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OR Mosfet circuit with high current consumption

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bmalbusca

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I'm using the mosfet circuit presented below to switch the power supply in the PCB (low power). The problem is the current consumption during the ON state of P1, which is 0.5mA to keep-on the pmos (The current nmos drains is controlled by the resistor R). The current is controlled by the resistor R. However, due to the noise levels, I'm not able to increase the value of that resistor in order to decrease the current drain (I tried experimentally other values such 15k, 20k, ...). I began to design a circuit to disconnect the pull-up using pnp when PMOS is turned on, but the circuit became complex and possibly more current consuming. Maybe there is a clever way to do it.

Resuming the circuit functioning: S1 and S2 signals can be used to drive the P1 pmos. When S1 or S2 are both high, the pmos' gate is set to 0V, and the pmos turn on. When S1 and S2 are both low, 5V appears at the pmos' gate trough R pull-up.



ORcircuit.png
 
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Hi,

you say "high current" ... what range is this current? And where in your circuit does this current flow?

*****
I see two major currents:
* the load current: What range is it?
* the resistor current

******
In detail all depends on your requirements.
* input voltage range (btw: where do the 5V come from?)
* idle current
* ON current
* voltage drop across P1
* switching speed / frequency
* S1 / S2 voltage levels.
(please: numbers with units. No text)


Klaus
 

Hi,

you say "high current" ... what range is this current? And where in your circuit does this current flow?

*****
I see two major currents:
* the load current: What range is it?
* the resistor current

******
In detail all depends on your requirements.
* input voltage range (btw: where do the 5V come from?)
* idle current
* ON current
* voltage drop across P1
* switching speed / frequency
* S1 / S2 voltage levels.
(please: numbers with units. No text)


Klaus
Hello @KlausST,
Thank you for the note. I'm trying to reduce the current to keep-on the pmos, which flows through the resistor R and nmos drains. At this moment the current drain is 0.5mA limited by R, however for a low power design and comparatively to other circuits in the same board is considerable high. I was wondering if there is an clever way to reduce this current.
 

Hi,

yes, surely there are better ways.

You say you can´t increase resistor because of noise. I´ve done many projects - even with 6000A RMS switching nearby - thus I can´t believe that you can´t go higher than 10k.

Tell us your requirements and I will give you a suitable solution.

Klaus
 

Hi,

yes, surely there are better ways.

You say you can´t increase resistor because of noise. I´ve done many projects - even with 6000A RMS switching nearby - thus I can´t believe that you can´t go higher than 10k.

Tell us your requirements and I will give you a suitable solution.

Klaus
Ok, thank you once more. The requirements are:
- P-MOS Source voltage: 5V (from DC\DC output)
- ON current (P-MOS Drain): 140mA
- Voltage drop across P1: <100mV
- Idle current: <0.1mA
- Switching speed / frequency: N.A
- S1 / S2 voltage levels: 3.3V
 

Hi,

Why worring about 500uA while the load draws 140mA. It´s not worth to worry about it.

For your requirements:
I´d use a 74AHCT1G02 instead of R, N1, N2

Klaus
 
If noise is the reason you can't raise the value of R, maybe you should look at your noise source. That has to be an incredibly aggressive noise if it can turn a MOSFET on.

Is this battery powered? If not, then, as Klaus says, why are you worrying about a current that's less than 0.5% of your load?
 

Use a CD4000 or 74HC series CMOS NOR gate to drive the MOSFET (will turn on MOSFET when either control input is high).
Requires no resistor, draws only leakage current, and will be highly resistant to noise.
 
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If noise is the reason you can't raise the value of R, maybe you should look at your noise source. That has to be an incredibly aggressive noise if it can turn a MOSFET on.

Is this battery powered? If not, then, as Klaus says, why are you worrying about a current that's less than 0.5% of your load?
Hello @barry, yes, PCB is powered by a small LiPo battery.
The noise seems to came from the dc\dc switching and the Wifi module. However, I have isolated grounds planes for HF and LF; and I took attention to the return current paths as well adding ferrite and caps to the DC\DC output.
--- Updated ---

Thank you both @KlausST, @crutschow and @barry, Indeed, the logic gate solution consumes less power than my mosfet circuit with logic gate always turned on (the current leakage of the transistors is close to the logic gate's current consumption).
 

Hi,
or 74HC series
Sometimes I see this... but 5V powered 74HC series is not compatible with 3V logic input.

--> use at least 74HCT series.
***

LiPo: are you switching the (3.7V nom) LiPo voltage, or do you have a 5V boost circuit?
If direct LiPo, then 74HCT is no suitable.

Klaus
 
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