Datasheet
LIM,max
RSET,min
259.26
I 1.11
R
= +
LIM,min
RSET,max
232.19
I 0.06
R
= -
( )
RSET,max TOL
SET,min
185.58
R 1 R
I 0.13
= - ´
+
( )
RSET,min TOL
SET,max
213.68
R 1 R
I 0.13
= + ´
-
SET,max
RSET,min
213.68
I 0.13
R
= +
SET,min
RSET,max
185.58
I 0.13
R
= -
TPS2421-1
TPS2421-2
SLUS907H –JANUARY 2009–REVISED JANUARY 2014
www.ti.com
Overcurrent Shutdown
Overcurrent shutdown occurs when the output current exceeds I
SET
for the duration of the fault timer. Figure 8
shows a step rise in output current which exceeds the I
SET
threshold but not the I
LIM
threshold. The increased
current is on for the duration of the timer. When the timer expires, the output is turned off.
Programming the Fault (I
SET
) and Current-limit (I
LIM
) Thresholds
The I
SET
and I
LIM
thresholds is user programmable with a single external resistor connected to ISET and the I
LIM
threshold is internally set according to the I
LIM
/I
SET
ratio specified in the electrical characteristics table. The
TPS2421 device uses an internal regulation loop to provide a regulated voltage on the ISET pin. The fault and
current-limit thresholds are proportional to the current sourced out of ISET. The recommended 1% resistor range
is 49.9 kΩ ≤ R
RSET
≤ 200 kΩ to ensure the rated accuracy. Many applications require that minimum fault and
current limits are known or that maximum current limit is bounded. Considering the tolerance of the fault and
current limit thresholds, as well as R
RSET
when selecting values is important. See the ELECTRICAL
CHARACTERISTICS table for specific fault and current limit settings.
Using the data for I
SET
and I
LIM
from the ELECTRICAL CHARACTERISTICS, equations are generated and used
for other set points. Equation 12 and Equation 13 are used to calculate minimum and maximum I
SET
where
R
RSET,max
and R
RSET,min
include R
RSET
tolerances. Equation 14 and Equation 15 calculate R
RSET,max
and R
RSET,min
where R
TOL
is the 1% resistor tolerance.
(12)
(13)
(14)
(15)
Equation 16 and Equation 17 are used to calculate minimum and maximum I
LIM
where R
RSET,max
and R
RSET,min
include R
RSET
tolerances.
(16)
(17)
Design Example
A typical design is shown in Figure 22 with the following requirements:
• Nominal input voltage, V
VIN
: 12 V
• Maximum expected load current, I
VOUT
: 2.1 A
• Load capacitance, C
LOAD
: 220 µF
• Expected resistive load, R
LOAD
during start up: 15 Ω
• Example calculations are shown in the TPS2421 Design Calculator Tool (SLUC427).
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