NB621 – 8A, 25V, 600kHz SYNCHRONOUS STEP-DOWN CONVERTER WITH INTEGRATED LDO

NB621 Rev. 0.9

www.MonolithicPower.com

12

12/10/2013

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the schematic on front page).

The feedback resistor R1 also sets the feedback

loop bandwidth with the internal compensation

capacitor (see Figure 1). Choose R1 to be

around 31.6kΩ for optimal transient response. R2

is then given by:

1

V

8

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

31.6 (1%)

25.5 (1%)

2.5

31.6 (1%)

14.7(1%)

3.3

31.6 (1%)

10(1%)

5

31.6 (1%)

6.04 (1%)

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

of at least 25% percent higher than the maximum

load

current

is

recommended

for

most

applications. For highest efficiency, the inductor

DC resistance should be less than 15mΩ. For

most designs, the inductance value can be

derived from the following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose inductor current to be approximately

30% if the maximum load current, 8A. The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, larger

inductance

is

recommended

for

improved

efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low ESR

capacitors for the best performance. Ceramic

capacitors with X5R or X7R dielectrics are highly

recommended because of their low ESR and

small

temperature

coefficients.

For

most

applications, a 22µF capacitor is sufficient.

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 by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors,

a

small,

high

quality

ceramic

capacitor, i.e. 0.1μF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V