Datasheet

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Capacitor Selection
1
2
 L
O


I
O

2

1
2
C
O

V
OS
2
 V
O
2

(6)
C
MIN

I
O
 t
 V
OUT
 6.1  F
(7)
TPS40222
SLUS642A – OCTOBER 2005 – REVISED JANUARY 2006
One constraint on the capacitance is the overshoot allowed by a sudden load change. The worst case for a
transient load release occurs at the time when the inductor has just finished a t
ON
pulse. At this point, the
inductor is operating at maximum current. When the output load is suddenly removed, all of the inductor current
must be absorbed by the output capacitance. With a typical output voltage overshoot requirement of 2% at 3.3-V,
the minimum capacitance required to remain in specification is calculated using Equation 6 .
where
• V
OS
is the maximum overshoot voltage
• L
O
= 2.2 µ H
• I
O
= 1.5 A
• V
O
= 3.3 V
Solving this relationship, the minimum required output capacitance C
O
is 11- µ F.
The other load transition extreme is from no load to full load that occurs just after a minimum on-time cycle has
started. At this point, the controller has to support this load for the remainder of the cycle with a minimum of
current available from the inductor. In this example, the minimum on-time with a 3.3-V output is 528 ns and the
off-time is 800 ns minus 528 ns = 272 ns. Using the relationship shown in Equation 7 ;
where
• ∆ V
OUT
is a 2% specified output voltage droop
• I
O
= 1.5 A
• ∆ t = 272 ns
Ceramic capacitors with a low ESR are used to achieve the lowest voltage ripple. For example, current 1206,
6.3-V capacitors that provide 22 µ F and an ESR of 2 m Ω are available.
13