Voltage Regulator with other power sources

[chrischarles]

Hello everybody,

I am new to this forum but have been reading bits of it every now and again on some of my projects. I have been designing circuits for a while now and am venturing into the PIC world at the moment. I look forward to becoming a more regular poster now I am a member.

Anyway, I have a question that I just cant figure out on my own....

I have a circuit that uses a tl750m05 voltage regulator (basically a low dropout 7805) to allow a phone charger of 6 volts to power it anywhere in my house. Also on my PCB is a USB connection that I use to configure the EEPROM and an ICSP header for full reprogramming.

When I run the circuit through the voltage regulator everything is fine and the regulator stays cool to the touch, however, when I run the circuit through USB power or ICSP then the regulator gets pretty hot (still touchable but HOT).

I initially checked the internal resistance (from output to ground) of the regulator and it is 20kOhms (at 5V that means 1.25mW) so I cant figure out why it would be getting so hot?

Does anybody have any ideas?

The same happens with other voltage regulators that I have, hence it cant be a duff component. I have the regulator connected correctly (i.e. output connects to the 5V rail on my PCB which has the USB/ICSP power connectors). There is nothing connected to the input of the voltage regulator when USB/ICSP power is being supplied.

 

[crutschow]

Applying 5V to the output of the regulator with no input voltage may cause it to conduct high current. To prevent this try connecting a diode from the output to the input of the regulator (anode to output) so the input is not at zero volts.

 

[chrischarles]

Apologies for my late reply to this post but it has taken me a little while to get hold of a collection of diodes to try.

Unfortunately none of them when connected in the way described stop the regulator from getting hot. The only thing in my head is to put a diode in series with the regulator such that no voltage can get back to the output pin of the regulator, however it is then not 5V and hence completely useless...

I would greatly appreciate some other thoughts. Thanks

 

[flat5]

If you also put a diode in series with the ground pin would it then output 5V?

 

[chemelec]

Hello everybody,

I am new to this forum but have been reading bits of it every now and again on some of my projects. I have been designing circuits for a while now and am venturing into the PIC world at the moment. I look forward to becoming a more regular poster now I am a member.

Anyway, I have a question that I just cant figure out on my own....

I have a circuit that uses a tl750m05 voltage regulator (basically a low dropout 7805) to allow a phone charger of 6 volts to power it anywhere in my house. Also on my PCB is a USB connection that I use to configure the EEPROM and an ICSP header for full reprogramming.

When I run the circuit through the voltage regulator everything is fine and the regulator stays cool to the touch, however, when I run the circuit through USB power or ICSP then the regulator gets pretty hot (still touchable but HOT).

I initially checked the internal resistance (from output to ground) of the regulator and it is 20kOhms (at 5V that means 1.25mW) so I cant figure out why it would be getting so hot?

Does anybody have any ideas?

The same happens with other voltage regulators that I have, hence it cant be a duff component. I have the regulator connected correctly (i.e. output connects to the 5V rail on my PCB which has the USB/ICSP power connectors). There is nothing connected to the input of the voltage regulator when USB/ICSP power is being supplied.

One Problem with the TL750M05 is: IF your Input Voltage Goes too Low, the TL750M05 will Draw EXCESSIVE CURRENT.

Check this out.

 

[chrischarles]

Ah ha, thankyou very much chemelec, I had not spotted that and had not realised it from the data sheets since it is clearly not intended operation. However Im sure plenty of people have encountered this before when doing ICSP programming of a PIC...

Do you have any LDO voltage regulators that would work with my situation - having other power sources (USB/ICSP)?

Thanks again

 

[chemelec]

Ah ha, thankyou very much chemelec, I had not spotted that and had not realised it from the data sheets since it is clearly not intended operation. However Im sure plenty of people have encountered this before when doing ICSP programming of a PIC...

Do you have any LDO voltage regulators that would work with my situation - having other power sources (USB/ICSP)?

Thanks again

That Depends on your Current Requirement?

One LDR I Especially like is a MC33269TD-XX series, But these are Surface Mount Parts.
In your case the XX = 05 for 5 Volts.
All these MC33269's are rated at 800 mA, I Believe.

(I only have 12 volt ones here, MC33269DT-012)

 

[3v0]

The LM2931AZ5 is one of may favorite regulators. This is the first regulator I reach for when 100mA is enough.

It will not conduct backward. I use the junebug to power boards with this regulator even with the regulator unpowered.

The dropout is low enough that 5V in will still provide a voltage that is nearly 5V when output current is low. As current nears 100mA the voltage drops off to about 4.5V. I think (should check) that most of my pic circuits run at about 4.85 when powered through this regulator from an already regulated 5V source.

 

[chrischarles]

I need somewhere between 400-500mA and really want it in a through-hole package of some sort. One with the same footprint as the 7805 would be ideal (IN, GND, OUT) though I can do some modifications if this isnt the case. Performance isnt that important so long as there is no backwards conduction and the dropout voltage is low.

Thank you all so much once again!

 

[3v0]

The regulators made for automotive use are very robust. I think not conducting backwards is part of that.
The little fellow I mentioned was one of these.

In another thread about regulators I found an automotive regulator that did a half amp. Check mouser or digikey.

Do not let SMD parts stop you. They are not that hard to deal with.

 

[chrischarles]

I have no problems with surface mount really but Ive already designed and manufactured the PCB and I would rather keep things looking as good as possible. Ill investigate automotive parts as I can see why they would be good for the purpose though interested in other peoples choices.

What specification is it that would stop backwards conduction?

Thanks

 

[chrischarles]

I have eventually settled on the L4940V5 from STMicroelectronics. After a great deal of searching I managed to find a PIC with the same footprint and same pinout as a 7805. It has a dropout of about 0.3V and it does not backwards conduct. Just wanted to say thanks again for your help.

Chris