SNVS774L – MAY 2004 – REVISED FEBRUARY 2011
In general, the best type of capacitors to use is solid tantalum. Solid tantalum capacitors have low impedance
even at high frequencies. Depending upon capacitor construction, it takes about 25
μF
in aluminum electrolytic to
equal 1μF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies; but some
types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, 0.01
μF
disc may
seem to work better than a 0.1
μF
disc as a bypass.
Although the LM117 is stable with no output capacitors, like any feedback circuit, certain values of external
capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1
μF
solid
tantalum (or 25
μF
aluminum electrolytic) on the output swamps this effect and insures stability. Any increase of
the load capacitance larger than 10
μF
will merely improve the loop stability and output impedance.
Load Regulation
The LM117 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 (case) 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.
shows the effect of resistance between the regulator and 240Ω set resistor.
Figure 26. 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 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.
10
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