ML4895

8

DESIGN CONSIDERATIONS (cont.)

INPUT CAPACITOR SELECTION

The choice of the input capacitor is based on its ripple

current and voltage ratings rather than its capacitance

value. The input capacitor should be a low ESR type and

located as close to the source of the P-MOS switch as

possible. The input capacitor’s ripple current is

determined by the load current and input voltage, with

II

I

VV

V

V

RMS C

SENSE MAX

L MAX

OUT

IN

OUT

IN

IN

(

)

()

()

()

≈+

×

×−

1

2

∆

bg

The capacitor’s voltage rating is based on the maximum

recommend derating the capacitor voltage rating by 20%

to 50% for aluminum electrolytic types and 50% to 70%

for tantalum types.

In high current applications it may necessary to add a

small 0.1

ML4895.

OUTPUT CAPACITOR SELECTION

The output capacitors determine the loop stability and the

output ripple voltage. Use only low ESR capacitors

intended for switching power supply applications, such as

AVX TPS, Sprague 593D, Sanyo OS-CON, or Nichicon PL

series. To ensure stability, the minimum capacitance value

is given by:

C

V

T

R

OUT

OUT

ON MAX

SENSE

≥×

43

.

()

(14)

The maximum ESR value can be estimated using:

ESR

V

I

OUT

LMAX

≤

∆

∆

()

(15)

The selected capacitor must meet both the capacitance

and ESR requirements. As a final check, make sure the

I

I

RMS

LMAX

≈

∆

()

12

(16)

OUTPUT VOLTAGE

The output of the buck converter is adjustable and can be

set to any voltage between 2.5V and 4V by connecting a

resistor divider to the feedback pin as shown in Figure 1.

The resistor values R1 and R2 can be calculated using the

following equation:

VV

RR2

R2

+

250

1

.

(17)

The value of R2 should be 475k

Ω or less to minimize bias

current errors.

It is important to note that the accuracy of these resistors

directly affects the accuracy of the output. Use precision

resistors and set the nominal voltage approximately 1% to

2% high in order to make up for the load regulation. This

offset results in the best overall output accuracy over line

and load.