MP38872 – 6A, 21V, 600kHz STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER

MP38872 Rev.1.11

www.MonolithicPower.com

7

9/21/2011

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OPERATION

IN

EN/SYNC

FB

SW

VCC

BG

GND

PG

BST

REGULATOR

OSCILLATOR

600KHz

DRIVER

CURRENT SENSE

AMPLIFIER

ERROR

AMPLIFIER

COMP

CURRENT

LIMIT

COMPARATOR

PWM

COMPARATOR

DRIVER

D

REGULATOR

POWER

GOOD

REFERENCE

--

+

--

+

--

+

S

R

R

Q

Q

54pF

1pF

0.74V

Figure 1—Functional Block Diagram

The

MP38872

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 6A 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 MP38872 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.