MP28258 – 3A, 4.2V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)

MP28258 Rev 1.14

www.MonolithicPower.com

10

9/25/2012

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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 charges from VCC

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

FET turns on and turns off when the LS-FET

turns off.

Switching Frequency

MP28258 uses constant-on-time (COT) control

because there is no dedicated oscillator in the IC.

The input voltage is feed-forwarded to the on-

time one-shot timer through the resistor R7. The

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 (k )

V (V)

T(ns)

V(V) 0.4

V

(V)

=

×Ω

×+

−

(3)

equals approximately 40ns.

MP28258 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

HS-FET driver, as shown in Figures 5 and 6.

Jitter can affect system stability, with noise

directly affect noise immunity.

V

REF

V

FB

HS Driver

Jitter

V

SLO PE1

Figure 5—Jitter in PWM Mode

V

FB

HS Driver

Jitter

V

REF

V

SLO PE2

Figure 6—Jitter in Skip Mode

Ramp with Large ESR Cap

In the case of POSCAP or other types of

capacitor with larger ESR is applied as output

capacitor. The ESR ripple dominates the output

ripple, and the slope on the FB is quite ESR

related. Figure 7 shows an equivalent circuit in

PWM mode with the HS-FET off and without an

external ramp circuit. Turn to application

information section for design steps with large

ESR caps.

Figure 7—Simplified Circuit in PWM Mode

without

External

Ramp

Compensation