Datasheet

TPS40055-EP
www.ti.com SGLS310D –JULY 2005–REVISED FEBRUARY 2012
INPUT VOLTAGE INPUT VOLTAGE
vs vs
BP5 VOLTAGE BP10 VOLTAGE
Figure 8. Figure 9.
SELECTING THE INDUCTOR VALUE
The inductor value determines the magnitude of ripple current in the output capacitors as well as the load current
at which the converter enters discontinuous mode. Too large an inductance results in lower ripple current but is
physically larger for the same load current. Too small an inductance results in larger ripple currents and a greater
number of (or more expensive output capacitors for) the same output ripple voltage requirement. A good
compromise is to select the inductance value such that the converter does not enter discontinuous mode until the
load approximated somewhere between 10% and 30% of the rated output. The inductance value is described in
Equation 4.
(4)
where:
V
O
is the output voltage
ΔI is the peak-to-peak inductor current
CALCULATING THE OUTPUT CAPACITANCE
The output capacitance depends on the output ripple voltage requirement, output ripple current, as well as any
output voltage deviation requirement during a load transient.
The output ripple voltage is a function of both the output capacitance and capacitor ESR. The worst case output
ripple is described in Equation 5.
(5)
The output ripple voltage is typically between 90% and 95% due to the ESR component.
The output capacitance requirement typically increases in the presence of a load transient requirement. During a
step load, the output capacitance must provide energy to the load (light-to-heavy load step) or absorb excess
inductor energy (heavy-to-light load step) while maintaining the output voltage within acceptable limits. The
amount of capacitance depends on the magnitude of the load step, the speed of the loop and the size of the
inductor.
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