6

ML4871

OUTPUT CAPACITOR

The output capacitor filters the pulses of current from the

switching regulator. Since the switching frequency will

vary with inductance, the minimum output capacitance

required to reduce the output ripple to an acceptable level

will be a function of the inductor used. Therefore, to

maintain an output voltage with less than 100mV of ripple

at full load current, use the following equation:

C

L

V

OUT

OUT

=

™

44

(2)

The output capacitor’s Equivalent Series Resistance (ESR)

and Equivalent Series Inductance (ESL), also contribute to

the ripple. Just after the NMOS transistor, Q1, turns off, the

current in the output capacitor ramps quickly to between

0.5A and 1.5A. This fast change in current through the

capacitor’s ESL causes a high frequency (5ns) spike to

appear on the output. After the ESL spike settles, the output

still has a ripple component equal to the inductor

discharge current times the ESR. To minimize these effects,

choose an output capacitor with less than 10nH of ESL

and 100m

W of ESR.

Suitable tantalum capacitors can be obtained from the

following vendors:

AVX

(207) 282-5111

Kemet

(846) 963-6300

Sprague

(207) 324-4140

1000

800

600

400

200

0

VIN (V)

1.0

2.0

3.0

5.0

4.0

VOUT = 3.3V

VOUT = 5V

90

80

70

60

IOUT (mA)

1

10

100

1000

VOUT = 3.3V

VOUT = 5V

VIN = 2.4V

90

60

30

0

VIN (V)

1.0

2.0

3.0

5.0

4.0

VOUT = 3.3V

VOUT = 5V