LINEAR 15VIN, 4MHz Monolithic Synchronous Buck Regulator Specifications - Design Note 467 by Tom Gross

15VIN, 4MHz Monolithic Synchronous Buck Regulator Delivers 5A in 4mm × 4mm QFN – Design Note 467 Tom Gross Introduction produces an out-of-phase clock signal so that parallel

The LTC®3605 is a high effi ciency, monolithic synchronous devices can be interleaved to reduce input and output step-down switching regulator that is capable of delivering current ripple. A multiphase, or Poly Phase®, design 5A of continuous output current from input voltages of also generates lower high frequency EMI noise than a 4V to 15V. Its compact 4mm × 4mm QFN package has single-phase design, due to the lower switching currents very low thermal impedance from the IC junction to the of each phase. This confi guration also helps with the PCB, such that the regulator can deliver maximum power thermal design issues normally associated with a single without the need of a heat sink. A single LTC3605 circuit high output current device.

can power a 1.2V microprocessor directly from a 12V

1.8VOUT , 2.25MHz Buck Regulator

rail—no need for an intermediate voltage rail.

The LTC3605 is specifi cally designed for high effi ciency

The LTC3605 employs a unique controlled on-time/con- at low duty cycles such as 12VIN-to-1.8VOUT at 5A, as stant frequency current mode architecture, making it ideal shown in Figure 1. High effi ciency is achieved with a low for low duty cycle applications and high frequency opera- RDS(ON) bottom synchronous MOSFET switch (35mΩ) and tion. There are two phase-lock loops inside the LTC3605: a 70mΩ RDS(ON) top synchronous MOSFET switch. one servos the regulator on-time to track the internal

This circuit runs at 2.25MHz, which reduces the value and

oscillator frequency, which is determined by an external

size of the output capacitors and inductor. Even with the

timing resistor, and the other servos the internal oscillator

high switching frequency, the effi ciency of this circuit is

to an external clock signal if the part is synchronized. Due

about 80% at full load.

to the controlled on-time design, the LTC3605 can achieve

very fast load transient response while minimizing the Figure 2 shows the fast load transient response of the number and value of external output capacitors. application circuit shown in Figure 1. It takes only 10μs to recover from a 4A load step with less than 100m V

The LTC3605’s switching frequency is programmable from

of output voltage deviation and only two 47μF ceramic

800k Hz to 4MHz, or the regulator can be synchronized to

output capacitors. Note that compensation is internal, set

an external clock for noise-sensitive applications.

up by tying the compensation pin (ITH) to the internal

Furthermore, multiple LTC3605s can be used in parallel L, LT, LTC, LTM, Poly Phase, Linear Technology and the Linear logo are registered

trademarks of Linear Technology Corporation. All other trademarks are the property of

to increase the available output current. The LTC3605

their respective owners.

PGOOD PHMODE ITH INTVCC

VIN C5

PVIN BOOST L1

PVIN 0.33μH

SW 1.8V

SW C3 C4 5A SW 47μF 47μF

LTC3605

SVIN SW 6.3V 6.3V

R3 C6 RUN

CLKIN SW

CLKOUT VON FB

RT TRACK MODE SGND PGND PGND R2 20.0k

DN467 F01

Figure 1. 12V to 1.8V at 5A Buck Converter Operating at 2.25MHz