MP8758H–22V, HIGH-CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758H Rev.1.0

www.MonolithicPower.com

12

10/13/2015

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OPERATION

PWM Operation

The MP8758H is a fully integrated, synchronous,

rectified, step-down, switch-mode converter that

employs a

constant-on-time (COT) control

scheme to provide fast transient response and

ease loop stabilization. At the beginning of each

cycle, the high-side MOSFET (HS-FET) is turned

insufficient output voltage. The on period is

determined by both the output and input voltages

to make the switching frequency constant over

the input voltage range.

After the on period elapses, the HS-FET turns off

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 the conduction loss. There will

be a dead short between the input and GND if

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

at the same time. This is called a shoot-through.

In order to avoid a shoot-through, a dead time

(DT) is generated internally between the HS-FET

off and the LS-FET on time period or the LS-FET

off and the HS-FET on time period.

Internal compensation is applied for COT control

to ensure more stable operation, even when

ceramic capacitors are used as output capacitors.

This internal compensation improves the jitter

performance without affecting the line or load

regulation.

MODE Selection

The MP8758H has MODE selection. When

MODE is pulled high, the part works in PFM

mode. When MODE is pulled low, the part works

in forced PWM mode. MODE is pulled up

internally. Floating MODE sets PFM mode.

Heavy-Load Operation

Continuous conduction mode (CCM) occurs

when the output current is high and the inductor

current is always above zero amps. CCM (see

turned on for a fixed interval, which is determined

by a one-shot on-timer. The one-shot timer is

controlled by the input and output voltages, so

that the switching frequency can be fairly fixed at

500kHz for different input and output conditions.

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

on until the next period.

Figure 2: Heavy-Load Operation

PWM mode occurs in CCM operation, where the

switching frequency is fairly constant.

Light-Load Operation

Forced PWM Mode Operation

The MP8758H enters continuous conduction

mode (CCM) when working in forced PWM mode.

In this mode, the HS-FET and the LS-FET repeat

the on/off operation, even if the inductor current

drops to zero or a negative value. The switching

PFM Mode Operation

The inductor current decreases as the load

decreases. Once the inductor current reaches

zero, the operation transitions from continuous

conduction

mode

(CCM)

to

discontinuous

conduction mode (DCM).

When the MP8758H works in PFM mode during

light-load operation, the switching frequency is

reduced automatically to maintain high efficiency

FET is turned on for a fixed interval. When the

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

until the inductor current reaches zero.

the inductor current approaches zero. The LS-

FET driver turns into tri-state (high Z) whenever

the inductor current reaches zero. As a result, the

efficiency at a light-load condition is improved

greatly. At a light-load condition, the HS-FET is

not turned on as often as it is at a heavy load

condition. This is called skip mode.