MP38874 – 8A, 21V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38874 Rev. 1.1

www.MonolithicPower.com

7

10/13/2010

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© 2010 MPS. All Rights Reserved.

OPERATION

IN

EN/SYNC

FB

SW

VCC

BG

GND

PG

BST

REGULATOR

OSCILLATOR

600KHz

DRIVER

CURRENT SENSE

AMPLIFIER

x40

ERROR

AMPLIFIER

CURRENT

LIMIT

COMPARATOR

PWM

COMPARATOR

DRIVER

D

REGULATOR

REFERENCE

--

+

--

+

--

+

S

R

R

Q

Q

POWER

GOOD

Figure 1—Functional Block Diagram

The

MP38874

is

a

fixed

frequency,

synchronous, step-down switching regulator

with an integrated high-side power MOSFET

and a gate driver for a low-side external

MOSFET. It achieves 8A continuous output

current over a wide input supply range with

excellent load and line regulation. It provides a

single highly efficient solution with current mode

control for fast loop response and easy

compensation.

The MP38874 operates in a fixed frequency,

peak current control mode to regulate the

output voltage. A PWM cycle is initiated by the

internal clock. The integrated high-side power

MOSFET is turned on and remains on until its

current reaches the value set by the COMP

voltage. When the power switch is off, it

remains off until the next clock cycle starts. If, in

90% of one PWM period, the current in the

power MOSFET does not reach the COMP set

current value, the power MOSFET will be

forced to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal 0.8V reference (REF) and

outputs a current proportional to the difference

between the two. This output current is then used

to charge or discharge the internal compensation

network to form the COMP voltage, which is used

to control the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component counts and

simplifies the control loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes the

VIN input and operates in the full VIN range.

When VIN is greater than 5.0V, the output of the

regulator is in full regulation. When VIN is lower

than 5.0V, the output decreases. Since this

internal regulator provides the bias current for the

bottom gate driver that requires significant

amount of current depending upon the external

MOSFET selection, a 1uF ceramic capacitor for

decoupling purpose is required.