Limit Vcc of IC

[okivrak]

Hi everyone;

I want to keep the supply to an IC within 8-20 volts interval while the possible DC supply occurs within 10-30 volts from a battery. I used a 2.4 volts zener diode in series with the Vcc pin of IC to achieve the lower limit 8V theoretically but unable to limit the supply to IC to 20 volts when the DC input exceeds over that value.

Any considerations without using a regulator is needed.

Thanks.

 

[IRstuff]

do a search for crowbar circuits:

http://www.dogpile.com/info.dogpl/search/web/crowbar+circuit/1/20/1/-/0/0/0/1/1/1

TTFN

 

[okivrak]

Actually I want to keep the 20 volt level, not to ground it. Crowbar circuits, as I examined, disables the supply. So,I want the supply not to be turned off.

 

[VEBill]

"...wihout using a regulator..."

Too bad - a 'low dropout' regulator set to something just over 8 volts would seem to be the otherwise-ideal solution (assuming that it meets your other, unstated, requirements).

I believe that some of these LDO regulators offer very low loss - the only significant power loss is due to the voltage drop across it (input to output). For a linear regulator, that is unavoidable in any case.

 

[BrianG]

You need a way to get rid of the excess voltage which takes your Vcc pin out of range. As I see it there are only two ways to prevent the maximum voltage from exceeding 20V at the Vcc pin of the IC:

1)use a 20V zener diode (Vcc to Gnd) in series with a current limiting resistor from the variable supply input. This is still a "regulator" - but a shunt regulator - it may be more acceptable than a three-terminal device but will still dissipate the excess voltage as heat in the zener and limiting resistor when the supply goes over 20V.

2) design a special custom switching regulator which only starts to PWM-switch for voltages above 20V. Rather than always giving a fixed output voltage like a conventional switcher this design needs to be turned fully on below 20V so that it tracks the 10-20V area, then clamps at 20V and progressively chops the input supply at voltages above 20V to reduce power loss.

 

[CarlPugh]

Why can't you use a series regulator.
A series regulator is cheap and simple to design and build.
Assuming you have a positive voltage, why not use a LM317? You should be able to get a data sheet at Nationial Semiconductor. (?

http://www.national.com)

or Digi-key. (

http://www.digikey.com)

 

[buzzp]

CarlPugh has the solution. These regulators can have a variable input with a fixed output. I believe the max input voltage to the LM317 is 30 volts so you may want to put a zener on the input side to ground with some zener voltage less than 30V to protect the 317. Pretty easy to use with a couple of supporting resistors.

 

[VEBill]

Depending on the current and temperature, the LM317 may not provide sufficient 'low drop out voltage' performance.

'Okivrak' mentioned that the solution had to work with as little as a 2 volt differential (10v in, 8v out). The LM317 gets close to, or even over, that 2v limit in some circumstances (high current and/or low temperature).

Even if his I & T conditions fit (barely), the margin would be better with a LDO regulator. Also, with a LDO reg., he might be able to up the output to something better than 'just barely' 8.0 volts (margin again).

 

[buzzp]

VE1BLL, I agree with your statements concerning operation at 10V into the LM317. It could be marginal. Perhaps the user can operate the "IC" at a more standard voltage. Not knowing what the "IC" is, its hard to say. Its also unclear why he wants to operate at 8V. If the IC can operate up to 20V then why add a zener to drop it to 8V when the DC supply is at 10V? There is more here than what were being told. Otherwise, another easy solution is too put a zener to ground on the IC supply to ground with a zener voltage less than 20V, problem solved (with extra heating due to the zener).

 

[nbucska]

Please read FAQ 240-1032
How much current do you need? How sensitive is your IC for
AC ? How many circuit do you need (one or mass production?)

<nbucska@pcperipherals DOT com> subj: eng-tips