6

ML4872

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