MP4575 datasheet(19/22 Pages) MPS

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MP4575

– 5A, 55V, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4575 Rev. 1.0

www.MonolithicPower.com

8/16/2016

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

Use

the

following

steps

design

the

compensation:

1. Choose R3 to set the desired crossover

frequency. R3 can be determined with

Equation (18):

OUT





(18)

Where

the

desired

crossover

frequency.

2. Choose C3 to achieve the desired phase

margin.

For

applications

with

typical

inductor values, set the compensation zero

(fZ1) to <0.25xfC to provide a sufficient phase

margin. C3 can be calculated with Equation

(19):





(19)

3. Determine if C4 is required. C4 is required if

the ESR zero of the output capacitor is

located at <0.5xfSW, or Equation (20) is

valid:

ESR





(20)

If this is the case, use C4 to set the pole

(fP3) at the location of the ESR zero.

Determine C4 with Equation (21):

ESR





(21)

External Bootstrap Diode

For high duty-cycle operations where VOUT/VIN

> 65%, the time period available to the

bootstrap charging is less, so the bootstrap

capacitor may not be charged sufficiently. This

affects efficiency and normal operation. An

external bootstrap diode from the 3V - 5V rail to

BST can help charge the bootstrap capacitor

and enhance efficiency (see Figure 2). The

output voltage is a good choice for this power

supply if it is in above range. The bootstrap

diode can be a low-cost one such as IN4148 or

BAT54.

CBST

COUT

BST

3V-5V Rail

1N4148

VOUT

Figure 2: External Bootstrap Diode

At no load or light load, the converter may

operate in pulse-skipping mode to maintain the

output-voltage regulation. Under this condition,

VSW = VOUT for most of the time, so the diode

from VOUT to BST cannot charge the bootstrap

capacitor. For a sufficient gate voltage during

pulse-skipping mode, VIN - VOUT should be no

less than 3V. For example, if VOUT = 3.3V, then

VIN must exceed 3.3V + 3V = 6.3V to maintain

a sufficient bootstrap voltage at no load or light

load. To meet this requirement,

EN can

program the input UVLO voltage to VOUT + 3V.

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