question about LDO with 1.5V fixed output

[Chethan]

Hi all,
I am supposed to design an LDO with an output voltage of 1.5V (Fixed) while the input voltage is from 1.8V to 3.3V. The output current is 100mA(max). Is this specsification possible by using an PMOS pass transistor. I had heard that for very low supply voltages the pass transistor should be NMOS type. can someone guide me in this regard.

thanx in advance

chethan

 

[bluesmaster]

300mv is enough !! if u use NMOS, 1.8v VDD will not apply. because the gate of NMOS shoule be 1.5+vth > 1.8v.

 

[v_c]

If you are allowd to design a charge pump (to allow generating higher voltages) then you can use an NMOS.

 

[jutek]

I had heard that for very low supply voltages the pass transistor should be NMOS type. can someone guide me in this regard.

thanx in advance

chethan

for low supply you need a PMOS transistor. low voltages can easily turn on pass device, but with NMOS 1.8V will not turn on it

regards

 

[willyboy19]

I agree: PMOS pass transistor can be used since you still have 300mV headroom even when the supply is 1.8V. You probably don't want to mess up with another charge pump if you need a 'quiet' regulated output voltage at 1.5V. One thing to remeber is to carefully calculate/simulate the on-resistance of the PMOS transistor since you have 100mA output current, which means that the on-resistance should be less than 3-ohm if the PMOS is operating in the linear region.

 

[maddy]

I have done a LDO with similar spec with a PMOS pass transistor. No probs. Go ahead and do it.
Maddy

 

[nagula suresh]

for low voltage applications pmos as a passdevice is a better option coz it needs low gate drive and low leakeage current.
but ,the width is quiet high.,
u have to be careful in compensation,coz of large pole associated with the pmos device.

 

[limingchou]

yes, use pmos pass device. but when u design it, u need consider the phase margin, because the pmos transistor have potential instable. select the good compensation.