MP38870 – 3A, 14V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38870 Rev. 1.1

www.MonolithicPower.com

6

10/22/2009

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© 2009 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

C1

50pF

C1

1pF

POWER

GOOD

Figure 1—Functional Block Diagram

The MP38870 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 3A

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 MP38870 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.