ML4869

6

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

(3)

The output capacitor’s Equivalent Series Resistance (ESR)

and Equivalent Series Inductance (ESL), also contribute to

the ripple. Just after the Q1 turns off, the current in the

output capacitor ramps quickly to between 0.3A and

0.9A. 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 product of inductor

discharge current and the ESR. To minimize these effects,

choose an output capacitor with less than 10nH of ESL

and less than 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

VIN (V)

600

500

400

300

200

100

0.0

1.50

2.50

5.00

4.00

2.00

3.00

4.50

3.50

VOUT = 5V

VOUT = 3.3V

80

70

60

50

40

30

20

VIN (V)

35

4

12

VOUT = 5V

VOUT = 3.3V

95.0

90.0

85.0

80.0

75.0

IOUT (mA)

0.1

1

10

1000

100

VOUT = 5V

VOUT = 3.3V

250

200

150

100

50

0

VIN (V)

01

2

3

4

5

8

6

7

VOUT = 5V

VOUT = 3.3V