Datasheet

( )
( )
OUT OUT
RIPPLE
OUT max
V 0.015 V
ESR 60 m
I 0.75 I
´
= » ´ W
´
( )
( )
( )
(
)
( )
f
OUT OUT
IN max
TRIP
IND peak
DS on IN max
V V V
V
1
I
R 2 L V
- ´
= + ´
´ ´
( )
( )
(
)
( )
( )
( )
(
)
( )
f
OUT OUT OUT OUT
IN max IN max
IND
IND ripple f IN max OUT max IN max
V V V V V V
1 3
L
I V I V
´
- ´ - ´
= ´ = ´
´
( )
( )
( )
( )
( )
IN min
ON(max)
T on
12
OUT
V
T 50ns
3
R
2
V 150mV
19 10
-
-
= ´ ´ W
+
´
( )
( )
f
OUT
ON max
IN min
V
1
T
V
= ´
OUT
V 0.75
R1 R2
0.75
-
= ´
Not Recommended for New Designs
TPS51315
www.ti.com SLUS881B –MAY 2009–REVISED AUGUST 2012
External Component Selection
External component selection is a simple process in D-CAP™ Mode.
1. Determine the value of R1 and R2.
The recommended R2 value is 10 kΩ to 100 kΩ. Calculate R1 by using Equation 8.
(8)
2. Choose R
T(on)
The switching frequency is usually determined by overall view of the dc-dc converter designer in
consideration of size, efficiency or cost, and mostly dictated by external component constraints such as the
size of inductor and/or the output capacitor. If an extremely low or high duty factor is expected, the minimum
on-time or off-time must also be considered to satisfy the required duty factor. Once the switching frequency
is decided, R
T(on)
can be determined by Equation 9 and Equation 10.
(9)
(10)
3. Choose the inductor.
To begin, choose an inductance value where the ripple current is approximately 1/4 to 1/2 of the maximum
output current.
(11)
For applications that require a fast transient response with a minimum of V
OUT
overshoot, consider a smaller
inductance value than calculated in Equation 11. The cost of a small inductance value is higher steady-state
ripple, larger line regulation, and higher switching loss.
The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above the
peak inductor current before saturation. The peak inductor current can be estimated by Equation 12.
(12)
4. Choose output capacitor(s).
Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine an ESR
to meet the required ripple voltage. A quick approximation is shown in Equation 13.
(13)
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