Hi,

Finally got some of my biggest projects done, and I'm moving onto something new.

I never took any EE classes, so this might be easy...but not for me.

I would like to use this low drop out regulator: http://www.fairchildsemi.com/ds/KA/KA78R05C.pdf

You will notice it is a 4 terminal device. I'm confused as to what exactly VDis is and what it does...

Where does it get attached to in the power supply circuit?

Thanks.

 

It's to disable/ enable the output. Looks like if the voltage on this pin is higher than 2 V then output is active (datasheet under electrical characteristics)

 

Hi,

Finally got some of my biggest projects done, and I'm moving onto something new.

I never took any EE classes, so this might be easy...but not for me.

I would like to use this low drop out regulator: http://www.fairchildsemi.com/ds/KA/KA78R05C.pdf

You will notice it is a 4 terminal device. I'm confused as to what exactly VDis is and what it does...

Where does it get attached to in the power supply circuit?

Thanks.

pin 4 is disable pin. you need 2-35V on pin 4 to get voltage out. if you leave it disconnected, you will never get anything out. If you don't need the disable function, I think you could just tie it to pin1. see figure 3 Vo Vs. Vdis

 

It's to disable/ enable the output. Looks like if the voltage on this pin is higher than 2 V then output is active (datasheet under electrical characteristics)

stop doing that to me! lol

 

Look at page 2 and 3 of datasheet you posted.

Vdis absolute maximum input: 35V

Disable voltage voltage high: minumum 2V=output active

Disable voltage voltage low: maximum .8V=output disabled

Summation: Disable pin fed between 2v-35v output active.
Disable pin taken below .8v output disabled.

Edit: Holy cow it's an answer storm. I'm too slow.

 

Okay thanks....but where exactly is it "measuring" the voltage?

 

Okay thanks....but where exactly is it "measuring" the voltage?

for pin 4? between pin 4 and GND (pin3)

EDIT: don't be confused by the diagram where it shows a little battery @1.4V; that's just representative, not really inside the IC. It's just there to represent that you need >1.4V (specs say 2V) to switch the output on.

had post the quick & dirty answer, then come back and expound with the EDIT so I wouldn't get beaten to the punch again.

 

stop doing that to me! lol

?? did I write something funny

 

?? did I write something funny

no, you remember back on this post where we were both typing the same answer at the same time and you beat me to the punch? I was just joking with you now about it happening again.

 

no, you remember back on this post where we were both typing the same answer at the same time and you beat me to the punch? I was just joking with you now about it happening again.

Actually, I look if you're online and only then I write something... Just kidding...

 

Gahhh. I'm an idiot. Speaking of quick and dirty....exactly where does pin 4 get connected to?

Thanks. Sorry if I seem ignorant or something.

 

Gahhh. I'm an idiot. Speaking of quick and dirty....exactly where does pin 4 get connected to?

Thanks. Sorry if I seem ignorant or something.

connected to pin 1

 

connected to pin 1

Right. Is there a reason why they aren't connected internally?

 

It's just an additional functionality of this particular IC. So you can disable the output. Could be quite useful I guess.

 

Right. Is there a reason why they aren't connected internally?

pin 4 is what you would use to shut off the output from somewhere else in the circuit with a low voltage (or high voltage, up to 35V) signal. You would remove said signal to stop the output. I assume you don't need this feature, which is why I said connect it to pin1. Using this feature could save you a component by means of not having to have a component (transistor, relay, other) to disconnect your incoming voltage (pin1).

for instance, if you used this with a microcontroller, the microcontroller could shut off power by dropping the signal on pin4 (draws 20μA). Otherwise you would need a seperate transistor.

 

pin 4 is what you would use to shut off the output from somewhere else in the circuit with a low voltage (or high voltage, up to 35V) signal. You would remove said signal to stop the output. I assume you don't need this feature, which is why I said connect it to pin1. Using this feature could save you a component by means of not having to have a component (transistor, relay, other) to disconnect your incoming voltage (pin1).

for instance, if you used this with a microcontroller, the microcontroller could shut off power by dropping the signal on pin4 (draws 20μA). Otherwise you would need a seperate transistor.

Thanks. I suspected you would need a mcu that could use a serial signal or something.

This regulator sort of reminds me of a transistor, imagine pin4 is the base.