MPQ8623 – 16V, 6A, HIGH EFFICIENCY SYNC STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT

MPQ8623 Rev. 1.01

www.MonolithicPower.com

14

10/2/2018

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© 2018 MPS. All Rights Reserved.

Figure 4 shows pulse-skip mode 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 pulse-skip mode operation, FB

does not reach the superimposed COMP when

the inductor current approaches zero. The LS-

FET driver turns into tri-state (high-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. At light-

load condition, the HS-FET is not turned on as

frequently in pulse-skip mode as it is in forced

CCM. As a result, the efficiency in pulse-skip

mode is improved greatly compared to that in

forced CCM operation.

PWM

REF

RAMP

FB

COMP

HS-FET

Driver

LS-FET

Driver

Figure 4: Pulse Skip in Light Load

As the output current increases from the light-

load condition, the time period the current

modulator regulates in 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)

The MPQ8623 enters PWM mode once the

output

current

exceeds

the

critical

level.

Afterward, the switching frequency remains fairly

constant over the output current range.

The MPQ8623 can be configured to operate in

forced CCM even in light-load condition (see

Table 1).

Mode Selection

The MPQ8623 provides both forced CCM

operation and pulse-skip operation in light-load

condition. The MPQ8623 has three options for

switching frequency. 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

1100Hz

Soft Start (SS)

The minimum soft-start time is limited to 1.5ms.

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):

17

~

3

.

3

)

(



nF

C

REF

V

A

ms

t

nF

C

ss

REF

9

.

0

10

)

(

)

(





