MP8865

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8865 Rev. 1.03

www.MonolithicPower.com

14

1/18/2018

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OPERATION

The MP8865 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 6A continuous

output current with excellent load and line

regulation over a wide input-supply range.

The

MP8865

has

three

working

modes:

advanced asynchronous modulation (AAM),

discontinuous conduction mode (DCM), and

continuous conduction mode (CCM). When the

MP8865 works in CCM constantly.

AAM Control Operation

In a light-load condition, MP8865 works in AAM

fixed voltage when input and output voltages

which represents the peak inductor current

internal clock is blocked. This causes the

MP8865 to skip pulses, achieving the light-load

power save. For additional details, refer to

AN032.

side MOSFET (HS-FET) turns on and remains

The light-load feature in this device is optimized

for 12V input applications.

56

4

Rt

R1

R2

R9

Figure 3. Simplified AAM Control Logic

DCM Control Operation

current increases. When its minimum value

mode, the internal clock initiates the PWM

cycle, the HS-FET turns on and remains on

(after a period of dead time), and 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 Fig. 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, the HS-FET turns on and

LS-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% (600kHz switching frequency) of

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

Internal Regulator

A 5V internal regulator powers most of the

than 5.0V, the output voltage decreases. A

0.22

μF ceramic capacitor is required for

decoupling.

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

connected to the EA non-inverter input. The

optimized

internal

compensation

network

minimizes the external component count and

simplifies the control loop design.