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

failure occurs in the internal power transistor structure.

Zener 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

current 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.

**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.

Maximum Average

Power Dissipation (LM109K)

DS007138-16

Maximum Average

Power Dissipation (LM309K)

DS007138-17

Output Impedance

DS007138-18

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