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LM109KSTEEL Scheda tecnica(PDF) 3 Page - National Semiconductor (TI) |
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LM109KSTEEL Scheda tecnica(HTML) 3 Page - National Semiconductor (TI) |
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3 / 8 page  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 OUT by ≈ 0.05V. 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 www.national.com 3 |
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