Datasheet

0.947
ILIM
1.0028
ILIM
1.0708
ILIM
99038V
OSmax
R k
111704V
OSnom
R k
127981V
OSmin
R k
I (mA) =
I (mA) =
I (mA) =
W
W
W
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
20 30 40 50 60 70 80 90 100 110 120 130 140 150
R – CurrentLimitResistor – k
ILIM
Ω
Current-LimitThreshold – mA
I
OS(min)
I
OS(typ)
I
OS(max)
1.0708
ILIM
OSmin
127981V
OSmin
R k
1
1.0708
ILIM
OSmin
ILIM
I (mA) = 3000mA
I (mA) =
127981V
R (k ) =
I mA
R (k ) = 33.3k
W
W
W W
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TPS2556
TPS2557
www.ti.com SLVS931A – NOVEMBER 2009– REVISED FEBRUARY 2012
PROGRAMMING THE CURRENT-LIMIT THRESHOLD
The overcurrent threshold is user programmable via an external resistor. The TPS2556/57 uses an internal
regulation loop to provide a regulated voltage on the ILIM pin. The current-limit threshold is proportional to the
current sourced out of ILIM. The recommended 1% resistor range for R
ILIM
is 20 kΩ ≤ R
ILIM
≤ 187 kΩ to ensure
stability of the internal regulation loop. Many applications require that the minimum current limit is above a certain
current level or that the maximum current limit is below a certain current level, so it is important to consider the
tolerance of the overcurrent threshold when selecting a value for R
ILIM
. The following equations approximate the
resulting overcurrent threshold for a given external resistor value R
ILIM
). Consult the Electrical Characteristics
table for specific current limit settings. The traces routing the R
ILIM
resistor to the TPS2556/57 should be as short
as possible to reduce parasitic effects on the current-limit accuracy.
(1)
Figure 19. Current-Limit Threshold vs. R
ILIM
APPLICATION 1: DESIGNING ABOVE A MINIMUM CURRENT LIMIT
Some applications require that current limiting cannot occur below a certain threshold. For this example, assume
that 3 A must be delivered to the load so that the minimum desired current-limit threshold is 3000 mA. Use the
I
OS
equations and Figure 19 to select R
ILIM
.
(2)
Select the closest 1% resistor less than the calculated value: R
ILIM
= 33.2 kΩ. This sets the minimum current-limit
threshold at 3000 mA . Use the I
OS
equations, Figure 19, and the previously calculated value for R
ILIM
to
calculate the maximum resulting current-limit threshold.
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