MP8774

– 18V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8774 Rev. 1.0

www.MonolithicPower.com

15

1/15/2018

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© 2018 MPS. All Rights Reserved.

OPERATION

The MP8774 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

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

provide fast transient response and ease loop

stabilization. Figure 2 shows the simplified ramp

compensation block in the MP8774. At the

beginning of each cycle, the high-side MOSFET

(HS-FET) is turned on when the feedback

voltage. The on period is determined by both

the output voltage and input voltage to make

the switching frequency fairly constant over the

input voltage range.

After the on period elapses, the HS-FET is

turned off. The HS-FET is turned on again

operation in this way, 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

shoot-through. To avoid shoot-through, a dead

time (DT) is generated internally between the

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

FET off and HS-FET on period.

Internal compensation is applied for COT

control to provide a more stable operation, even

when ceramic capacitors are used as output

capacitors.

This

internal

compensation

improves jitter performance without affecting

the line or load regulation.

VOUT

L

Cout

R1

R2

On

Timer

Logic

Control

REF

SW

RAMP

FB

RAMP

GENERATOR

PWM

BUF

Figure 2: Simplified Ramp Compensation Block

Heavy-Load Operation

Continuous conduction mode (CCM) is when

the output current is high and the inductor

current is always above zero amps (see Figure

fixed interval determined by the one-shot on-

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

FET is turned on until the next period.

Figure 3: Heavy-Load Operation

In CCM operation, the switching frequency is

fairly

constant.

This

is

called

pulse-width

modulation (PWM) mode.

Light-Load Operation

When the MP8774 works in pulse-frequency

modulation (PFM) during light-load operation,

the MP8774 reduces the switching frequency

automatically to maintain high efficiency, and

the inductor current drops almost to zero. When

the inductor current reaches zero, the LS-FET

driver goes into tri-state (Hi-Z) (see Figure 4).

Therefore, the output capacitors discharge

slowly to GND through the LS-FET, R1, and R2.

This

operation

improves

device

efficiency

greatly when the output current is low.

Ton is constant

Figure 4: Light-Load Operation