Datasheet

fL
I
V
O
V
O
V
L
I
´
-
´=D
1
2
m a xm a x
L
I
O
I
L
I
D
+=
TPS62110-HT
SLVSAO9B –DECEMBER 2010–REVISED FEBRUARY 2011 www.ti.com
Table 2. Advantages and Disadvantages When Designing the Inductor and Output Capacitor
INFLUENCE ON STABILITY ADVANTAGE DISADVANTAGE
Less output voltage ripple
Increase Cout (>22 µF) Uncritical None
Less output voltage overshoot /
undershoot during load transient
Higher output voltage ripple
Critical High output voltage overshoot /
Decrease Cout (<22 µF) Increase inductor value >6.8 µH None undershoot during load
also transient
Less gain and phase margin
Less inductor current ripple More energy stored in the
inductor → higher voltage
overshoot during load transient
Higher dc output current possible if Smaller current rise → higher
Increase L (>6.8 µH) Uncritical
operated close to the current limit voltage undershoot during load
transient → do not decrease the
value of Cout due to these
effects
Critical High inductor current ripple
Small voltage overshoot / undershoot
especially at high input voltage
Decrease L (<6.8 µH)
Increase output capacitor value >
during load transient
and low output voltage
22 µF also
As it is shown in Table 2, the inductor value can be increased to higher values. For good performance, the
peak-to-peak inductor current ripple should be less than 30% of the maximum dc output current. Especially at
input voltages above 12 V, it makes sense to increase the inductor value to keep the inductor current ripple low.
In such applications, the inductor value can be increased to 10 µH or 22 µH. Values above 22 µH should be
avoided to keep the voltage overshoot during load transient in an acceptable range.
After choosing the inductor value, two additional inductor parameters should be considered:
1. current rating of the inductor
2. dc resistance
The dc resistance of the inductance directly influences the efficiency of the converter. Therefore, an inductor with
lowest dc resistance should be selected for highest efficiency. To avoid saturation of the inductor, the inductor
should be rated at least for the maximum output current plus the inductor ripple current which is calculated as:
(3)
Where:
f = Switching frequency (1000 kHz typical)
L = Inductor value
ΔI
L
= Peak-to-peak inductor ripple current
I
L
(max) = Maximum inductor current
The highest inductor current occurs at maximum V
I
. A more conservative approach is to select the inductor
current rating just for the maximum switch current of the TPS62110, which is 2.4 A (typically). See Table 1 for
recommended inductors.
OUTPUT CAPACITOR SELECTION
A 22-μF (typical) output capacitor is needed with a 6.8 μH inductor. For an output voltage greater than 5 V, a
33-μF (minimum) output capacitor is required for stability. For best performance, a low ESR ceramic output
capacitor is needed.
The RMS ripple current is calculated as:
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