Datasheet

L =
36 - 3.3
200kHz x 1.2A
x
3.3
36
= 12.5PH
1.2A =
36 - 3.3
L x 200kHz
x
3.3
36
I
rip
=
36 - 3.3
200kHz x 5x10
-6
3.3
36
x
= 3A
L
min
=
36 - 3.3
200kHz x 36
3.3 x 0.02
.060
x
= 5PH
LM5642, LM5642X
www.ti.com SNVS219K –JUNE 2003–REVISED APRIL 2013
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:
(15)
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 frequency is very close to Re. In the case of
ceramic capacitors, replace Re with the true impedance at the switching frequency.
Example: Vin = 36V, Vo = 3.3V, V
RIP
= 60 mV, Re = 20 mΩ, F = 200 kHz.
(16)
The actual selection process usually involves several iterations 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 inductance 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 selecting an output inductor, it is a good idea to double-
check this value. The equation is:
(17)
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 power loss in the inductor.
Example: Vin = 36V, Vo = 3.3V, F = 200 kHz, L = 5 µH, 3A max I
OUT
(18)
3A is 100% ripple which is too high.
In this case, the inductor should be reselected on the basis of ripple current.
Example: 40% ripple, 40% • 3A = 1.2A
(19)
(20)
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 LM5642 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 system. Input capacitors must be selected that can handle both the maximum ripple RMS current at
highest ambient temperature as well as the maximum input voltage. In applications 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 channels' input current pulses.
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