LM7815CT - Operating with Vin < 17V

Jeff Starkell

I realize that with less than 17Vin this device will not be regulating the output.

My question is will the 7815 be damaged if operated for an extended period with Vin in the range of 10-14V while delivering 500-700mA?

Please don't suggest a comparable switch mode part, I simply want to know if this device will be damaged or simply put out about 2V less than the input voltage?

over 9 years ago

0 Donald Jones over 9 years ago

TI__Mastermind 22610 points

No, the LM7815 will not be damaged.

0 Jeff Starkell over 9 years ago in reply to Donald Jones

Intellectual 505 points

Donald,
It seems when Vin << Vreg the NPN pass transistor (Q16) simply gets fully biased on striving to increase the output voltage, is that correct?

0 Donald Jones over 9 years ago in reply to Jeff Starkell

TI__Mastermind 22610 points

" ... or simply put out about 2V less than the input voltage?"

Yes, typically Vout will be about 2V lower.

Unlike PNP (or PMOS) pass element in a typical LDO, the LM7815 pass element is Darlington NPN, so the minimum Vin-Vout will be about (Vbe x 2) = (0.65V x 2) = 1.3V at low output currents, plus some voltage offset from the series current sense resistor (0.3 ohms)

Plus, ~maybe~, the effects of Vce(sat) of the high current output transistor if Vce(sat) exceeds about 0.6V

You will be operating in an area where behavior is not ensured, but this will not damage the LM7815 regulator.

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LM7815CT - Operating with Vin < 17V