Reference Voltage for LDO

[Mounikap]

Hello All,

I am designing 2 LDO's.
1. Power supply = 1.8, LDO output put = 1.2V and max current load =100ma
2. Power supply = 1.8, LDO output put = 0.7V and max current load =10ma

For these LDO's how can i decide the reference voltage (BGR) ? Suppose let say if Vref is 600mv then why not 570mv or 620mv ? Can anyone please tell me how to decide the value of Vref for our required LDO ? Based on which parameters shall we decide BGR value ? Do we need to test at every point such as 550mv, 560mv, 590mv and so on ... ?

 

[nitishn5]

A Voltage mode BGR without any Curvature Compensation will give a typical value of about 1.23V.
A Current mode BGR will allow you to set "any" voltage that you want.
(PS Yes there are other architectures which give other voltages)

If you are using a Voltage mode BGR, you have to divide the VBGR voltage to say 0.7V and have one LDO in unity Gain and the other with a gain of 12/7.
If you are using a Current mode BGR, you can generate both 0.7V and 1.2V directly and have both the LDOs in unity gain mode.

The choice of the VREF will be from what you can generate and how convenient is it to have some gain on your LDO (is it stable with all that)

The next question is about trimming away any offsets. Then you need some mechanism to change either your Gain factor OR the VREF input to your LDOs or both.
If so then you have to check again which voltage is more convenient. Where do you implement the trimming and how much. And this will determine what VREF you need to start with.

Once you have those calculated, then just simulate within those paramaters and you should be good.

 

[dick_freebird]

Explain what you mean by current mode voltage reference.

 

[nitishn5]

Voltage Mode BGR - Something that generates the Bandgap voltage directly by adding the PTAT and CTAT voltages.

Current Mode BGR - Where the PTAT and CTAT currents are added to get the final current and voltage.
In the above by splitting R4 we can get different voltages from the same circuit.