SNVS774O – MAY 2004 – REVISED JANUARY 2014
Figure 23. Regulator with Line Resistance in Output Lead
With the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by using two
separate leads to the case. However, with the TO-39 package, care should be taken to minimize the wire length
of the output lead. The ground of R2 can be returned near the ground of the load to provide remote ground
sensing and improve load regulation.
Protection Diodes
When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to
prevent the capacitors from discharging through low current points into the regulator. Most 10
μF
capacitors have
low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is
enough energy to damage parts of the IC.
When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge
into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage
of the regulator, and the rate of decrease of V
IN
. In the LM117, this discharge path is through a large junction that
is able to sustain 15A surge with no problem. This is not true of other types of positive regulators. For output
capacitors of 25
μF
or less, there is no need to use diodes.
The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs
when
either
the input, or the output, is shorted. Internal to the LM117 is a 50Ω resistor which limits the peak
discharge current. No protection is needed for output voltages of 25V or less and 10
μF
capacitance.
shows an LM117 with protection diodes included for use with outputs greater than 25V and high values of output
capacitance.
D1 protects against C1
D2 protects against C2
Figure 24. Regulator with Protection Diodes
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11
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