Help me design a power supply for bandgap

[hmmm]

Power Supply for bandgap

hi!

i wonder if anyone has done this before?

i have a bandgap that supplies Vref=1.25 . i need to implement a circuit that can do the following:

1) power up the bandgap ref circuit from source Va when souce Vb is less than 2.5V
2) power up the badngap ref circuit from source Vb and cut out the supply from Va when Vb =2.5V

can anyone kindly recommend me any books, journals or

 

[steer]

Re: Power Supply for bandgap

I've not seet such a circuit, but it seems pretty easy to implement.
Just put two forward biased diodes on supplies Va and Vb, than whatever supply is higher would take on the BG reference power.
Am I missing something in your spec?

 

[kwkam]

Power Supply for bandgap

I think you have a problem is how to define the 2.5V, is it from external. Or it have generated internally. It make the circuit use different idea to design.

 

[JPR]

Re: Power Supply for bandgap

I am assuming that Va is always present (such as an unregulated supply), and that Vb is a regulated supply that is not always present.

What I would recommend is using a comparitor. The supply for the comparitor would be Va (if it is always present and greater than Vb), or using a peak detector to decide between Va and Vb to supply the comparitor if Va can be lower than Vb.

The reference for the comparitor could be a crude secondary voltage reference (2nd untrimmed bandgap, vbe multiplier, resistor divider from supply, zener voltage, diode stack, etc., depending upon how accurate it needs to determine the 2.5V supply, you can determine how good this reference needs to be.)

The comparitor determines if Vb is greater than this reference voltage, and controls switches to control the supply for your bandgap.

Of course, if my assumptions are not true, the solution may not work for you.

 

[lastdance]

Re: Power Supply for bandgap

What steer mentioned is actually diode oring.
But unless you have good Schottkys in your process, the drop across the diode is going to hurt your circuit's headroom, especially across corners.
If headroom is important, you should use pass transistors to do the switching. Use a comparator, (I don't think a conventional one can do the job, you can try common gate amplifier), and use the output to switch on/off PMOS switches. U do not have drop (actually, there's the rdson, but small if u sized them correctly) like in the case of diode.
Beware of bulk being forward biased in the PMOS switch, use well-switching to bias the bulk.

 

[rongli]

Power Supply for bandgap

add a TTL switch .

 

[mady79]

Re: Power Supply for bandgap

Generate 2.5 V using any of the power supplies (Use resistor ladder or zener diode ) or get it externally .
Use a hysteresis comparator to compare Vb with the 2.5V and get a digital output .
This digital output connects the supply to Va or Vb .Use some cap at the supply if needed to store the charge from Va/Vb .

Hope this answer's your query .

 

[mike_bihan]

Power Supply for bandgap

I guess this is happening during chip power up. When the VCC is ramping up, internal regulator is not enabled. So the VCC will be connected to the bandgap.

After that, you want to hot switch the bandgap from VCC to your regulated voltage source.

Big "back to back switch" can help you to do the switching. It can eliminate the forward bias in PMOS.

Thx