TC110

DS21355B-page 6

© 2002 Microchip Technology Inc.

3.5

Output Capacitor

The effective series resistance of the output capacitor

directly affects the amplitude of the output voltage

ripple. (The product of the peak inductor current and

the ESR determines output ripple amplitude.) There-

fore, a capacitor with the lowest possible ESR should

be selected. Smaller capacitors are acceptable for light

loads or in applications where ripple is not a concern.

The Sprague 595D series of tantalum capacitors are

among the smallest of all low ESR surface mount

capacitors

available.

Table 4-1

lists

suggested

components and suppliers.

3.6

Inductor Selection

Selecting the proper inductor value is a trade-off

between physical size and power conversion require-

ments. Lower value inductors cost less, but result in

higher ripple current and core losses. They are also

more prone to saturate since the coil current ramps

faster and could overshoot the desired peak value. This

not only reduces efficiency, but could also cause the

current rating of the external components to be

exceeded. Larger inductor values reduce both ripple

current and core losses, but are larger in physical size

and tend to increase the start-up time slightly.

A22

µH inductor is recommended for the 300kHz

versions and a 47

µH inductor is recommended for the

100kHz versions. Inductors with a ferrite core (or

equivalent) are

also

recommended. For highest

efficiency, use inductors with a low DC resistance (less

than 20 m

Ω).

The inductor value directly affects the output ripple

voltage. Equation 3-3 is derived as shown below, and

can be used to calculate an inductor value, given the

required output ripple voltage and output capacitor

series resistance:

EQUATION 3-1:

where ESR is the equivalent series resistance of the

Expressing di in terms of switch ON resistance and

time:

EQUATION 3-2:

Solving for L:

EQUATION 3-3:

Care must be taken to ensure the inductor can handle

peak switching currents, which can be several times

load currents. Exceeding rated peak current will result

in core saturation and loss of inductance. The inductor

should be selected to withstand currents greater than

Calculating the peak inductor current is straightforward.

Inductor current consists of an AC (sawtooth) current

centered on an average DC current (i.e., input current).

Equation 3-6 calculates the average DC current. Note

that minimum input voltage and maximum load current

values should be used:

EQUATION 3-4:

Re-writing in terms of input and output currents and

voltages:

EQUATION 3-5:

Solving for input curent:

EQUATION 3-6:

The sawtooth current is centered on the DC current

level; swinging equally above and below the DC current

calculated in Equation 3-6. The peak inductor current is

the sum of the DC current plus half the AC current.

Note that minimum input voltage should be used when

calculating the AC inductor current (Equation 3-9).

EQUATION 3-7:

EQUATION 3-8:

EQUATION 3-9:

Combining the DC current calculated in Equation 3-6,

with half the peak AC current calculated in Equation 3-

9, the peak inductor current is given by:

EQUATION 3-10:

L

≈

L

=

Output Power

Efficiency

Input Power

Efficiency

=

L(di)

dt

V

=

V(dt)

dt

di

L

di =