MPQ4480 – 36V, 6A, SYNC STEP-DOWN CONVERTER FOR AEC1-Q100 QUALIFIED

MPQ4480 Rev. 1.0

www.MonolithicPower.com

18

7/2/2019

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 2).

The feedback resistor (R1) also sets the

feedback loop bandwidth with the internal

compensation capacitor. R2 is determined with

Equation (4):

(4)

Figure 7 shows the external resistor divider.

Figure 7: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

VOUT (V) R1 (kΩ)R2 (kΩ)

3.3

40.2

17.4

5

40.2

10

9

40.2

4.99

Selecting the Inductor

For most applications, use an inductor with a

DC current rating at least 25% higher than the

maximum load current. Select an inductor with

a small DC resistance for optimum efficiency.

The inductor value for most designs can be

calculated with Equation (5):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

(5)

Where ∆IL is the inductor ripple current.

Choose the inductor ripple current to be

approximately 30% of the maximum load

current. The maximum inductor peak current

can be determined with Equation (6):

2

I

I

I

L

LOAD

)

MAX

(

L







(6)

Selecting the Buck Input Capacitor

The step-down converter has a discontinuous

input current, and requires a capacitor to supply

the AC current while maintaining the DC input

voltage. Use low-ESR capacitors for optimal

performance. Ceramic capacitors with X5R or

X7R dielectrics are highly recommended

because

of

their

low

ESR

and

small

temperature coefficients. For CLA application, a

100µF electrolytic capacitor and two 10µF

ceramic capacitors are recommended.

Since the input capacitor (C1) absorbs the input

switching current, it requires an adequate

ripple-current rating. The RMS current in the

input capacitor can be estimated with Equation

(7):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(7)

The worst-case condition occurs at VIN = 2VOUT,

which can be calculated with Equation (8):

2

I

I

LOAD

1

C 

(8)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum,

or ceramic. When using an electrolytic capacitor,

place two additional high-quality ceramic

capacitors as close to IN as possible. Estimate

the input voltage ripple caused by the

capacitance with Equation (9):







 







LOAD

OUT

OUT

IN

IN

SW

IN

IV

V

V1

fC1

V

V

(9)

Selecting the Buck Output Capacitor

The MPQ4480 requires an output capacitor (C2)

to maintain the DC output voltage. Estimate the

output voltage ripple with Equation (10):







 









 

 

OUT

OUT

OUT

ESR

SW

1

IN

S

VV

1

V1

R

fL

V

8 f

C2

(10)

Where L1 is the inductor value and RESR is the

equivalent series resistance (ESR) value of the

output capacitor.