A 10μH BIAS 0.22μF B240A SW3 DRIVE 21.5k 53.6k FB3 47μF 10k 20k FB4 680pF 10k 2.2μF VOUT4 2.5V 0.2A VC3 SHDN 107k TRK/SS1 RUN1 TRK/SS2 RUN2 TRK/SS3 RUN3 RT/SYNC TRK/SS4 GND 0.22μF DN441 F01 Figure 1. A 4-Output Supply Including a Low Noise LDO noise rail. Figure 1 shows a typical application for four outputs—1.8V at 2.4A, 3.3V at 1.3A, 5.0V at 1.5A and 2.5V at 0.2A—from a 6V-36V input supply. Low Ripple High Frequency Operation Even at High VIN/VOUT Ratios High frequency operation minimizes solution size, but one obstacle to high voltage (36V), high frequency (MHz) operation of monolithic step-down regulators is the minimum on-time constraint. Due to an internal logic propagation delay, a step-down regulator must remain on for a minimum time interval for proper operation. Otherwise, the converter operates in pulse-skipping mode at high input-to-output ratios, which has the undesirable side effect of increasing output ripple. For example, the application in Figure 1 best operates at 450kHz when the input is 36V and the output is 1.8V. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 8V TO 18V 36V TRANSIENT CMDSH-3 VOUT1 5V 1A 2.7μH VIN1 VIN2 VIN3 PGOOD1 PGOOD2 PGOOD3 BOOST2 BOOST1 100k 11k 0.22μF 1μH B340A SW1 SW2 FB1 FB2 B240A 12.7k 680pF 20k 680pF LT3507 VC1 0.33μF TRK/SS1 0.1μF DN441 F03 0.22μF 10k 6.81k GND CMDSH-3 53.6k 10μF 100k LT3507 TRK/SS2 TRK/SS3 49.9k TRK/SS4 PGOOD1 OVLO UVLO VINSW 22μF 20k VOUT2 1.8V 1.3A Figure 3. External Resistors Program the Input Voltage Lockout; PGOOD1 Determines Sequencing and Tracking 10μF 10k 5VOUT 1V/DIV VC2 CMDSH-3 BOOST3 VOUT3 1.2V 1.5A 1μH BIAS B240A SW3 DRIVE 8.66k 4.99k FB3 22μF 1.8VOUT 1.5VOUT 0.5V/DIV 1.2VOUT VOUT1 0.22μF 10k FB4 680pF 20k 10k 2.2μF VOUT4 1.5V 0.2A 20ms/DIV VC3 SHDN 15.8k RUN1 TRK/SS1 RUN2 TRK/SS2 RUN3 TRK/SS3 RT/SYNC TRK/SS4 GND 0.22μF DN441 F04 Figure 4. Channel 1 Starts First, the Others Follow and Track DN441 F02 Figure 2. Cascading Supplies Maintain High Frequency Operation Even with High VIN/VOUT Ratios An additional feature of LT3507 is the low noise LDO output. Figure 5 shows the LDO output ripple is reduced from the preregulated Channel 2 output. However, the LT3507 has a built-in solution to this problem. By cascading the first converter and the other two, as shown in Figure 2, all three converters can be operated at 2MHz without pulse-skipping mode. Input Voltage Lockout and Sequencing The LT3507’s under- and overvoltage lockouts can be programmed with external resistors. When the schematic in Figure 2 is modified as in Figure 3, the LT3507 will accept VIN up to 36V but operate only when VIN is between 8V to 18V. This prevents the IC from operating during unintended or fault conditions, allowing the circuit designer to reduce the size of the external components. Figure 3 also shows a simple sequencing scheme: channel 1’s power good indicator is tied to the tracking pins of the other three channels. Figure 4 shows the resulting start-up sequence, with channel 1 starting first and the remaining channels tracking during their start-up. Other sequencing and tracking examples can be found in the data sheet. VOUT2 20mV/DIV LDO OUTPUT 20mV/DIV 2μs/DIV DN441 F05 Figure 5. Low Noise LDO Output: Top Trace is Channel 2 Output, Bottom Trace is Channel 4 LDO Output Conclusion The LT3507 integrates three buck regulators and an LDO driver in a QFN (5mm × 7mm) package, offering a compact solution for multiple-rail systems. Separate inputs for each converter offer wide design freedom, while separate PG indicators and TRK/SS pins further extend tracking and sequencing flexibility. Data Sheet Download For applications help, call (408) 432-1900, Ext. 3223 www.linear.com Linear Technology Corporation dn441f LT/TP 0508 392K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●