NB669, 24V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER WITH LDO

NB669 Rev. 1.01

www.MonolithicPower.com

11

7/23/2013

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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.

The HS-FET is turned ON more frequently.

Hence,

the

switching

frequency

increases

correspondingly. The output current reaches the

critical level when the current modulator time is

zero. The critical level of the output current is

determined as follows:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V

−×

=

××

×

(1)

It turns into PWM mode once the output current

exceeds the critical level. After that, the switching

frequency stays fairly constant over the output

current range.

Jitter and FB Ramp Slope

Jitter occurs in both PWM and skip modes when

HS-FET driver, as shown in Figures 4 and 5.

Jitter can affect system stability, with noise

directly affect noise immunity.

V

RE F

V

FB

HS D river

J itter

V

S L O PE1

Figure 4—Jitter in PWM Mode

V

FB

HS D river

Jitter

V

REF

V

SL O P E 2

Figure 5—Jitter in Skip Mode

Selecting the Output Capacitors

The traditional constant-on-time control scheme

is intrinsically unstable if output capacitor’s ESR

is not large enough as an effective current-sense

resistor. Ceramic capacitors usually can not be

used as output capacitor.

Figure 6 shows an equivalent circuit in PWM

mode with the HS-FET off. To realize the stability,

the ESR value should be chosen as follow:

SW

ON

ESR

OUT

TT

0.7

2

R

C

+

×π

≥

(2)

R

R

ESR

CAP

FB

L

VOUT

SW

Figure 6—Simplified Circuit in PWM Mode

The

NB669

has

built

in

internal

ramp

compensation to make sure the system is stable

even without the help of output capacitor’s ESR;

and thus the pure ceramic capacitor solution can

be applicant. The pure ceramic capacitor solution

can significantly reduce the output ripple, total

BOM cost and the board area.

Configuring the EN Control

The NB669 has two enable pins to control the

on/off of the internal regulators.

ENLDO is used to enable or disable the whole