LM26 datasheet(17/20 Pages) NSC

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Inductor Selection

The size of the output inductor can be determined from the

desired output ripple voltage, Vrip, and the impedance of the

output capacitors at the switching frequency. The equation to

determine the minimum inductance value is as follows:

In the above equation, Re is used in place of the impedance

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

impedance of the output capacitors at the switching fre-

quency is very close to Re. In the case of ceramic capaci-

tors, replace Re with the true impedance.

Example: Vin (max)= 30V, Vnom = 5.0V, Vrip = 40mV, Re

=20m

Ω, f = 300kHz

Lmin = 7µH

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 Lmin is selected, make sure that the

Cmin requirement is not violated.

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:

Where D is the duty cycle, defined by V

nom/Vin.

Also important is the ripple content, which is defined by Irip

/Inom. Generally speaking, a ripple content of less than 50%

is ok. Larger ripple content will cause too much loss in the

inductor.

Example: Vin = 12V, Vnom = 5.0V, f = 300kHz, L = 8µH

Given a maximum load current of 3A, the ripple content is

1.2A / 3A = 40%.

When choosing the 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 fact that the two switching channels of the LM2642 are

180˚ out of phase will reduce the RMS value of the ripple

current seen by the input capacitors. This will help extend

input capacitor life span and result in a more efficient sys-

tem. Input capacitors must be selected that can handle both

the maximum ripple RMS current at highest ambient tem-

perature as well as the maximum input voltage. In applica-

tions in which output voltages are less than half of the input

voltage, the corresponding duty cycles will be less than 50%.

This means there will be no overlap between the two chan-

nels’ input current pulses. The equation for calculating the

maximum total input ripple RMS current for duty cycles

under 50% is:

where I1 is maximum load current of Channel 1, I2 is the

maximum load current of Channel 2, D1 is the duty cycle of

Channel 1, and D2 is the duty cycle of Channel 2.

Example: Imax_1 = 3.6A, Imax_2 = 3.6A, D1 = 0.42, and D2

= 0.275

Choose input capacitors that can handle 1.66A ripple RMS

current at highest ambient temperature. In applications

where output voltages are greater than half the input voltage,

the corresponding duty cycles will be greater than 50%, and

there will be overlapping input current pulses. Input ripple

current will be highest under these circumstances. The input

RMS current in this case is given by:

Where, again, I1 and I2 are the maximum load currents of

channel 1 and 2, and D1 and D2 are the duty cycles. This

equation should be used when both duty cycles are ex-

pected to be higher than 50%.

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 require-

ments. Bench testing for individual applications is still the

best way to determine a reliable input capacitor value. The

input capacitor 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 on and

off at almost zero voltage. Therefore, only conduction losses

are present in the bottom FET. The most important param-

eter when selecting the bottom FET is the on resistance

(Rdson). The lower the on resistance, the lower the power

loss. The bottom FET power loss peaks at maximum input

voltage and load current. The equation for the maximum

allowed on resistance at room temperature for a given FET

package, is:

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