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LM338K Datasheet(PDF) 7 Page - STMicroelectronics

Part No. LM338K
Description  THREE-TERMINAL 5 A ADJUSTABLE VOLTAGE REGULATORS
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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will be 0.05
Ω (1 + R2/R1) or in this case, 11.5 times
worse.
Figure 2 shows the effect of resistance between the
regulator and 140
Ω set resistor.
With the TO-3 package, it is easy to minimize the re-
sistance from the case to the set resistor, by using
2 separate leads to the case. The ground of R2 can
be returned near the ground of the load to provide
remote ground sensing and improve load regula-
tion.
PROTECTION DIODES
When external capacitors are used with any IC regu-
lator it is sometimes necessary to add protection
diodes to prevent the capacitors from discharging
through low current points into the regulator. Most
Figure 2 : Regulator with Line Resistance in Out-
put Lead.
20
µF capacitors have low enough internal series re-
sistance to deliver 20A spikes when shorted. Al-
though the surge is short, there is enough energy to
damage parts of the IC.
When an output capacitor is connected to a regula-
tor and the input is shorted, the output capacitor will
discharge into the output of the regulator. The dis-
charge current depends on the value of the capaci-
tor, the output voltage of the regulator, and the rate
of decrease of VI. In the LM338 this discharge path
is through a large junction that is able to sustain 25A
surge with no problem. This is not true of other types
of positive regulators. For output capacitors of
100
µF or less at output of 15V or less, there is no
need to use diodes.
The bypass capacitor on the adjustment terminal
can discharge through a low current junction. Dis-
charge occurs when either the input or output is
shorted. Internal to the LM338 is a 50
Ω resistor
which limits the peak discharge current. No protec-
tion is needed for output voltages of 25V or less and
10
µF capacitance. Figure 3 shows an LM338 with
protection diodes included for use with outputs
greater than 25V and high values of output capacit-
ance.
Figure 3 : Regulator with Protection Diodes.
LM138-LM238-LM338
7/12




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