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

MPQ8623 Rev. 1.01

www.MonolithicPower.com

13

10/2/2018

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2018 MPS. All Rights Reserved.

OPERATION

CONSTANT-ON-TIME (COT) Control

The MPQ8623 employs constant-on-time (COT)

control to achieve fast load transient response.

Figure 2 shows the details of the control stage of

the MPQ8623.

The operational amplifier (AMP) corrects any

error voltage between FB and VREF. The

MPQ8623 can use AMP to provide excellent

load regulation over the entire load range

whether it is operating in forced continuous

conduction mode (CCM) or pulse-skip mode.

The dedicated VSNS pin helps provide the

feature of the differential output voltage remote

sense. The pair of the remote sense trace should

be kept at a low impedance to achieve the best

performance.

The

MPQ8623

has

internal

RAMP

compensation to support low ESR MLCC output

capacitor solutions. The adaptive internal RAMP

is optimized so that the MPQ8623 is stable in the

entire operating input/output voltage range with

proper design of the output L/C filter.

Adaptive

RAMP

ON

Timer

PWM

VREF

FB

PWM

Comparator

AGND

AMP

Vsens+

Vsens-

Σ

+

L

VIN

PGND

SW

HSFET

LSFET

Figure 2: COT Control

PWM Operation

Figure 3 shows how the pulse-width modulation

(PWM) signal is generated. AMP corrects any

error between FB and REF and generates a

fairly smooth DC voltage (COMP). The internal

RAMP is superimposed onto COMP. The

superimposed COMP is compared with the FB

signal.

Whenever

FB

drops

below

the

superimposed COMP, the integrated high-side

MOSFET (HS-FET) is turned on and remains on

for a fixed turn-on time. The fixed on time is

determined by the input voltage, output voltage,

and selected switching frequency. After the on

period elapses, the HS-FET turns off. The HS-

FET turns on again when FB drops below the

superimposed

COMP.

By

repeating

this

operation, the MPQ8623 regulates the output

voltage. The integrated low-side MOSFET (LS-

FET) turns on when the HS-FET is in its off state

to minimize conduction loss. A dead short occurs

between VIN and PGND if both the HS-FET and

the LS-FET are turned on at the same time

(shoot-through). In order to avoid a shoot-

through, a dead time (DT) is generated internally

between the HS-FET off and the LS-FET on

period or the LS-FET off and the HS-FET on

period.

PWM

REF

RAMP

FB

COMP

HS-FET

Driver

LS-FET

Driver

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 zero amps (see Figure

3). The MPQ8623 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 almost constant throughout the entire

load range.

Pulse-Skip Operation

At light-load condition, the MPQ8623 can be

configured to work in pulse-skip mode to

optimize efficiency. When the load decreases,

the inductor current decreases as well. Once the

inductor current reaches zero, the MPQ8623

transitions from CCM to pulse-skip mode if the

MPQ8623 is configured in this way. See the

Mode Selection section on page 14 for details.