MP4572

– 60V, 2A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4572 Rev. 1.0

www.MonolithicPower.com

26

3/9/2020

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider connected to the FB

pin sets the output voltage (see the Typical

Application Circuits on page 28). The feedback

resistor (R1) must account for both stability and

dynamic response, so it cannot be too large or

too small. Choose an R1 value of about 40k

Ω.

R2 is then estimated with Equation (2):





OUT

R1

R2

V

1

0.8

(2)

Figure

4

shows

the

recommended

T-type

feedback network.

R1

R2

VOUT

FB

MP4572

R3

5

Figure 4: Feedback Network

R3 + R1 is used to set the loop bandwidth. A

higher R3 + R1 indicates a lower bandwidth. To

ensure loop stability, it is strongly recommended

to limit the bandwidth between 1/10 of the

switching frequency and 100kHz.

The calculated resistance may need fine-tuning

via bench testing. Table 1 lists the recommended

feedback divider resistor values for common

output voltages. Use check loop analysis before

using the device in an application, and change

the resistance of R3 for loop stability if necessary.

Table 1: Resistor Values for Typical VOUT

VOUT (V)

R1 (kΩ)

R2 (kΩ)

3.3

41.2

13

5.0

41.2

7.68

Selecting the Inductor

The inductor must supply constant current to the

output load while being driven by the switching

input voltage. For the highest efficiency, choose

an inductor with a low DC resistance. High

inductance will result in less ripple current and

lower output ripple voltage. However, a larger

inductance value results in a physically larger

inductor, higher series resistance, and lower

saturation current.

A good rule to determine the ideal inductance

value is to make the inductor ripple current about

30% of the maximum load current. Ensure that

the peak inductor current is below the device

peak current limit. The inductance value can be

calculated with Equation (3):



 



OUT

OUT

SW

L

IN

VV

L

(1

)

f

I

V

(3)

Where

ΔIL is the peak-to-peak inductor ripple

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. Calculate

the peak inductor current with Equation (4):





 



OUT

OUT

LP

OUT

SW

IN

VV

I

I

(1

)

2f

L

V

(4)

Selecting the Input Capacitor

The step-down converter has a discontinuous

input current, and requires a capacitor to supply

the AC current to the converter while maintaining

the DC input voltage. Use low-ESR capacitors for

the best performance. Ceramic capacitors with

X5R

or

X7R

dielectrics

are

strongly

recommended because of their low ESR and

small temperature coefficients. Other capacitors,

such as Y5V and Z5U, should not be used since

they lose too much capacitance with frequency,

temperature, and bias voltage.

Place the input capacitors as close to the IN pin

as possible. For most applications, a 22µF

capacitor is sufficient. For higher output voltages,

use a

47μF capacitor to improve system stability.

To maintain a small solution size, choose a

properly sized capacitor that has a voltage rating

compliant with the input spec.

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current rating that should exceed

the converter’s

maximum input ripple current. The input ripple

current can be estimated with Equation (5):





 

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV

(5)