Datasheet

OUT _ TOL T _ TOL ON
CT
(1 C C ) t
(1 0.2 0.1) 0.012
C 0.48 F
40000 40000
+ + ´
+ + ´
= = = m
RSET,max
185.58
R 0.99 82.39k
2.1 0.13
= ´ = W
+
80.6k
C
OUT
0
.
5
6
u
F
TPS2421-1
TPS2421-2
www.ti.com SLUS907G –JANUARY 2009–REVISED MAY 2013
Figure 22. Design Example Schematic
1. Calculate maximum R
RSET
to ensure that minimum I
SET
is above maximum operating load current using
Equation 15 as shown below in Equation 18.
(18)
● Choose a standard 1% value below R
RSET,max
for R
RSET
= 80.6kΩ
● I
SET,min
= 2.15A using Equation 12 and will meet the maximum operating current requirement of 2.1A
without starting the fault timer during maximum steady state operation for R
RSET
= 80.6kΩ, 1%.
● I
SET,max
= 4.359A using Equation 13 for R
RSET
= 80.6Ω, 1%.
2. Calculate minimum and maximum I
LIM
.
● I
LIM,min
= 2.792A and I
LIM,max
= 4.359A using Equation 16 and Equation 17 for R
RSET
= 80.6kΩ, 1%.
3. Minimum R
LOAD
at start up using Equation 10 is 12Ω. Since R
LOAD
= 15Ω is present during circuit start up,
use t
ON
= 15ms from Table 3 for C
OUT
= 220µF and R
LOAD
= 15Ω.
● Calculate C
CT
= 0.48µF including C
OUT
and C
CT
tolerances (C
OUT_TOL
= 20% and C
CT_TOL
= 10%) using
Equation 19.
(19)
Transient Protection
The need for transient protection in conjunction with hot-swap controllers should always be considered. When
the TPS2421 interrupts current flow, input inductance generates a positive voltage spike on the input and output
inductance generates a negative voltage spike on the output. Such transients can easily exceed twice the supply
voltage if steps are not taken to address the issue. Typical methods for addressing transients include;
• Minimizing lead length/inductance into and out of the device
• Voltage Suppressors (TVS) on the input to absorb inductive spikes
• Shottky diode across the output to absorb negative spikes
• A combination of ceramic and electrolytic capacitors on the input and output to absorb energy
• Use PCB GND planes
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