Datasheet

LM117HV, LM317HV
SNVS773C –APRIL 2000–REVISED APRIL 2013 www.ti.com
LOAD REGULATION
The LM117HV is capable of providing extremely good load regulation but a few precautions are needed to obtain
maximum performance. The current set resistor connected between the adjustment terminal and the output
terminal (usually 240Ω) should be tied directly to the output of the regulator rather than near the load. This
eliminates line drops from appearing effectively in series with the reference and degrading regulation. For
example, a 15V regulator with 0.05Ω resistance between the regulator and load will have a load regulation due to
line resistance of 0.05Ω × I
L
. If the set resistor is connected near the load the effective line resistance will be
0.05Ω (1 + R2/R1) or in this case, 11.5 times worse.
Figure 19 shows the effect of resistance between the regulator and 240Ω set resistor.
Figure 19. 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-5 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 LM117HV, 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 output is shorted. Internal to the LM117HV 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. Figure 20
shows an LM117HV with protection diodes included for use with outputs greater than 25V and high values of
output capacitance.
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