On chip power supply source

[Teddy]

I would like to create on-chip dc-dc or similar voltage source to use for a sensitive analog circuitry. I would like to keep the internal VCC at 3.0 V while device itself has just 1 VCC pin which by spec. can go from 3-5.5V
Obviously I would like to have it stable over process/temp/VCC and able to provide enough current (10mA and more) for the circuitry.

Any ideas?

 

[electronrancher]

for that small amount of current, i would use a linear regulator. A 10mA pass device would be pretty small, and 10mA * 2.5v worst case power dissipation is only 250mW. Not too bad, maybe 5-10 degrees temperature rise at peak dissipation.

For higher efficiency, you could always try a switched-cap solution, but your caps will need to be external if it needs to provide 10mA, and you will need about 4 10mA switches. Plus, it's much noisier, maybe no good for your sensitive circuitry.

If you want, we can start to work out a simple LDO design - does your chip have a bandgap reference? You should describe your application a little bit, and we can get it started.

 

[Teddy]

I have simple bandgap - 5uA to 50uA output; +/-0.3uA over process/V/T - no big deal.

In fact I would like to place analog portion of PLL (Active filter, VCO)
to stable internal voltage. VCO range 10MHz- 250MHz

LDO would work great - I just never even seen one - do you have a schematic?

 

[IanP]

You can use the old but still very good uA723 voltage regulator. I always use it as refference voltage (it is adjustable) and the temperature coefficient is very, very low.

 

[electronrancher]

I think he needs a design for on-chip, rather than an external LDO..


Anyway, an LDO is basically an amplifer and a pass device. Look for the Rincon-Mora thesis on LDO's for a good starter, but I will show you one here.

I used a generic 1um CMOS process, assuming the process could withstand the VIN range up to 5.5v. I made a little 2-stage amp that drives a 250/1 sized PMOS, plenty of PMOS for 10mA.

The amp gets a bandgap as one input, and a resistor divider of the output as the second input. If the output is low, the PMOS is turned on harder, bringing the output into regulation.

My 2-stage amp has pretty good DC gain (83dB), so it is very accurate but needs an external cap to make it stable, probably a good idea to use an external cap anyway since you have switching circuits on board, else you won't be able to get rid of all the VCO noise and it will pollute your analog supply. You can make the resistor divider internal, so all you need is a VOUT pin to connect the cap to. If you make your PMOS a little bigger (and use a bigger cap) you can probably supply external circuitry, always a nice addition.

Anyway, this is just an example. Take a look and let me know if you have any questions. With 80dB gain, the line regulation from 3-5V and the load regulation from 0-10mA is quite awesome, much less than 1mV.

The nice thing about this much gain is that as long as your bandgap is relatively flat over temperature, the amp will drive out almost all temperature effects of the LDO, giving you a nice flat LDO output over temp. One caveat is the pass device, which may add a little tempco, you'll need to draw your own LDO and check it.

I'll be available for questions, and I'd like to see your circuit when you get it working. You can PM if you want, but I'd rather you kept your questions in this thread so others can learn!

 

[selvaraja]

Hi

you can use on chip voltage regulator ....much more reliable.....

regards,

selvarajah

 

[albertyin]

Besides internal regulator or bandgap, you can also add resistor/cap before the analog module.

 

[jinsin]

the answer is good...