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
SLUS907G –JANUARY 2009–REVISED MAY 2013
www.ti.com
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 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
49.9kΩ ≤ R
RSET
≤ 200kΩ to ensure the rated accuracy. Many applications require that minimum fault and current
limits are known or that maximum current limit is bounded. It is important to consider the tolerance of the fault
and current limit thresholds, as well as R
RSET
when selecting values. Consult the Electrical Characteristics table
for specific fault and current limit settings.
Using the data for I
SET
and I
LIM
from the Electrical Characteristics table, equations can be 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
: 12V
• Maximum expected load current, I
VOUT
: 2.1A
• Load capacitance, C
OUT
: 220uF
• Expected resistive load, R
LOAD
during start up: 15Ω
• Example calculations are shown in the TPS2421 Design Calculator Tool (SLUC427).
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