Turning On and Off Voltage Regulator with small transistor

[nmbg011]

Hi

I have one problem and some questions about voltage regulator :



On picture say that is LM7805.


1. How to implement small transistor to complete turn on turn off this voltage regulator and capacitors?
2. If disconnect gnd of voltage regulator, voltage on Vout will be the same as Vin ?

 

[FvM]

1. You need to place a series transistor before the voltage regulator.
2. No. there will be at least a voltage drop. Behaviour of the voltage regulators with floating GND/Adjust pin isn't exactly specified, however.

 

[emontllo]

Hello,

1. Use a P-MOS transistor before Vin. You will turn the circuit on by connecting the gate to 0 V. If the circuit input voltage is higher than Vgate max use a voltage divider. Use a nmos or npn transistor as a "pull down" to drive this "enable input". Also place a high value resistor between gate and source of the P-MOS.

2. Yes, approximately. So take care not to destroy the output circuits since they will be connected to Vin. Ref: **broken link removed** Page 11.

Good luck!

 

[tpetar]

When the ground pin alone becomes disconnected, the output approaches the unregulated input, causing possible damage to other circuits connected to VOUT. If ground is reconnected with power “ON”, damage may also occur to the regulator. This fault is most likely to occur when plugging in regulators or modules with on card regulators into powered up sockets. Power should be turned off first, thermal limit ceases operating, or ground.

 

[nmbg011]

1. Use a P-MOS transistor before Vin. You will turn the circuit on by connecting the gate to 0 V. If the circuit input voltage is higher than Vgate max use a voltage divider. Use a nmos or npn transistor as a "pull down" to drive this "enable input". Also place a high value resistor between gate and source of the P-MOS.

Can I use this PMOS for that :

TSM3401CX P-MOSFET, 30V, 3A, 0.06R, SOT-23

and circuit is like this :





please correct me if I'm wrong

thx

 

[alexan_e]

You can but you have to invert the source / drain of the mosfet because as it is placed in you schematic the mosfet diode will always conduct.

You also need a pullup resistor from source to the gate to force the mosfet off when the gate is floating.

Alex

 

[nmbg011]

You mean like this:



Will this work ?

 

[FvM]

Perfect, as long as Vin doesn't exceed maximum Vgs of the transistor.

 

[alexan_e]

Exactly.
The P mosfet turns on when the gate is grounded and turns off when the switch opens and the gate voltage is pulled to the source level (Vgs=0)
Note that max Vgs is 20v , I suppose that your input voltage is lower than that because the input voltage will actually be used as Vgs.

Alex

 

[nmbg011]

Input is 12V from battery.

One more question. Will this resistor in Off state of circuit consume some current and how much ?

thx

 

[alexan_e]

The resistor discharges the gate capacitance and then the current is negligible (cause by gate leakage).

Alex

 

[nmbg011]

Is ok to put 10K resistor ? Voltage in is 12V.

 

[alexan_e]

Yes, it is fine, you can use a wide range of values.

Alex

 

[nmbg011]

Is this possible with N-MOSFET ?

 

[alexan_e]

In order to use a N-mosfet you need a gate voltage that at least 3 to 5v higher than the mosfet source.
You have to connect the input voltage to the drain , output to the 7805 to the mosfet source and then drive the gate with a voltage higher that the input voltage.
For example if the input is 10v you need a drain voltage of 13 or 15v, this will obviously complicate things and I don't think you need something like that.

Alex

 

[KerimF]

Is this possible with N-MOSFET ?

This may help.

Kerim

 

[alexan_e]

I assumed that he was asking for a "lossless" solution where the mosfet would be almost transparent when on.
The solution shown in the PDF can probably work if there is enough headroom in the input voltage.
Also note that the output voltage will be highly dependent of the specific mosfet model used and the Vgs characteristics.

Alex

 

[KerimF]

You are right Alex... That is why I said: It may help.
I just gave a possibility in case 12uA as an idle current is not a problem for his application.
And as you said the type of the NMOS should be chosen carefully for its Vgs and perhaps its rated drain voltage/current.
The one shown in the PDF is just a model I took as an example from the LTspice list.

Kerim

 

[nmbg011]

I think to complete stop current.