Application Hints
In operation, the LM117HV develops a nominal 1.25V refer-
ence voltage, VREF, between the output and adjustment ter-
minal. The reference voltage is impressed across program
resistor R1 and, since the voltage is constant, a constant cur-
rent I1 then flows through the output set resistor R2, giving an
output voltage of
20143805
FIGURE 1.
Since the 100 μA current from the adjustment terminal rep-
resents an error term, the LM117HV was designed to mini-
mize IADJ and make it very constant with line and load
changes. To do this, all quiescent operating current is re-
turned to the output establishing a minimum load current
requirement. If there is insufficient load on the output, the out-
put will rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A 0.1 μF disc or
1 μF solid tantalum on the input is suitable input bypassing for
almost all applications. The device is more sensitive to the
absence of input bypassing when adjustment or output ca-
pacitors are used but the above values will eliminate the
possibility of problems.
The adjustment terminal can be bypassed to ground on the
LM117HV to improve ripple rejection. This bypass capacitor
prevents ripple from being amplified as the output voltage is
increased. With a 10 μF bypass capacitor 80 dB ripple rejec-
tion is obtainable at any output level. Increases over 10 μF do
not appreciably improve the ripple rejection at frequencies
above 120 Hz. If the bypass capacitor is used, it is sometimes
necessary to include protection diodes to prevent the capac-
itor from discharging through internal low current paths and
damaging the device.
In general, the best type of capacitors to use are solid tanta-
lum. Solid tantalum capacitors have low impedance even at
high frequencies. Depending upon capacitor construction, it
takes about 25 μF in aluminum electrolytic to equal 1 μF solid
tantalum at high frequencies. Ceramic capacitors are also
good at high frequencies; but some types have a large de-
crease 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 LM117HV is stable with no output capacitors,
like any feedback circuit, certain values of external capaci-
tance 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 load capacitance larger
than 10 μF will merely improve the loop stability and output
impedance.
LOAD REGULATION
The LM117HV is capable of providing extremely good load
regulation but a few precautions are needed to obtain maxi-
mum performance. The current set resistor connected be-
tween the adjustment terminal and the output terminal
(usually 240Ω) should be tied directly to the output of the reg-
ulator 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 × IL. If the set
resistor is connected near the load the effective line resis-
tance will be 0.05Ω (1 + R2/R1) or in this case, 11.5 times
worse.
Figure 2 shows the effect of resistance between the regulator
and 240Ω set resistor.
20143806
FIGURE 2. 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. Al-
though 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 VIN. 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 pos-
itive regulators. For output capacitors of 25 μF or less, there
is no need to use diodes.
The bypass capacitor on the adjustment terminal can dis-
charge through a low current junction. Discharge occurs when
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LM117HVQML