MP24971 –1.5A, 50V, 100kHz, 5V FIXED OUTPUT STEP-DOWN CONVERTER

MP24971 Rev. 1.01

www.MonolithicPower.com

11

1/6/2012

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Selecting the Input Capacitor

An input capacitor reduces the surge current

drawn from the input and any switching noise

from the device. Chose a capacitor with a

switching-frequency impedance less than the

input

source

impedance.

Use

ceramic

capacitors with X5R or X7R dielectrics for their

low ESR and small temperature coefficients. A

capacitor value of 4.7µF will suffice for most

applications.

Selecting the Output Capacitor

An output capacitor limits the output voltage

and ensures regulator loop stability. Chose an

output capacitor with a low impedance at the

switching frequency; preferably ceramic with

X5R or X7R dielectrics.

Setting the Current Limit

The inductor’s DC resistance (DCR) sets the

hiccup current limit. Use a sense resistor with

high precision and accuracy for accurate sense

measurement.

In Figure 3a, the output current limit is:

OUT _ L

100mV

I

DCR



(10)

In Figure 3b, the output current limit is set as:

OUT _ L

SENSE

100mV

I

R



(11)

Programming the Short Circuit Current Limit

The hiccup current limit at output short

condition can be programmed to be lower by

in figure 4.

When output voltage is lower than 200mV, the

current limit is described by:

OUT _ SL

SENSE

S1

I

R

6.2 A R

100mV



 



(12)

The current limit at output short condition is:

S1

OUT _ SL

SENSE

100mV 6.2 A R

I

R

 



(13)

SW

ISP

ISN

SW

ISP

ISN

(a)

(b)

Figure 3—Current Sensing Methods

SW

ISP

ISN

200mV

6.2uA

Figure 4—Short Circuit Current Limit