MP28248 – 3A, 3.3V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN 2X3MM QFN

MP28248 Rev. 1.0

www.MonolithicPower.com

12

1/5/2012

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© 2012 MPS. All Rights Reserved.

BST

N1

LSG

VCC

C3

SW

L

COUT

C5

Figure 4: Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection with a rising

threshold of 2.2V and a hysteresis of 150mV.

The bootstrap capacitor is charged from VCC

through N1 (Figure 4). N1 turns on when the LS-

FET turns on and turns off when the LS-FET

turns off.

Switching Frequency

MP28248 uses constant-on-time control because

there is no dedicated oscillator in the IC. The

input voltage is feed-forwarded to the on-time

frequency is fairly constant over the input voltage

range.

The

switching

frequency

can

be

determined with the following equation:

6

SW

7IN

DELAY

IN

OUT

10

f (kHz)=

9.3 R (kΩ)V (V)

+t

(ns)

V (V)-0.4

V

(V)





(3)

approximately 40ns.

MP28248 is optimized to operate at high

switching frequency with high efficiency. High

switching frequency makes it possible to use

small-sized LC filter components to save system

PCB space.

Jitter and FB Ramp Slope

Jitter occurs in both PWM and skip modes when

the HS-FET driver, as shown in Figure 5 and

Figure 6. Jitter can affect system stability, with

noise immunity proportional to the steepness of

not directly affect noise immunity.

V

RE F

V

FB

HS D river

J itter

V

S L O PE1

Figure 5: Jitter in PWM Mode

V

FB

HS D river

Jitter

V

REF

V

SL O P E 2

Figure 6: Jitter in Skip Mode

Ramp with Large ESR Capacitor

For POSCAP or other types of capacitors with

large ESR as the output capacitors, the ESR

ripple dominates the output ripple, and the slope

on the FB is related to the ESR. Figure 7 shows

an equivalent circuit in PWM mode with the HS-

FET off and without an external ramp circuit. Go

to the application information section for design

recommendations for large ESR capacitors.

Figure 7: Simplified Circuit in PWM Mode without

External Ramp Compensation

To realize a stable output without an external

ramp, select an ESR value using the following

equation:







SW

ON

ESR

OUT

tt

0.7

2

R

C

(4)