Datasheet

ILIM
OSmax
0.947
ILIM
OSmax
0.947
OSmax
R (k ) = 33.2k
99038V
I (mA) =
R k
99038V
I (mA) =
33.2 k
I (mA) = 3592mA
W W
W
W
0.947
ILIM
OSmax
99038V
OSmax
R k
1
0.947
ILIM
OSmax
ILIM
I (mA) = 5000mA
I (mA) =
99038V
R (k ) =
I mA
R (k ) = 23.4k
W
W
W W
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23.7
ILIM
OSmin
1.0708
ILIM
OSmin
1.0708
OSmin
R (k ) = 23.7k
127981V
I (mA) =
R k
127981V
I (mA) =
k
I (mA) = 4316mA
W W
W
W
TPS2556
TPS2557
SLVS931A – NOVEMBER 2009– REVISED FEBRUARY 2012 www.ti.com
(3)
The resulting maximum current-limit threshold is 3592 mA with a 33.2 kΩ resistor.
APPLICATION 2: DESIGNING BELOW A MAXIMUM CURRENT LIMIT
Some applications require that current limiting must occur below a certain threshold. For this example, assume
that the desired upper current-limit threshold must be below 5000 mA to protect an up-stream power supply. Use
the I
OS
equations and Figure 19 to select R
ILIM
.
(4)
Select the closest 1% resistor greater than the calculated value: R
ILIM
= 23.7kΩ. This sets the maximum
current-limit threshold at 5000 mA . Use the I
OS
equations, Figure 19, and the previously calculated value for
R
ILIM
to calculate the minimum resulting current-limit threshold.
(5)
The resulting minimum current-limit threshold is 4316 mA with a 23.7 kΩ resistor.
ACCOUNTING FOR RESISTOR TOLERANCE
The previous sections described the selection of R
ILIM
given certain application requirements and the importance
of understanding the current-limit threshold tolerance. The analysis focused only on the TPS2556/57
performance and assumed an exact resistor value. However, resistors sold in quantity are not exact and are
bounded by an upper and lower tolerance centered around a nominal resistance. The additional R
ILIM
resistance
tolerance directly affects the current-limit threshold accuracy at a system level. The following table shows a
process that accounts for worst-case resistor tolerance assuming 1% resistor values. Step one follows the
selection process outlined in the application examples above. Step two determines the upper and lower
resistance bounds of the selected resistor. Step three uses the upper and lower resistor bounds in the I
OS
equations to calculate the threshold limits. It is important to use tighter tolerance resistors, e.g. 0.5% or 0.1%,
when precision current limiting is desired.
Table 1. Common R
ILIM
Resistor Selections
Resistor Tolerance Actual Limits
Ideal Closest 1%
Desired Nominal
Resistor Resistor
IOS MIN IOS Nom IOS MAX
Current Limit (mA)
1% low (kΩ) 1% high (kΩ)
(kΩ) (kΩ)
(mA) (mA) (mA)
750 146.9 147 145.5 148.5 605 749 886
1000 110.2 110 108.9 111.1 825 1002 1166
1250 88.2 88.7 87.8 89.6 1039 1244 1430
1500 73.6 73.2 72.5 73.9 1276 1508 1715
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