NB650/NB650H

– 6A, 28V, FAST-TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTERS

NB650/NB650H Rev. 1.13

www.MonolithicPower.com

9

10/7/2019

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

Preliminary Specifications Subject to Change

© 2019 MPS. All Rights Reserved.

OPERATION

PWM Operation

The

NB650/NB650H

is

a

fully-integrated,

synchronous, rectified, step-down, switch-mode

converter with dynamic output voltage control. It

offers a very compact solution to achieve a 6A

continuous output current over a wide input

supply range, with excellent load and line

regulation. The NB650/NB650H operates at high

efficiency over a wide output current load range.

Constant-on-time (COT) provides a fast transient

response and easy loop stabilization. At the

beginning of each cycle, the high-side MOSFET

(HS-FET) turns on when the feedback voltage

(VFB) falls below the reference voltage (VREF),

which indicates an insufficient output voltage.

The input voltage and the frequency-set resistor

determine the ON as follows:

FREQ

ON

DELAY1

IN

9.6 R

(k )

t

(ns)

t

(ns)

V (V) 0.4







(1)

Where tDELAY1 is the 20ns delay of a comparator

in the tON module.

For best results, select tON ≥120ns.

After the ON period elapses, the HS-FET turns

off to enter the OFF state. The part turns ON

again when VFB drops below VREF. By repeating

this operation, the converter regulates the output

voltage. The integrated low-side MOSFET (LS-

FET) turns on when the HS-FET is OFF to

minimize conduction loss. There is a dead short

between input and GND (shoot-through) if both

HS-FET and LS-FET turn on at the same time.

An internally-generated dead-time (DT) between

HS-FET OFF and LS-FET ON, or LS-FET OFF

and HS-FET OFF avoids shoot-through.

Heavy-Load Operation

As shown in Figure 2, the HS-FET and LS-FET

repeatedly turn on/off when the output current is

high, and the inductor current never goes to zero.

It’s called continuous-conduction-mode (CCM)

operation. In CCM operation, the switching

frequency (fSW) is fairly constant.

Figure 2: Heavy-Load Operation

Light-Load Operation

When

the

load

current

decreases,

the

NB650/NB650H

automatically

reduces

the

switching frequency to maintain high efficiency.

Figure 3 shows the light-load operation. VFB does

not

reach

VREF

when

the

inductor

current

approaches zero. As the output current drops

from heavy-load condition, the inductor current

also decreases and eventually approaches zero.

The LS-FET driver enters a tri-state (high-Z)

whenever the inductor current reaches zero. A

current modulator takes control of the LS-FET

and limits the inductor current to less than 600

μA

to slowly discharge the output capacitors to GND

through LS-FET as well as R1 and R2A, R2B

and R2C. The HS-FET does not turn ON as

frequently as in heavy-load condition. As a result,

the efficiency at light-load condition increases

greatly. This operation mode is also called skip

mode.

Figure 3: Light-Load Operation

As the output current increases from the light-

load condition, the time period within which the

current modulator regulates becomes shorter.

As the part exits light-load mode, the HS-FET

turns on more frequently to increase the switching

frequency. The output current reaches critical

when the current modulator time is zero. The