MP8846 – 6A, 8V SYNCHRONOUS STEP-DOWN CONVERTER

MP8846 Rev. 1.0

www.MonolithicPower.com

14

5/19/2015

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OPERATION

The MP8846 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 MP8846 has three working modes:

advanced asynchronous modulation (AAM)

mode, discontinuous conduction mode (DCM),

and continuous conduction mode (CCM). When

MP8846 works in CCM mode constantly.

AAM Control Operation

In a light-load condition, the MP8846 works in

internally fixed voltage when input and output

output, which represents the peak inductor-

the MP8846 to skip pulses, achieving the light-

load power save. Refer to AN032 for additional

details.

side MOSFET (HS-FET) turns on and remains

FIGURE 2. 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 until

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. 3).

FIGURE 3. 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

0.22μF 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-inverted input. The

optimized

internal

compensation

network

minimizes the external component count and

simplifies the control loop design.