7

ML4875

1.0

3.0

4.0

2.0

100

90

80

70

60

50

0

ML4875-5.0

5.0

L = 56

µH

L = 27

µH

L = 15

µH

1.0

3.0

2.0

100

90

80

70

60

50

0

ML4875-3.3

L = 56

µH

L = 27

µH

L = 15

µH

OUTPUT CAPACITOR

The choice of output capacitor is also important, as it

controls the output ripple and optimizes the efficiency of

the circuit. Output ripple is influenced by three capacitor

parameters: capacitance, ESR, and ESL. The contribution

due to capacitance can be determined by looking at the

change in capacitor voltage required to store the energy

delivered by the inductor in a single charge-discharge

cycle, as determined by the following formula:

∆V

TV

LC

V

V

OUT

ON

IN

OUT

IN

=

×

×× ×

−

22

2(

)

(4)

For a 2.4V input, and 5V output, a 27µH inductor, and a

47µF capacitor, the expected output ripple due to

capacitor value is 87mV.

Capacitor Equivalent Series Resistance (ESR) and

Equivalent Series Inductance (ESL), also contribute to the

output ripple due to the inductor discharge current

waveform. Just after the NMOS transistor turns off, the

output current ramps quickly to match the peak inductor

current. This fast change in current through the output

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

can be over 1V in magnitude. After the ESL spike settles,

the output voltage still has a ripple component equal to

the inductor discharge current times the ESR. This

component will have a sawtooth shape and a peak value

equal to the peak inductor current times the ESR. ESR also

has a negative effect on efficiency by contributing

I-squared R losses during the discharge cycle.

An output capacitor with a capacitance of 100µF, an ESR

of less than 0.1ý, and an ESL of less than 5nH is a good

general purpose choice. Tantalum capacitors which meet

these requirements can be obtained from the following

suppliers:

Matsuo

(714) 969-2491

Sprague

(603) 224-1961

If ESL spikes are causing output noise problems, an EMI

filter can be added in series with the output.

1.0

3.0

2.0

100

90

80

70

60

50

0

ML4875-3.0

L = 56

µH

L = 27

µH

L = 15

µH