MP8675 – 6A, 42V, 420KHZ STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER

MP8675 Rev.1.2

www.MonolithicPower.com

7

1/26/2010

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2010 MPS. All Rights Reserved.

OPERATION

IN

EN/SYNC

FB

SW

VCC

BG

GND

BST

REGULATOR

OSCILLATOR

420KHz

DRIVER

CURRENT SENSE

AMPLIFIER

ERROR

AMPLIFIER

COMP

CURRENT

LIMIT

COMPARATOR

PWM

COMPARATOR

DRIVER

D1

REGULATOR

REFERENCE

--

+

--

+

--

+

S

R

R

Q

Q

Figure 1—Functional Block Diagram

The MP8675 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 MP8675 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 1µF ceramic

capacitor for decoupling purpose is required.