MP8756 – 26V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8756 Rev. 1.1

www.MonolithicPower.com

12

4/13/2017

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

© 2017 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP8756 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

Constant-on-time (COT) control is employed to

provide a fast transient response and ease loop

stabilization. At the beginning of each cycle, the

high-side MOSFET (HS-FET) is turned on when

insufficient output voltage. The on period is

determined by the output voltage and input

voltage to make the switching frequency constant

over the input voltage range.

After the on period elapses, the HS-FET is turned

off or enters an off state. It is turned on again

operation, the converter regulates the output

voltage. The integrated low-side MOSFET (LS-

FET) is turned on when the HS-FET is in its off

state to minimize conduction loss. There is a

dead short between the input and GND if both

the HS-FET and LS-FET are turned on at the

same time. This is called a shoot-through. To

avoid a shoot-through, a dead time (DT) is

generated internally between HS-FET off and LS-

FET on, or LS-FET off and HS-FET on.

Internal compensation is applied for COT control

to make a more stable operation, even when

ceramic capacitors are used as output capacitors.

This

internal

compensation

improves

jitter

performance without affecting line or load

regulation.

Heavy-Load Operation

Continuous conduction mode (CCM) occurs

when the output current is high and the inductor

current is always above zero amps (see Figure 2).

for a fixed interval. When the HS-FET is turned

off, the LS-FET is turned on until the next period.

Figure 2: Heavy-Load Operation

In CCM operation, the switching frequency is in

pulse-width modulation (PWM) mode and is fairly

constant.

Light-Load Power Save Mode

The inductor current decreases as the load

decreases. Once the inductor current reaches

zero, the operation switches from continuous

conduction

mode

(CCM)

to

discontinuous

conduction mode (DCM).

The power save mode operation is shown in

turned on for a fixed interval, which is determined

by a one-shot on-timer, as shown in Equation 1.

When the HS-FET is turned off, the LS-FET is

turned on until the inductor current reaches zero.

the inductor current is approaching zero. The LS-

FET driver switches to tri-state (high-Z) whenever

the inductor current reaches zero. As a result, the

efficiency at light load is greatly improved. In

light-load condition, the HS-FET is not turned on

as frequently as in heavy-load condition. This is

called skip mode.

At light-load or no-load condition, the output

drops very slowly, and the MP8756 reduces the

switching frequency to achieve high efficiency.

Figure 3: Light-Load Operation

As the output current increases from light-load

condition, the current modulator regulation time

period becomes shorter. The HS-FET is turned

on more frequently, so the switching frequency

increases correspondingly. The output current

reaches

critical

levels

when

the

current

modulator time is zero. The critical level of the

output current is determined with Equation (1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V









(1)

The device enters PWM mode once the output

current exceeds critical levels. Afterward, the

switching frequency remains fairly constant over

the output current range.