Application Hints
1.
Bypass the input
of the LM109 to ground with
≥
0.2 µF
ceramic or solid tantalum capacitor if main filter capaci-
tor is more than 4 inches away.
5.
Preventing latchoff
for loads connected to negative
voltage:
If the output of the LM109 is pulled negative by a high cur-
rent supply so that the output pin is more than 0.5V negative
with respect to the ground pin, the LM109 can latch off. This
can be prevented by clamping the ground pin to the output
pin with a germanium or Schottky diode as shown. A silicon
diode (1N4001) at the output is also needed to keep the
positive output from being pulled too far negative. The 10Ω
resistor will raise +V
OUT
by
≈
0.05V.
Avoid insertion of regulator into “live” socket
if input
voltage is greater than 10V. The output will rise to within
2V of the unregulated input if the ground pin does not
make contact, possibly damaging the load. The LM109
may also be damaged if a large output capacitor is
charged up, then discharged through the internal clamp
zener when the ground pin makes contact.
3.
The output clamp zener
is designed to absorb tran-
sients only. It will not clamp the output effectively if a fail-
ure occurs in the internal power transistor structure. Ze-
ner dynamic impedance is
≈
4Ω. Continuous RMS
current into the zener should not exceed 0.5A.
4.
Paralleling of LM109s
for higher output current is not
recommended. Current sharing will be almost nonexist-
ent, leading to a current limit mode operation for devices
with the highest initial output voltage. The current limit
devices may also heat up to the thermal shutdown point
(≈ 175˚C). Long term reliability cannot be guaranteed
under these conditions.
2.
DS007138-7
Crowbar Overvoltage Protection
Input Crowbar
Output Crowbar
DS007138-8
DS007138-9
*Zener is internal to LM109.
*
*
Q1 must be able to withstand 7A continuous current if fusing is not used
at regulator input. LM109 bond wires will fuse at currents above 7A.
†
Q2 is selected for surge capability. Consideration must be given to filter
capacitor size, transformer impedance, and fuse blowing time.
††
Trip point is
≈
7.5V.
Typical Performance Characteristics
Maximum Average
Power Dissipation (LM109K)
Maximum Average
Power Dissipation (LM309K)
Output Impedance
DS007138-18
DS007138-16
DS007138-17
3
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