MP8756 – 26V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8756 Rev. 1.1

www.MonolithicPower.com

16

4/13/2017

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© 2017 MPS. All Rights Reserved.

Calculate R2 first, and then calculate R1 with

Equation (6):

1

FB

2OUT

FB

4

9

1

R=

V1

-

R(V

-V ) R +R

(6)

Usually, R9 is set to 499Ω. It should be five times

smaller than R1//R2 to minimize its influence on

immunity with Equation (7):

9

4SW

1

R

2C

2F







(7)

Table 1 lists the recommended resistor values for

common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

(V)

R1

(kΩ)

R2

(kΩ)

L

(μH)

R4

(kΩ)

C5

(pF)

1

48.7

66.5

0.68

499

220

1.2

52.3

47

0.95

499

220

1.5

82.5

47

0.95

499

220

1.8

115

47

0.95

499

220

2.5

43.2

12.7

1.2

499

330

3.3

43

9.63

1.5

NS

NS

5

41.2

5.6

1.5

NS

NS

Selecting the Inductor

The inductor is necessary for supplying a

constant current to the output load while being

driven by the switched input voltage. An inductor

with a larger value results in less ripple current

and a lower output ripple voltage. However, it

also has a larger physical footprint, higher series

resistance, and lower saturation current. A good

rule for determining the inductance value is to

design the peak-to-peak ripple current in the

inductor to be in the range of 30% to 40% of the

maximum output current to ensure that the peak

inductor current is below the maximum switch

current limit. The inductance value can be

calculated with Equation (8):

OUT

OUT

SW

L

IN

VV

L(1

)

FI

V

 



(8)

current.

The inductor should not saturate under the

maximum inductor peak current. The peak

inductor current can be calculated with Equation

(9):

OUT

OUT

LP

OUT

SW

IN

VV

II

(1

)

2F

L

V



 



(9)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous and therefore requires a capacitor

to supply AC current to the step-down converter

while maintaining the DC input voltage. Use

ceramic capacitors placed as close to VIN as

possible for best performance. Capacitors with

X5R

and

X7R

ceramic

dielectrics

are

recommended because they are fairly stable with

temperature fluctuations.

The capacitors must also have a ripple current

rating greater than the maximum input ripple

current of the converter. The input ripple current

can be estimated with Equation (10):

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV



 

(10)

shown in Equation (11):

OUT

CIN

I

I

2



(11)

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.

The input voltage ripple can be estimated with

Equation (12):

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V





 



(12)

use Equation (13):

OUT

IN

SW

IN

I

1

V

4F

C

 



(13)