MP8794

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

MP8794 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. In pulse skip mode

operation,

VFB

does

not

reach

the

superimposed COMP when the inductor current

approaches zero. The LS-FET driver swtiches

tri-state

(Hi-Z)

when

the

inductor

current

reaches zero.

A current modulator takes over the control of

the LS-FET and limits the inductor current to

below -1mA. Therefore, the output capacitors

discharge slowly to PGND through the LS-FET.

In a light-load condition, 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 in 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 critical levels when the

current modulator time is zero. The critical level

of the output current can be determined with

Equation (1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2 L f

V





 



(1)

Where fSW is the switching frequency.

The MP8794 enters PWM mode once the

output

current

exceeds

the

critical

level.

Afterward, the switching frequency remains

fairly constant over the output current range.

The MP8794 can be configured to operate in

forced CCM, even in light-load conditions (see

Table 1).

Mode Selection

The

MP8794

provides

both

forced

CCM

operation and pulse skip mode operation in

light-load conditions. The MP8794 has three

selectable switching frequency options: 600kHz,

800kHz, and 1000kHz. Selecting the operation

mode and switching frequency under light-load

conditions is done 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

243kΩ (±20%)

to GND

Pulse skip

800kHz

121kΩ (±20%)

to GND

Pulse skip

1000kHz

GND

Forced CCM

600kHz

30.1kΩ (±20%)

to GND

Forced CCM

800kHz

60.4kΩ (±20%)

to GND

Forced CCM

1000kHz

Soft Start (SS)

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

increased by adding an SS capacitor between

REF/TRK and VSNS-.

The total SS capacitor value can be determined

with Equation (2):

ss

SS

t (ms) 36 A

C (nF)

0.6(V)





(2)

Output Voltage Tracking and Reference

The MP8794 provides an analog input pin

(REF/TRK) to track another power supply or

accept an external reference. When an external

voltage signal is connected to REF/TRK, it acts

as a reference for the MP8794 output voltage.