Datasheet

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R -k
ILIM
W
Current-LimitThreshold-mA
I
OC(max)
I
OC(typ)
I
OC(min)
I
OS(min)
APPLICATION 1: DESIGNING ABOVE A MINIMUM CURRENT-LIMIT
APPLICATION 2: DESIGNING BELOW A MAXIMUM CURRENT-LIMIT
TPS2550
TPS2551
SLVS736B – FEBRUARY 2008 – REVISED NOVEMBER 2008 ........................................................................................................................................ www.ti.com
Figure 21. Current-Limit Threshold Vs.R
ILIM
Some applications require that current-limiting cannot occur below a certain threshold. For this example, assume
that 1 A must be delivered to the load so that the minimum desired current-limit threshold is 1000 mA. Use the
I
OS
equations and Figure 21 to select R
ILIM
.
• I
OS(min)
(mA) = 1000 mA
• I
OS(min)
(mA) = (39700 V) /
ILIM
(k Ω ))
1.342
• R
ILIM
(k Ω ) = [(39700 V) / (I
OS(min)
(mA))]
1/1.342
• R
ILIM
= 15.54 k Ω
Select the closest 1% resistor less than the calculated value: R
ILIM
= 15.4 k Ω . This sets the minimum current-limit
threshold at 1 A . Use the I
OC
equations, Figure 21 , and the previously calculated value for R
ILIM
to calculate the
maximum resulting current-limit threshold.
• R
ILIM
= 15.4 k Ω
• I
OC(max)
(mA) = (24500 V) /
ILIM
(k Ω ))
0.975
• I
OC(max)
(mA) = (24500 V) / (15 (k Ω ))
0.975
• I
OC(max)
= 1703 mA
The resulting maximum current-limit threshold is 1.7 A with a 15.4 k Ω resistor.
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 1.25 A to protect an up-stream power supply. Use
the I
OC
equations and Figure 21 to select R
ILIM
.
• I
OC(max)
(mA) = 1250 mA
• I
OC(max)
(mA) = (24500 V) /
ILIM
(k Ω ))
0.975
• R
ILIM
(k Ω ) = [(24500 V) / (I
OC(max)
(mA))]
1/0.975
• R
ILIM
= 21.15 k Ω
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