Output Capacitor Selection

(Continued)

input voltage is the highest and when the present switching

cycle has just finished. The corresponding minimum capaci-

tance is calculated as follows:

(8)

Notice it is already assumed the total ESR, R

greater than R

root will be a negative value. Also, it is assumed that L has

already been selected, therefore the minimum L value

should be calculated before C

tor Selection below). Example: R

Ω,V

∆V

∆I

(9)

Generally speaking, C

∆I

∆V

output capacitance can therefore be chosen to be slightly

larger than the calculated value so that it is more easily

available. Here we would likely be fine choosing 220 µF.

Inductor Selection

The size of the output inductor can be determined from the

desired output ripple voltage,

∆V

the output capacitors at the switching frequency. The equa-

tion to determine the minimum inductance value is as fol-

lows:

(10)

In the above equation, R

ance of the output capacitors. This is because in most cases,

the impedance of the output capacitors at the switching

frequency is very close to R

capacitors, replace R

Example: V

∆V

=20m

Ω,f

(11)

The actual selection process usually involves several itera-

tions of all of the above steps, from ripple voltage selection,

to capacitor selection, to inductance calculations. Both the

highest and the lowest input and output voltages and load

transient requirements should be considered. If an induc-

tance value larger than L

C

Priority should be given to parameters that are not flexible or

more costly. For example, if there are very few types of

capacitors to choose from, it may be a good idea to adjust

the inductance value so that a requirement of 3.2 capacitors

can be reduced to 3 capacitors.

Since inductor ripple current is often the criterion for select-

ing an output inductor, it is a good idea to double-check this

value. The equation is:

(12)

Where D is the duty cycle, defined by V

Also important is the ripple current, which is defined by

∆I

L

/I

speaking, a ripple content of less than 50% is ok. Larger

ripple content causes excessive losses in the inductor.

Example: V

(13)

Given a maximum load current of 5A, the ripple content is

1.2A / 5A = 24%. When choosing an inductor, the saturation

current should be higher than the maximum peak inductor

current and the RMS current rating should be higher than the

maximum load current.

Input Capacitor Selection

The input capacitor must be selected such that it can handle

both the maximum ripple RMS current at highest ambient

temperature and the maximum input voltage. The equation

for the RMS current through the input capacitor is then

(14)

Where I

Example: I

(15)

The function D(1-D) has a maxima at D = 0.5. This duty cycle

corresponds to the maximum RMS input current that may be

used as a worst case in selecting an input capacitor. Input

capacitors must meet the minimum requirements of voltage

and ripple current capacity. The size of the capacitor should

then be selected based on hold up time requirements. Bench

testing for individual applications is still the best way to

determine a reliable input capacitor value. The input capaci-

tor should always be placed as close as possible to the

current sense resistor or the drain of the top FET.

MOSFET Selection

BOTTOM FET SELECTION

During normal operation, the bottom FET is switching at

almost zero voltage and therefore only conduction losses

are present in the bottom FET. This makes the on resistance

(R

bottom FET; the lower the on resistance, the lower the power

loss. The bottom FETs’ power losses peak at the maximum

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