MP8792 – 16V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8792 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 MP8792 employs constant-on-time (COT)

control to achieve fast load transient response.

Figure 2 shows the details of the

MP8792’s

control stage.

The operational amplifier (AMP) corrects any

voltage errors between the feedback voltage

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

MP8792 can use AMP to provide excellent load

regulation over the whole load range in either

forced continuous conduction mode (CCM) or

pulse skip mode.

The dedicated VSNS- pin provides differential

output voltage remote sensing. The pair of

remote-sense traces should be kept at low

impedance for optimal performance.

The MP8792 has internal ramp compensation,

and it supports a low-ESR MLCC output

capacitor solution. The adaptive internal ramp is

optimized so that the MP8792 remains stable in

the whole operating input/output voltage range

with 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

PWM Operation

Figure 3 shows the generation of the pulse-

width modulation (PWM) signal. AMP corrects

any error between VFB and VREF, and generates

a smooth DC voltage (COMP). The internal

ramp

is

superimposed

onto

COMP.

The

superimposed COMP is compared with 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 MP8792 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 the 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

2). The MP8792 can also 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 constant throughout the load

range.

Pulse Skip Operation

Under light-load conditions, the MP8792 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

part

transitions from CCM to pulse skip mode if the

MP8792 is configured to do so (see the Mode

Selection section on page 14 for details).