MP24943 –3A, 55V, 100kHz STEP-DOWN CONVERTER

MP24943 Rev. 0.83

www.MonolithicPower.com

10

12/11/2013

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider—R1 and R2—sets

front page). R1 also sets the feedback loop

bandwidth with the internal compensation

capacitors (see Figure 1). Choose R1 to be

around 300kΩ for optimal transient response.

Choose R2 as determined by:

1

V

8

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

301 (1%)

240 (1%)

2.5

301 (1%)

140 (1%)

3.3

301 (1%)

95.3 (1%)

5

301 (1%)

57.6 (1%)

Setting the Output OVP Threshold

An external resistor divider—R5 and R6 (see

the typical application circuit on the front

page)—connected to VO sets the output OVP

threshold. Choose R5 to be 301kΩ for reduced

power dissipation. Then R6 is given by:

OVP

OVREF

R5

R6

(k )

V

1

V

=Ω

−

Selecting the Inductor

Include an inductor with a value between 22µH

and 47µH and a DC current rating that is at

least 25% percent higher than the maximum

load

current

for

most

applications.

For

maximum efficiency, the inductor DC resistance

should be less than 200mΩ. For most designs,

the inductance value can be estimated from the

following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose the inductor current ripple to be

approximately 30% of the maximum load

current, 3A. The maximum inductor peak

current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light-load conditions of <100mA, larger

inductance values improve efficiency.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and also the switching

noise from the device. Choose a capacitor with

a switching-frequency impedance of less than

the input source impedance to prevent any

high-frequency switching current from flowing to

the input. Use ceramic capacitors with X5R or

X7R dielectrics for their low ESR and small

temperature coefficients. Use a 4.7µF capacitor

for most applications.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures regulator loop stability.

Select an output capacitor with a low switching-

frequency

impedance,

preferably

ceramic

capacitors with X5R or X7R dielectrics.

PC Board Layout

The high frequency path—GND, IN, and SW—

should be placed very close to the device with

short, direct, and wide traces. Place the input

capacitor as close as possible to the IN and

GND pins. Place the external feedback

resistors next to the FB pin. Keep the SW node

short and away from the feedback network.

External Bootstrap Diode

Add an external bootstrap diode when the

system has a fixed 5V input or when the power

supply generates a 5V output. This helps

improve the efficiency of the regulator. The

bootstrap diode can be a low-cost one such as

IN4148 or BAT54.