question about the pmos pass element in the ldo

Hi all,
I find that the pmos pass element is working at the linear region when the load current is maximum . And I have the following questions:
1. Is it undesirable that the pmos is at the linear region when the load current is maximum?
2. Is the pmos needed to work at the saturation region in the whole regulation region?

Thanks!!!

carlson said:

Hi all,
I find that the pmos pass element is working at the linear region when the load current is maximum . And I have the following questions:
1. Is it undesirable that the pmos is at the linear region when the load current is maximum?
2. Is the pmos needed to work at the saturation region in the whole regulation region?

Thanks!!!

Click to expand...

1. It is normally that pmos is at the linear region when the load current is maximum.

2. Pmos works at the saturation region at light load and pmos works at the linear region at heavy load.

Thanks, gevy!
But how to sizing the pmos to satisfy the dropout voltage spec?
Is the size determined by sweeping the size at the maximum load current?
Or calculate the Vdsat at the sat. region at the maximum load current?

Regards

carlson said:

Thanks, gevy!
But how to sizing the pmos to satisfy the dropout voltage spec?
Is the size determined by sweeping the size at the maximum load current?
Or calculate the Vdsat at the sat. region at the maximum load current?

Regards

Click to expand...

hello

pass device may work in the saturation region also in a heavy load condidions, but it has to be larger then. it has gain more than one and it can be used in an overall loop gain, it has also a better PSRR when works in saturation.

you can specify the size in that way. For the lowest possible Vin you can sweep gate voltage and temperature. For the PMOS case you give a resistor with a proper value which suits heavy load conditions (for example: Vout=1.2V and Iloadmax=100mA so RL=12Ohms) You can also add parasitic resistance for large current paths and bonding pads.

Thereafter you're doing dc analysis and set pass device's size, to get wanted dropout. Remember about corners.

regards

carlson said:

Hi all,
I find that the pmos pass element is working at the linear region when the load current is maximum . And I have the following questions:
1. Is it undesirable that the pmos is at the linear region when the load current is maximum?
2. Is the pmos needed to work at the saturation region in the whole regulation region?

Thanks!!!

Click to expand...

Both region is possible, at light load it will work at saturation region and it show better PSRR.
another approach to determine the size: you can use lowest supply voltage and max load to estimate the size. do DC sweep of VDD and Vout, check the precise margin for Vout meet your SPEC(such as, Load regulation=2%,VDD_min=3.3v, Vout=1.8V,load=100mA, dc sweep VDD and Vout and find the Vout=1.764v,and so on)
the temperature and process corners should be considered.
good luck,guy.

Thanks a lot!!!


Regards