Designer’s Guide Community

danmc

Community Member

Offline

Posts: 35
Boston

voltage limited current source
Aug 17^th, 2011, 6:29am

Sometimes it is useful to have a verilog-a or AMS model for something like a bias current generator but in those cases it is also good to provide voltage limiting. Is there a simple way to do this in Verilog-A or Verilog-AMS? The issue is that without voltage limiting, the current sources will simply force the current no matter what and sometimes that is really not what the rest of the circuit wants!

In the past I have done some stuff in spectre with hanging additional controlled sources that have limiting on my output node.

Thanks
-Dan

Back to top

IP Logged

boe Community Fellow Offline Posts: 615	Re: voltage limited current source Reply #1 - Aug 17^th, 2011, 12:39pm Danmc, you can do something like Code: if (V(ibias, vss) > 1) � I(ibias, vss) <+ CurrentValue; else � I(ibias, vss) <+ 0; However, for better convergence you should avoid the discontinuity of the current and use a soft transition. B O E
Back to top	IP Logged

danmc Community Member Offline Posts: 35 Boston	Re: voltage limited current source Reply #2 - Aug 17^th, 2011, 1:17pm you mean use something like like a tanh function instead of hard compare?
Back to top	IP Logged

Ken Kundert

Global Moderator

Offline

Posts: 2384
Silicon Valley

Re: voltage limited current source
Reply #3 - Aug 17^th, 2011, 3:46pm

I don't recommend following BOE's suggestion. That is a recipe for convergence problems. I have seen two approaches to this problem that works well. One is to carefully model current/voltage limiting. You can use http://www.designers-guide.org/VerilogAMS/functional-blocks/regulator/regulator.... as a model. Alternately, you can model a two terminal clamp that you place in parallel with the current source. It would have infinite impedance within the compliance range of the current source, but then would look like a small resistor outside that range. It would be designed so that the IV characteristics were piecewise continuous.

-Ken

Back to top

IP Logged

boe Community Fellow Offline Posts: 615	Re: voltage limited current source Reply #4 - Aug 18^th, 2011, 3:25am danmc wrote on Aug 17^th, 2011, 1:17pm: you mean use something like like a tanh function instead of hard compare? Yes; and that seems to work quite well, too. B O E
Back to top	IP Logged

danmc

Community Member

Offline

Posts: 35
Boston

Re: voltage limited current source
Reply #5 - Aug 19^th, 2011, 8:36am

Ken Kundert wrote on Aug 17^th, 2011, 3:46pm:

I don't recommend following BOE's suggestion. That is a recipe for convergence problems. I have seen two approaches to this problem that works well. One is to carefully model current/voltage limiting. You can use http://www.designers-guide.org/VerilogAMS/functional-blocks/regulator/regulator.... as a model. Alternately, you can model a two terminal clamp that you place in parallel with the current source. It would have infinite impedance within the compliance range of the current source, but then would look like a small resistor outside that range. It would be designed so that the IV characteristics were piecewise continuous.

-Ken

Thanks Ken. I read those 2 pages from your book (which was only about arms length away) and implemented 2 boundaries instead of 1 and it seems to work like a champ.

Thanks again.

-Dan

Back to top

IP Logged