LINEAR Monolithic Synchronous Step-Down Regulator for High Current Output and Wider Input Voltage Range - Design Note 457

Monolithic Synchronous Step-Down Regulator Delivers up to 12A from a Wide Input Voltage Range – Design Note 457 Charlie Zhao and Henry Zhang

Introduction Typical Application Example The LTC®3610 is a high power monolithic synchronous Figure 1 shows a typical application schematic of the step-down DC/DC regulator that can deliver up to 12A LTC3608. This 7mm × 8mm regulator supplies 2.5V of continuous output current from a 4V to 24V (28V at 8A maximum load current from a 4.5V to 20V input maximum) input supply. It is a member of a high current source. This application switches at a nominal 650k Hz, monolithic regulator family (see Table 1) that features which allows the use of low profi le inductor and capaci-

integrated low RDS(ON) N-channel top and bottom tors while maintaining high effi ciency. The switching MOSFETs. This results in a high effi ciency and high frequency can be easily adjusted by a resistor (RON in power density solution with few external components. Figure 1, connected to the ION pin). Figure 2 shows the This regulator family uses a constant on-time valley operating effi ciency. current mode architecture that is capable of operating

The FCB pin is connected to ground to force continuous

at very low duty cycles at high frequency and with very

mode operation at light load for both low noise and small

fast transient response. All are available in low profi le

output ripple. The FCB pin can also be tied to INTVCC

(0.9mm max) QFN packages.

to enable discontinuous mode for higher effi ciency at

Table 1. Comparison of LTC36XX Family Members light load. Soft-start is programmable with a capacitor

LTC3608 LTC3609 LTC3610 LTC3611 from the RUN/SS pin to ground. Forcing the RUN/SS

INPUT VOLTAGE 4 to 20 4.5 to 32 4 to 24 4.5 to 32 pin below 0.8V shuts down the device. RANGE (V)

MAX LOAD The LTC36XX family are valley current mode regu-

CURRENT (A) lators so they inherently limit the cycle-by-cycle QFN PACKAGE 7 × 8 × 7 × 8 × 9 × 9 × 9 × 9 ×

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

SIZE (mm) 0.9 0.9 0.9 0.9

All other trademarks are the property of their respective owners.

4.5V TO 20V

10μF 10μF 10μF 1μH 25V 25V 25V IHLP2525CZ-01 VOUT

PVIN SW 2.5V

CMDHS-3 25V LTC3608

INTVCC SW 4.7μF

BOOST INTVCC 10V VIN

INTVCCRUN/SS

VONVOUT 1Ω SVIN

PGOOD SVIN

VRNG EXTVCC 25V

ITH VFB VOUT

ION PGND SGNDFCB 19.1k 2200p F 1%

RON DN457 F01 162k

f SW = 650k Hz 1%

Figure 1. 4.5V to 20V Input to 2.5V/8A High Density Step-Down Converter