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.

2.

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.

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 10

Ω

resistor will raise +V

Crowbar Overvoltage Protection

Typical Performance Characteristics

DS007138-7

Input Crowbar

DS007138-8

Output Crowbar

DS007138-9

*Zener is internal to LM109.

at regulator input. LM109 bond wires will fuse at currents above 7A.

capacitor size, transformer impedance, and fuse blowing time.

≈ 7.5V.

Maximum Average

Power Dissipation (LM109K)

DS007138-16

Maximum Average

Power Dissipation (LM309K)

DS007138-17

Output Impedance

DS007138-18

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