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ML4894

Synchronous Buck Controller

Micro Linear Corporation 

ML4894

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

the worst case condition occurring at VIN = 2 x VOUT:

IRM S(CIN )

≈ (ISENSE(M AX)

+

1

2

∆IL(M AX)

)

×

b g 5V × VIN − 5V

VIN

The capacitor’s voltage rating is based on the maximum

input voltage, VIN(MAX). Capacitor manufacturers typically

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 0.1µF ceramic capacitor to bypass VIN (pin 8) of the

ML4894.

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 OUT

≥

4.3

VOUT

× TON(MAX)

R SENSE

(14)

The maximum ESR value can be estimated using:

ESR ≤ ∆VOUT

∆IL( MA X)

(15)

The selected capacitor must meet both the capacitance

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

output capacitor can handle the ripple current, IRMS:

IRMS

≈

∆IL( MA X)

12

(16)

8

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