5962R9951705VXA Datasheet, PDF (18/36 Pages) Texas Instruments – LM117QML 3-Terminal Adjustable Regulator

LM117QML, LM117QML-SP
SNVS356D – MARCH 2006 – REVISED APRIL 2013
www.ti.com
Figure 17. Regulator with Line Resistance in Output Lead
With the TO 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 PFM 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 10μF 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 25μF 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 LM117 is a 50Ω resistor which limits the peak discharge
current. No protection is needed for output voltages of 25V or less and 10μF capacitance. Figure 18 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 18. Regulator with Protection Diodes
When a value for θ_(H−A) is found using the equation shown, a heatsink must be selected that has a value that is
less than or equal to this number.
θ_(H−A) is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots
temperature rise vs power dissipation for the heatsink.
18
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