Power Switch

[Anand P Paralkar]

We have a power sequencing requirement in our system.
Power will be switched on to different parts of the system
at certain time instances through a power switch.
The arrangement is quite simple:

Power In ----------

-----------| |--------> Power
| Switch | Out

----------
^
|
From Controller


Power In : 3.3V, 500 mA
Power Out : 3.3V, 500 mA

I need some advice on the choice of the switch:

1. Is it advisable to use a suitable transistor
as a switch or is it better to use a switch
IC (the kind that Micrel makes).

2. In case of a transistor used as a switch,
what are the design considerations.

Thanks,
Anand

 

[Tim Wescott]

Anand P Paralkar wrote:

We have a power sequencing requirement in our system.
Power will be switched on to different parts of the system
at certain time instances through a power switch.
The arrangement is quite simple:

Power In ----------
-----------| |--------> Power
| Switch | Out
----------
^
|
From Controller


Power In : 3.3V, 500 mA
Power Out : 3.3V, 500 mA

I need some advice on the choice of the switch:

1. Is it advisable to use a suitable transistor
as a switch or is it better to use a switch
IC (the kind that Micrel makes).

2. In case of a transistor used as a switch,
what are the design considerations.

Thanks,
Anand

A switch IC is good when you need something that will work over the full
voltage range and will conduct current in both directions. For what
you're doing a power MOSFET will probably do, and will probably give you
a lower channel resistance in a smaller package.

Not knowing what else is in your design I'd say you want to find a
logic-level P-channel MOSFET. You'll have a trade-off between channel
resistance and size, but you can get an SO-8 part with a 20m-ohm channel
resistance. This will drop 10mV at 1/2 amp, which you should be able to
work around.

I just found one part: On Semiconductor's NTMS10P02R2. There's scads of
them out there, you'll have to find the one that's best for you. Look
at Vishay, On Semi, Zetex, Fairchild, Phillips, etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com