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Author Topic: question about disabling a circuit  (Read 1349 times)
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jeanninemtv
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« on: April 19, 2012, 12:40:35 00:40 »

hello

so i'm again with another question ...

I was trying to choose a QUAD opamp (or more exactly a comparator) with a 'chip enable' option in order to save power in a battery operated mcu system with multiple opamp signal conditioning stages...  but as i found in datasheets most of the comparators sold (microchip mcp series of opamps for example) have just a single opamp model for each family with that option. (and in maxim as i searched is something similar)

How risky could be to use a mosfet switch to open/close the power rail of the quad chip then disabling/enabling it by doing this?   can i use a simple mosfet switch? or is not a good way to do that?

thanks in advance!



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Faros
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« Reply #1 on: April 21, 2012, 03:32:21 03:32 »

Yes , Jenny you can use a MOSFET as a switch to Shutdown part of a circuit, provided that you go after the following:

1- Use High Side P-channel MOSFET , the N-channel will not be appropriate since it will create another Ground point higher than the common circuit Ground by Vds.
2- The P-channel should be logic gate voltage level type, the standard MOSFET needs higher voltage and will not be directly driven by your MCU.
3- The P-channel should have an Rsd ON as low as possible to minimize the drop across MOSFET drain-source.
4- Choose the MOSFET  Vgate that match your working voltage (i.e. 3.3V MCU requires lower Vg than those operating at 5V).

The good news is that such MOSFET are now available and doesn’t cost much. For example the ROHM - RTQ025P02TR - MOSFET, P, 20V, 2.5A will switch at Gate threshold voltage = -0.7V and full switching to 1A at Vg = -2V, it just cost £0.073 Qty:1 (Farnell).

Attached is a Proteus circuit example, please check and keep me posted; I am always available for my old friends.

Regards,

Faros.
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jeanninemtv
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« Reply #2 on: April 21, 2012, 09:16:04 09:16 »

Smiley  ok then my supposition was not wrong ...  but i didn't knew about the benefit of using the P instead of the n one. (And the low Vsd of your simulation is nice ^^ ) Smiley

just three offline questions:
1:  is marked 'low voltage drive 2.5V'  in the datasheet, so, it means its suitable to work with the 5V Vdd and logic levels from the mcu.
2:  in the case of not using the mcu, can i bypass the 100K resistor if i use a dipswitch?  or i can just put the smd dipswith tied to Vdd to select manually the disabling of the circuit (i'm affraid about the max current of the typical dipsw , and as i understand i must not tie it to Vdd  to switch a load that use > 200mA?
3:  i suppose you tested the pmosfet solution on real application.  how problematic is  the startup current peak on switching?


BTw!  thanks a lot for the explanations! 
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zab
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« Reply #3 on: April 21, 2012, 11:37:53 11:37 »

Hi,
There should be a resistor across drain and gate to keep it off in no connection mode. just disconnect the control point, the 100k resistor will switch it on. I suppose this resistor should be at gate drain pins.
N type Mosfet are used for switching power lines as well but these require special switching arrangements like mosfet drivers. These are mainly used for high power.
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solutions
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« Reply #4 on: April 21, 2012, 07:21:26 19:21 »

300nA....http://www.linear.com/products/micropower_comparators

No switch needed
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SteveyG
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« Reply #5 on: April 22, 2012, 12:24:02 00:24 »

Yes , Jenny you can use a MOSFET as a switch to Shutdown part of a circuit, provided that you go after the following:

1- Use High Side P-channel MOSFET , the N-channel will not be appropriate since it will create another Ground point higher than the common circuit Ground by Vds.

Could you elaborate slightly on this point? I'm not sure I fully understand the point you're making. Thanks Smiley
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Faros
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« Reply #6 on: April 22, 2012, 02:02:49 02:02 »

1:  is marked 'low voltage drive 2.5V'  in the datasheet, so, it means its suitable to work with the 5V Vdd and logic levels from the mcu.
Yes; the logic level MOSFET’s are designed to use the logic level to completely set the N-Channel ON or the P-Channel OFF (note that they are reversed). If your P-channel MOSFET has 2.5 V switch, it means that it will completely change status to OFF when the gate is driven by 2.5 V; suitable for the 3.3V or even the 2.7V MCU operation.

2:  in the case of not using the mcu, can i bypass the 100K resistor if i use a dipswitch?  or i can just put the smd dipswith tied to Vdd to select manually the disabling of the circuit (i'm affraid about the max current of the typical dipsw , and as i understand i must not tie it to Vdd  to switch a load that use > 200mA?
Unlike the bipolar transistor (current driven device) the MOSFET’s are voltage driven; accordingly if you provide the necessary voltage then the MOSFET will switch regardless of the current,  you can directly drive the gate with DIP switch and the current will be in the Micro-Amps range.

3:  i suppose you tested the pmosfet solution on real application.  how problematic is  the startup current peak on switching?
The inrush or peak current depends on the nature of load you are driving (inductive/capacitive); both ways; MOSFET’s as a rule has peak current values several times its continues rated current (refer to the datasheet of the selected transistor for this particular value).

I usually use the MOSFET to switch a complete section of the circuit to reduce its power consumption; not only a comparator chip, for example I can switch OFF  a set of  comparator;  Vref,  Voltage dividers, comparator pull up resistor and finally the load connected to it.  Linear Technology is great but their chips are overpriced,  the suggested 300 nA is priced 60 times the cost of low power version of common quad comparator.

Could you elaborate slightly on this point? I'm not sure I fully understand the point you're making. Thanks Smiley
If you checked the attached circuit; you will find that the comparator GND is equal to Vds (typical Open Drain); Voltage references will not be correct in this case and the troubleshooting will be a living hell for unaware technician to figure it out.  

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hate
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« Reply #7 on: April 22, 2012, 06:58:04 18:58 »

How risky could be to use a mosfet switch to open/close the power rail of the quad chip then disabling/enabling it by doing this?   can i use a simple mosfet switch? or is not a good way to do that?

I don't think that's a good idea. There will be lots of other things that you'll have to take care of unless you are using the opamps for some very basic operations (very low frequency 1s). I second Solutions here to use a low current consumption opamp instead of trying to add a MOSFET switch.

Regards...
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Regards...
jeanninemtv
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« Reply #8 on: April 23, 2012, 01:35:51 01:35 »

For example i want to enable/ disable from startup a  specific anaog circuit because i'm making a general purpose acq board for funny experiments Smiley wirh some onboard circuitry that i want to enable disable by a sipsw o  r with the mcu . That's why i'm talking about switch on or off the corresponding rails Smiley

 Best regards
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solutions
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« Reply #9 on: April 23, 2012, 05:33:32 05:33 »

Use jumpers if it's not a dynamic thing, if you must. 

You need to be really careful with some of the schemes suggested here...while they may work a few times for the hobbyist, some supply disconnects while powering inputs can latch the chip up, possibly burning it up.

For what you are doing you'll have more than several comparators worth of slop in the power dissipation of your microcontroller...just leave them on, enjoy the SNR, rail to rail, and PSRR you'll get by NOT switching them in and out actively and just select their outputs as you change modes.
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blauwhaar
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« Reply #10 on: April 30, 2012, 07:45:18 19:45 »

Possible solution, use for every circuit an separate regulator with ENable input.

http://www.ti.com/product/tps76933#feature

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slvs203e&fileType=pdf

May be this is what you can use to disable some current consuming circuits.

Best regards.

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