Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

power transistor should always work in satur in ldo design?

Status
Not open for further replies.

lxcpku

Junior Member level 3
Joined
Sep 7, 2007
Messages
30
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,508
ldo triode region

hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot
 

iamxo

Full Member level 4
Joined
Mar 22, 2006
Messages
227
Helped
16
Reputation
32
Reaction score
4
Trophy points
1,298
Location
Southeast Asia
Activity points
2,495
Re: power transistor should always work in satur in ldo desi

in heavy load, the pass transistor should be in saturation region, offcourse, to reduce dropout voltage.

while, there is zero load, the pass transistor can be in subthreshold region, but should also keep the output voltage.
 

DenisMark

Full Member level 6
Joined
Sep 16, 2005
Messages
325
Helped
104
Reputation
208
Reaction score
34
Trophy points
1,308
Location
Russia
Activity points
3,437
Re: power transistor should always work in satur in ldo desi

Some LDO circuitries allow pass transistor working in triode region. But this citcuits are utilized more complex frequency compensation. See reference for example.
 

lxcpku

Junior Member level 3
Joined
Sep 7, 2007
Messages
30
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,508
ok. thx a lot :)
 

hung_wai_ming@hotmail.com

Full Member level 6
Joined
Jan 5, 2004
Messages
383
Helped
52
Reputation
104
Reaction score
11
Trophy points
1,298
Activity points
2,464
To save space, you may consider triode region.
However, triode region means you lose PSRR easily
 

WindRay

Member level 1
Joined
Nov 7, 2007
Messages
38
Helped
4
Reputation
8
Reaction score
2
Trophy points
1,288
Location
sg
Activity points
1,546
Re: power transistor should always work in satur in ldo desi

If the pass transistor is in triode region, the output voltage response to load step change is poor I think.
 

bicave

Full Member level 1
Joined
Jun 19, 2006
Messages
97
Helped
4
Reputation
8
Reaction score
2
Trophy points
1,288
Activity points
1,910
Re: power transistor should always work in satur in ldo desi

lxcpku said:
hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot

PMOS not always in Saturations. Depend on VIN, VOUT. If
- Vout<VIN-Vdsat, it will be on sat
- Vout> Vin-Vdsat, it will be on triode

If PMOS in saturation PSRR is better.
 

lxcpku

Junior Member level 3
Joined
Sep 7, 2007
Messages
30
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,508
Re: power transistor should always work in satur in ldo desi

bicave said:
lxcpku said:
hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot

PMOS not always in Saturations. Depend on VIN, VOUT. If
- Vout<VIN-Vdsat, it will be on sat
- Vout> Vin-Vdsat, it will be on triode

If PMOS in saturation PSRR is better.

got it ,thanks :)
 

shaq

Full Member level 5
Joined
Jul 23, 2005
Messages
312
Helped
14
Reputation
28
Reaction score
4
Trophy points
1,298
Activity points
3,397
In triode can make the on-resistance (Ron) smaller.
 

allennlowaton

Full Member level 5
Joined
Oct 5, 2009
Messages
243
Helped
3
Reputation
6
Reaction score
3
Trophy points
1,298
Location
Taiwan
Activity points
2,957
Re: power transistor should always work in satur in ldo desi

hi guys..i try to confirm this:

PMOS not always in Saturations. Depend on VIN, VOUT. If
- Vout<VIN-Vdsat, it will be on sat
- Vout> Vin-Vdsat, it will be on triode

in the .lis file of HSPICE
there appears Vds and Vdsat...i'm wondering what to use...
because, if i'm going Vdsat...my result don't follow the equation given above..

thank you...
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top