MPQ8626 – 16V, 6A, HIGH EFFICIENCY SYNC, STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT

MPQ8626 Rev. 1.1

www.MonolithicPower.com

14

8/16/2022

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© 2022 MPS. All Rights Reserved.

Figure 4 shows PSM operation at light-load

condition.

When

FB

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 PSM operation, FB does not

reach the superimposed COMP when the

inductor current approaches zero. The LS-FET

driver turns into 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 less than

-1mA.

Therefore,

the

output

capacitors

discharge slowly to PGND through the LS-FET.

In light-load condition, the HS-FET is not turned

on as frequently in PSM as it is in forced CCM.

As a result, the efficiency in PSM is improved

greatly compared to that in 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

condition, the current modulator regulation time

period becomes shorter. The HS-FET is turned

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

SW

OUT

OUT

IN

OUT

V

F

L

2

V

)

V

V

(

I









−

=

(1)

Where FSW is the switching frequency.

The MPQ8626 enters PWM mode once the

output

current

exceeds

the

critical

level.

Afterward, the switching frequency remains

fairly constant over the output current range.

The MPQ8626 can be configured to operate in

forced CCM, even in light-load condition (see

Table 1).

MODE Selection

The MPQ8626 provides both forced CCM

operation and pulse-skip operation in light-load

condition. The MPQ8626 has three options for

switching frequency: 600kHz, 1100kHz, and

2000kHz. Selecting the operation mode under

light-load condition and the switching frequency

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

AGND

Forced CCM

1100kHz

30.1kΩ (±20%)

to AGND

Forced CCM

2000kHz

60.4kΩ (±20%)

to AGND

Forced CCM

600kHz

121kΩ (±20%)

to AGND

Pulse skip

600kHz

243kΩ (±20%)

to AGND

Pulse skip

2000kHz

VCC

Pulse skip

1100kHz

Soft Start (SS)

The minimum soft-start time is limited to 2.2ms.

This

can

be

increased

by

choosing

the

capacitance between TRK/REF and AGND. A

minimum value of 3.3nF for this capacitor is

always required to stabilize the reference

voltage.

The capacitance of this capacitor can be

determined with Equation (2) and Equation (3):

REF

C (nF) 3.3 ~ 33

=

(tSS = 2.2ms) (2)

ss

REF

t (ms) 10 A

C

(nF)

0.6V



=

(tSS > 2.2ms) (3)