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Kai Klaas
03/26/06 16:28
  03/26/06 16:30

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Msg Score: +2
 +1 Good Answer/Helpful
 +1 Informative
#113161 - Using a MOSFET makes sense
Responding to: Bruce Grenade's previous message
Bruce said:
I wish to switch off (less than a couple uA) an IC when not in use. I am currently using a MOSFET, but it's expensive for my design. I have never seen a BJT used for this purpose before. Is there any reason I cannot do so? See the circuit Below:

The reason is, that BJTs can show very high collector emitter saturation voltage at higher collector currents. Also, collector emitter saturation voltage of BJTs suffers from very high manufacturing tolerances, as a look into databook proves.

An example:
BC337 (or BC817 as SMD) can handle collector currents of up to 1000mA. But collector emitter saturation voltage can be more than 1V at this current! Even at much lower collector currents, from 100A to 100mA namely, collector emitter saturation voltage can be higher than 0.15V. This would be a loss of supply voltage of 3%, which is much for a 5V powered application.
Of course, typical collector emitter saturation voltage is much lower, only about 0.025V at 10mA collector current, for instance, but you must check this first, which makes it more expensive than taking a MOSFET right from the beginning.

Bruce said:
Basically, Vcc is 15VDC. The IC is a MOSFET driver that switches a motor at high speed. I believe the driver uses about 10mA during normal use.

This is only the static current consumption!! Take care, the TA4427 is a 1.5A dual high-speed power MOSFET driver. So, for brief periods up to 3A can flow through the supply pins and through your turn-off transistor!! Especially if the 10F decoupling cap is a bit older or is omitted by mistake.

Other advantages of MOSFETs are:

1. They don't need static power to be controlled, just in opposite to BJTs.

2. They often contain already a protection diode from drain to source, which can be very helpful in certain applications.

So, you see, using a MOSFET makes sense.


List of 7 messages in thread
Switching an IC off using BJT?      Bruce Grenade      03/25/06 21:36      
   Try using a PNP        Russell Bull      03/26/06 00:49      
   Good Good      Bruce Grenade      03/26/06 09:16      
      "expensive" is relative      Erik Malund      03/26/06 10:55      
         Of course      Bruce Grenade      03/26/06 11:47      
   Using a MOSFET makes sense        Kai Klaas      03/26/06 16:28      
      Good Info      Bruce Grenade      03/26/06 17:03      

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