MP8867 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8867 Rev.1.01

www.MonolithicPower.com

14

2/21/2017

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OPERATION

The MP8867 is a high-frequency, synchronous

rectified, step-down, switch-mode converter

with built-in power MOSFETs. It offers a very

compact solution that achieves an 8A output

current with excellent load and line regulation

over a wide input supply range.

The MP8867 has three working modes:

continuous conduction mode (CCM), advanced

asynchronous modulation (AAM) mode, and

discontinuous conduction mode (DCM). The

MP8867’s default operation mode is CCM. The

can be set to “0.”

CCM Control Operation

In CCM, the internal clock initiates the PWM

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

After a period of dead time, the LS-FET turns

on and remains on until the next clock cycle

begins. The device repeats this operation in

every clock cycle to regulate the output voltage.

within 95% (500kHz switching frequency) of

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

AAM Control Operation

In a light-load condition, the MP8867 works in

AAM mode if the mode bit is set to “0” (see

amplifier output, which represents the peak

and the MP8867 skips some pulses, achieving

the light-load power save. Refer to AN032 for

additional details.

The light-load feature in this device is optimized

for 12V input applications.

Figure 2: Simplified AAM Control Logic

DCM Control Operation

the device enters DCM.

In this mode, the

internal clock initiates the PWM cycle, and the

After a period of dead time, the LS-FET turns

on and remains on until the inductor current

value decreases to zero. The device will repeat

the same operation in every clock cycle to

regulate the output voltage.

Figure 3: DCM Control Operation

VCC Regulator

A 5V internal regulator powers most of the

than 5.0V, the output voltage decreases and

follows the input voltage. A 0.47μF ceramic

capacitor is required for decoupling purposes.

Error Amplifier (EA)

The error amplifier compares the FB voltage

against the internal reference (REF) for the FB

control loop and outputs the COMP voltage,

which controls the power MOSFET current. In

to an EA non-inverter input. The optimized

internal compensation network minimizes the

external component count and simplifies the

control loop design.