MP9459 – SYNCHRONOUS, STEP-DOWN CONVERTER W/ INTERNAL MOSFETS

MP9459 Rev. 1.0

www.MonolithicPower.com

12

1/13/2020

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage. First, choose a value for R2. Choose a

reasonable R2, since a small R2 leads to a

considerable quiescent current loss, while a

large R2 makes FB noise-sensitive. R1 can be

determined with Equation (2):

OUT

REF

REF

VV

R1

R2

V





(2)

Figure 4 shows the feedback circuit.

MP9459

FB

R1

R2

VOUT

RT

Figure 4: Feedback Network

Table 1 lists the recommended parameters for

common output voltages.

Table 1: Parameters for Common Output

Voltages (9)

VOUT (V)

R1

(kΩ)

R2

(kΩ)

RT

(kΩ)

L

(μH)

5

40.2

7.68

75

3.3

3.3

40.2

13

75

2.2

2.5

40.2

19.1

115

2.2

1.8

40.2

32.4

115

1.5

1.5

40.2

45.3

160

1.5

1.2

40.2

82

210

1

1

20.5

84.5

316

1

Note:

9)

For a detailed design circuit, see the Typical Application

Circuits on pages 15, 16, and 17.

Selecting the Inductor

An inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger-value

inductance results in less ripple current and

lower output ripple voltage. However, a larger

inductance has a larger physical size, higher

series resistance, and lower saturation current.

A good rule to determine the inductance value

is to design the peak-to-peak ripple current in

the inductor to be between 30% and 60% of the

maximum output current, and to keep the peak

inductor current below the maximum switch

current limit. The inductance value can be

calculated with Equation (3):

OUT

OUT

SW

L

IN

VV

L

(1

)

f

I

V



 



(3)

Where

∆IL is the peak-to-peak inductor ripple

current.

The inductor should not saturate under the

maximum inductor peak current. The peak

inductor

current

can

be

calculated

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

AC current to the converter while maintaining

the DC input voltage. For the best performance,

use ceramic capacitors placed as close to VIN

as possible. Capacitors with X5R and X7R

ceramic dielectrics are recommended because

they

are

fairly

stable

amid

temperature

fluctuations.

The capacitors must also have a ripple current

rating that exceeds 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)

The worst-case condition occurs at VIN = 2 x

VOUT, estimated with Equation (6):

OUT

CIN

I

I

2



(6)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The input capacitance value determines the

input voltage ripple of the converter. If there is

an input voltage ripple requirement in the

system, choose an input capacitor that meets

the specification.