UVLO for discrete LDO

[LightBlue]

I've built an LDO regulator for a high temperature oil and gas tool.  It converts 2.5V to 1.8V and 1.5V using an OP296 op-amp and PMOS pass transistors.  The op-amp is supplied by a local 3.3V supply.  I have tested the circuit and it is stable under all necessary load conditions and operates at high temperature.

My problem is the two different supply voltages.  In the event that the 3.3V rail comes up after the 2.5V rail (or not at all), the op-amp is unpowered and the gate of the PMOS transistor is not switched hard off.  Therefore there is a potential for the regulated outputs to be at 2.5V.

A simple 10K resistor from the gate of the PMOS to the 2.5V rail is OK but the output sits at about 100mV as there is obviously currently leaking back through the unpowered op-amp to 0V.

I'm interested to see if anyone has any elegant solutions to this problem (i.e. a p-channel bipolar clamping the gate of the PMOS until 3.3V is applied?)

[Alex Eisenhut]

I've built an LDO regulator for a high temperature oil and gas tool.  It converts 2.5V to 1.8V and 1.5V using an OP296 op-amp and PMOS pass transistors.  The op-amp is supplied by a local 3.3V supply.  I have tested the circuit and it is stable under all necessary load conditions and operates at high temperature.

My problem is the two different supply voltages.  In the event that the 3.3V rail comes up after the 2.5V rail (or not at all), the op-amp is unpowered and the gate of the PMOS transistor is not switched hard off.  Therefore there is a potential for the regulated outputs to be at 2.5V.

A simple 10K resistor from the gate of the PMOS to the 2.5V rail is OK but the output sits at about 100mV as there is obviously currently leaking back through the unpowered op-amp to 0V.

I'm interested to see if anyone has any elegant solutions to this problem (i.e. a p-channel bipolar clamping the gate of the PMOS until 3.3V is applied?)

I haven't eaten yet so my braining not too good.
But this sounds like a power sequencing problem.
First question, why not use a dedicated dual out LDO chip? Is it a temperature thing? I need numbers.
http://www.digikey.ca/product-detail/en/ISL9000AIRBCZ-T/ISL9000AIRBCZ-TCT-ND/4570019

Can you use a dual opamp with one section configured as a comparator to generate an enable signal?

If not, there are dedicated power supervisor ICs to check stuff like this.

Sorry, I stopped being elegant a while back, I'm just pragmatic now. Maybe if you show us a schematic with part #, I can suggest something elegant...

[David Hess]

I'm interested to see if anyone has any elegant solutions to this problem (i.e. a p-channel bipolar clamping the gate of the PMOS until 3.3V is applied?)

That is what I would try first.  A PNP transistor driven from the 3.3 volt supply to the 2.5 volt supply can clamp the p-channel MOSFET gate to 2.5 volts keeping it shut off when the 3.3 volt supply is below 2.5 volts.  Appropriate resistors for the base of the transistor and the output of the operational amplifier should be used.

[LightBlue]

Thanks for the replies.

Alex:

Yes it is a power sequencing issue.  My application is running at >175degC and the LDOs and controllers that I have tested do not operate within specification at that temperature, or have a thermal cut-out (or are not widely available, TPS7H1201-HT)

If I use a power sequencing chip I still have to disable the PMOS pass transistor without affecting normal operation, i.e. build an "enable" pin into my LDO..

David:
I think I'll work out some values for a p-channel bipolar clamp transistor and see how that goes.

Thanks for your help,
James

[tszaboo]

You could use a red LED as a " 2V voltage reference" with a LM3xx comparator? It will even shine for you.

[David Hess]

I think I'll work out some values for a p-channel bipolar clamp transistor and see how that goes.

I was thinking PNP bipolar transistor but a p-channel MOSFET would work if its gate to source threshold voltage is low enough.  The PNP transistor will be less expensive.

[mikerj]

Would a simple Schottky diode between the 2.5v and 3.3v rail suffice?

[David Hess]

Would a simple Schottky diode between the 2.5v and 3.3v rail suffice?

I have seen high voltage power supplies do that to control sequencing better but it depends on the details.  Powering 3.3 volt supply at 2.1 volts from the 2.5 volt supply may cause problems.