MP8864

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8864 Rev. 1.02

www.MonolithicPower.com

14

1/18/2018

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2018 MPS. All Rights Reserved.

OPERATION

The MP8864 is a high-frequency, synchronous,

rectified,

step-down,

switch-mode

converter

with built-in power MOSFETs. It offers a very

compact

solution

that

achieves

a

4

A

continuous output current with excellent load

and line regulation over a wide input supply

range.

The

MP8864

has

three

working

modes:

advanced

asynchronous

modulation

(AAM)

mode, discontinuous conduction mode (DCM),

and continuous conduction mode (CCM). When

the MODE bit

MP8864 always operates in CCM.

AAM Control Operation

In a light-load condition, the MP8864 works in

advanced

asynchronous

modulation

(AAM)

fixed

voltage

when

the

input and output

output, which represents the peak inductor-

the MP8864 to skip pulses, achieving the light-

load power save. Refer to AN032 for additional

details.

MOSFET (HS-FET) turns on and remains on

The light-load feature in this device is optimized

for 12 V input applications.

56

4

Rt

R1

R2

R9

Figure 3

—Simplified AAM control logic

DCM Control Operation

current increases. Once its minimum value

mode, the internal clock initiates the PWM

cycle, and the HS-FET turns on and remains on

After a period of dead time, the low-side

MOSFET (LS-FET) turns on and remains on

until the inductor current value decreases to

zero. The device repeats the same operation in

every clock cycle to regulate the output voltage

(see Figure 4).

HS-FET is on

Zero current detect

A clock cycle

LS-FET is on

HS/LS-FETs are off

Figure 4

—DCM control operation

CCM Control Operation

The device enters CCM from DCM once the

inductor current no longer drops to zero in a

clock cycle. In CCM, the internal clock initiates

the PWM cycle, and the HS-FET turns on and

FET turns on and remains on until the next

clock cycle starts. The device repeats the same

operation in every clock cycle to regulate the

output voltage.

within 95% (600 kHz switching frequency) of

one PWM period, the HS-FET is forced off.

Internal Regulator

A 5 V internal regulator powers most of the

than 5 V, the output voltage decreases. A

0.22

μF ceramic capacitor for decoupling is

required.

Error Amplifier (EA)

The error amplifier compares the FB voltage

against the internal reference (REF) in the FB

control loop and outputs the COMP voltage

—

which controls the power MOSFET current. In a

connected to the EA non-inverter input. The

optimized

internal

compensation

network

minimizes the external component count and

simplifies the control loop design.