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

MP8792 Rev. 1.0

www.MonolithicPower.com

14

12/2/2019

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© 2019 MPS. All Rights Reserved.

Figure 4 shows pulse skip mode operation

under light-load conditions. When VFB drops

below the superimposed COMP, the HS-FET

turns on for a fixed interval. When the HS-FET

turns off, the LS-FET turns on until the inductor

current reaches zero.

During pulse skip mode, VFB does not reach the

superimposed COMP when the inductor current

approaches zero. The LS-FET driver switches

to tri-state (HI-Z) when the inductor current

reaches zero.

A current modulator controls the LS-FET and

limits the inductor current to below -1mA. This

allows the output capacitors to discharge slowly

to PGND through the LS-FET. Under light-load

conditions, the HS-FET does not turn on as

frequently in pulse skip mode as it does in

forced CCM. As a result, the efficiency in pulse

skip mode is improved compared to forced

CCM operation.

PWM

REF

RAMP

FB

COMP

tON

HS-FET

Driver

LS-FET

Driver

IL

IO

Figure 4: Pulse Skip at Light-Load

As the output current increases from light load,

the time period during which the current

modulator regulates becomes shorter. The HS-

FET

turns

on

more

frequently,

and

the

switching frequency increases accordingly. The

output current reaches the critical level when

the current modulator time is zero. The critical

level of the output current can be calculated

with Equation (1):

IN

SW

OUT

OUT

IN

OUT

V

f

L

V

V

V

I













2

)

(

(1)

Where fSW is the switching frequency.

The MP8792 enters PWM mode once the

output

current

exceeds

the

critical

level.

Afterward, the switching frequency remains

fairly constant over the output current range.

The MP8792 can be configured to operate in

forced CCM, even under light-load conditions

(see Table 1).

Mode Selection

The

MP8792

provides

both

forced

CCM

operation and pulse skip mode operation under

light-load conditions. The MP8792 has three

options

for

switching

frequency:

600kHz,

800kHz, and 1000kHz. The operation mode

and

switching

frequency

are

selected

by

choosing the resistance value of the resistor

connected between MODE and AGND or VCC

(see Table 1).

Table 1: Mode Selection

Mode

Light-Load

Mode

Switching

Frequency

VCC

Pulse skip

600kHz

243

kΩ (±20%)

to GND

Pulse skip

800kHz

121

kΩ (±20%)

to GND

Pulse skip

1000kHz

GND

Forced CCM

600kHz

30.1

kΩ (±20%)

to GND

Forced CCM

800kHz

60.4

kΩ (±20%)

to GND

Forced CCM

1000kHz

Soft Start (SS)

The minimum soft-start time is 1ms; it can be

increased by adding a soft-start capacitor

between TRK/REF and VSNS-.

The total SS capacitor value can be estimated

with Equation (2):

SS

SS

t (ms) 36 A

C (nF)

0.6(V)





(2)

Output Voltage Tracking and Reference

The MP8792 provides an analog input pin

(TRK/REF) to track another power supply or

accept an external reference. When an external

voltage signal is connected to TRK/REF, it acts

as a reference for the MP8792 output voltage.

VFB follows this external voltage signal, and the

soft-start settings are ignored. The TRK/REF