MP8794

– 16V, 20A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8794 Rev. 1.0

www.MonolithicPower.com

13

12/2/2019

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© 2019 MPS. All Rights Reserved.

OPERATION

Constant-On-Time (COT) Control

The MP8794 employs constant-on-time (COT)

control to achieve a fast load transient response.

Figure 2 shows the details of the control stage

of the MP8794.

The operational amplifier (AMP) corrects any

voltage errors between the feedback voltage

(VFB) and the reference voltage (VREF). The

MP8794 can use AMP to provide excellent load

regulation over the entire load range in either

forced continuous conduction mode (CCM) or

pulse skip mode.

The dedicated VSNS- pin provides differential

output voltage remote sensing. The remote-

sense trace pair should be kept at low

impedance for optimal performance.

The MP8794 uses internal ramp compensation

to support a low-ESR MLCC output capacitor

solution.

The

adaptive

internal

ramp

is

optimized so that the MP8794 is stable across

the entire operating input and output voltage

ranges with a proper design of the output L/C

filter.

Adaptive

Ramp

On

Timer

PWM

COMP +

VREF

FB

PWM

Comparator

VSNS-

AMP

VSENS+

VSENS-

Σ

+

L

COUT

VIN

PGND

SW

HS-FET

LS-FET

Figure 2: COT Control

Pulse-Width Modulation (PWM) Operation

Figure 3 shows the generation of the pulse-

width modulation (PWM) signal. AMP corrects

any error between FB and REF, and generates

a fairly smooth DC voltage (COMP). The

internal ramp is superimposed onto COMP

when compared to the FB signal.

When VFB drops below the superimposed

COMP, the integrated high-side MOSFET (HS-

FET) turns on. The HS-FET remains on for a

fixed on time. The fixed on time is determined

by the input voltage, output voltage, and

selected switching frequency. After the on time

elapses, the HS-FET turns off. It turns on again

when VFB drops below the superimposed

COMP.

By

repeating

this

operation,

the

MP8794 regulates the output voltage.

To minimize conduction loss, the integrated

low-side MOSFET (LS-FET) turns on when the

HS-FET is off. A dead short occurs between

VIN and PGND if both the HS-FET and the 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 period and LS-FET on period,

or vice versa.

PWM

REF

RAMP

FB

COMP

tON

HS-FET

Driver

LS-FET

Driver

IL

IO

Figure 3: Heavy-Load Operation (PWM)

CCM Operation

Continuous conduction mode (CCM) occurs

when the output current is high, and the

inductor current is always above 0A (see Figure

3). The MP8794 can be configured to operate in

forced CCM operation when the output current

is low (see the Mode Selection section on page

14 for details).

In CCM operation, the switching frequency is

fairly constant (PWM mode), so the output

ripple remains almost constant throughout the

entire load range.

Pulse Skip Operation

In light-load conditions, the MP8794 can be

configured to work in pulse skip mode to

optimize efficiency. When the load decreases,

the inductor current also decreases. Once the

inductor current reaches zero, the MP8794

transitions from CCM to pulse skip mode if the

MP8794 is configured in this way (see the

Mode Selection section on page 14 for details).