MP8868GLE datasheet(14/27 Pages) MPS

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MP8868 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8868 Rev.1.1

www.MonolithicPower.com

5/15/2015

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

OPERATION

The MP8868 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 10A output

current with excellent load and line regulation

over a wide input supply range.

The MP8868 has three working modes: CCM

(Continues-Conduction Mode), AAM mode

(Advanced Asynchronous Modulation) and

DCM (Discontinues-Conduction Mode). The

MP8868 default operation mode is CCM. When

set MODE bit(reg01[0]) in the I2C register to “0”.

MP8868 can works in the AAM mode.

CCM Control Operation

In CCM the internal clock initiates the PWM

cycle, the HS-FET turns on and remains on

until VILsense reaches the value set by VCOMP,

after a period of dead time, the LS-FET will turn

on and remain on until the next clock cycle

starts. The device will repeat the same

operation in every clock cycle to regulate the

output voltage.

If within 95% (500kHz switching frequency) of

one PWM period, VILsense does not reach the

value set by VCOMP, the HS power MOSFET is

forced off.

AAM Control Operation

In the light load condition, MP8868 works in

AAM (Advanced Asynchronous Modulation)

mode if Mode bit is set to “0”. Refer to Figure 2,

the VAAM is an internal fixed voltage when input

and output voltages are fixed. VCOMP is the error

amplifier output which represents the peak

inductor current information. When VCOMP is

lower than VAAM, the internal clock is blocked,

thus the MP8868 skips some pulses and

achieves the light load power save. Refer to

AN032 for more detail.

The internal clock resets every time when VCOMP

is higher than VAAM. At the same time the HS-

FET(High-Side MOSFET) turns on and remains

on until VILsense reaches the value set by VCOMP.

The light load feature in this device is optimized

for 12V input applications.

Figure 2: Simplified AAM Control Logic

DCM Control Operation

The VCOMP voltage ramps up with the increasing

of the output current, when its minimum value

exceeds VAAM, the device will enter DCM

(Discontinues-Conduction Mode). In this mode

the internal clock initiates the PWM cycle, the

HS-FET turns on and remains on until VILsense

reaches the value set by VCOMP, after a period of

dead time, the LS-FET (Low-Side MOSFET)

will turn on and remain 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

internal circuitries. 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 voltage decreases and

follows the input voltage. A 0.47μF ceramic

capacitor for decoupling purpose is required.

Error Amplifier

The error amplifier compares the FB pin voltage

against the internal reference (REF) for FB

control loop and outputs the COMP voltage—

which controls the power MOSFET current. In

I2C mode, the FB pin is opened and VOUT pin

is connected to the EA non-inverter input. The

optimized

internal

compensation

network

MP8868GLE Datasheet(PDF) 14 Page - Monolithic Power Systems

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