TLV3501 and SN74LVC1G123: -

Hello,

With any CMOS device, you should always have the inputs terminated at either Vcc or GND if not being used. This is easiest accomplished with a pull-up or pull-down resistor.

Floating inputs can account for very large increases in supply current, and other abnormal behavior.

What pins are in use on the LVC1G123? What is done with the unused pins?

What are the component values and applied voltages?

Can you share scope shots of this system operating?

R0= 100 kohm, R1= 2 kohm, R2= 10 kohm

C1= 100 pF, C2= 100 pF

Vin is an analog signal with the amplitude from 40 mV to 1.5-2 V.

Also in SN74LVC1G123 monostable multivibrator:

Pin-2 (B) is considered as its input, pin-1 (not A) is connected to GND, pin-3 (not CLR) is connected to 5 V supply, pin-6 (Cext) is connected to GND

Scope shots cannot be achieved, but it seems the SN74LVC1G123 monostable multivibrator IC stops working.

Shirin Pourashraf said:

I did not say I did not use any pin.

That's unfortunate because that would have been an easy problem to solve. We've at least eliminated floating inputs as a source of added current though.

That 54mA might be coming from some kind of oscillation in the circuit -- I can't think of any other reason you'd see that much supply current from this circuit.

Without scope shots of the device operating, I'm afraid I won't be of much help.

It sounds to me like you have a system design issue that needs to be troubleshot.  I'll forward this along to our team that handles to TLV3501 to see if they have any inputs that might be helpful.

Shirin

by the look of the circuit, it looks like you have the comparator connected in a negative feedback loop.  This could potentially put the output stage in neither a high or low output state.  If the output stage is in some non-saturated output state, this could be explaining the higher than expected current draw.  Oddly, we frequently make the comment that amplifiers can be used as comparators but that is not generally recommended.  The same holds true for comparators; it is not recommended to put comparators in a linear output region (ie used as amplifiers).

Chuck

Thanks for the reply, 

So you think if I change the configuration (swapping positive and negative inputs) and put the comparator in positive feedback, the problem will be solved?

that changes the polarity of the response; it may reduce your current consumption but that will change how the circuit responds

if you need the comparator output to go high with a rising edge input, you cannot make that change.  I am just saying that using a comparator with negative feedback is more of an amplifier application.

Chuck

I understand it changes the polarity. I am just curious it will fix the problem!

Please note that those times that the circuit is working the current is 2-3 mA, but sometimes specially when I first turn on the 5 V supply then I turn on the Vinitial (please see the schematic), 54 mA which is about the limit current of comparator, passes through the supply to GND. My guess is that this much of current is a lot for Monostable Multivibrator IC and burns it out?? 

Shirin

if the comparator is consuming extra power, this does not impact the other device.  So I am not sure that I am understanding what you are asking.  The comparator will just output levels in the range of the supply.  However, in a negative feedback configuration, the device may be outputting a value that is somewhere between the two supply rails.  I do not know if that is a problem for the other device.

Chuck

The SN74LVC1G123 is a CMOS logic device, outputting either 0V or VCC at the Q output, depending on the input state.

The input is high-impedance, with the exception that if the input goes below -0.5V, a diode will activate clamping the input to about -0.7V. As long as the comparator is outputting between 0 and 5 V, you should not see excessive input current to the SN74LVC1G123.

For the configuration Shirin has explained, a low -> high transition (rising edge) will trigger the device to output a pulse of approximately 1us.

The only other issue I could think of that might cause a malfunction would be attempting to retrigger the device too quickly - there is a limitation on the retrigger time:

I doubt that this is the case though - at 5V operation with a 100pF capacitor, the retrigger time should be just a few nanoseconds.

Shirin - do both devices have bypass capacitors placed at their supply pins?

Emrys

I was just concerned that connecting the comparator with negative feedback could cause the output stage to be in the linear range (neither high or low).  How would the SN74LVC1G123 respond in that case?  I guess there is a small possibility that it gets stuck right in the middle of high and low.  Could that cause the SN74LVC1G123 to overheat somehow?  I am asking because it sounds like the SN74LVC1G123 is getting damaged and our comparator will not output signals below V- which I assume is ground.

Chuck

Hey Chuck,

The SN74LVC1G123 can draw some extra current when an input is held between the rails, however the B input that Shirin is using has a Schmitt-trigger architecture, so I wouldn't expect just that one input to cause overheating, even if it was held at Vcc/2 for an extended time. It's designed to handle very slow inputs and would draw something around 4mA of extra supply current.

I don't think just having a slow input could account for the failure.

If the input were held at a mid-voltage value (ie not high enough to reach the positive input threshold), then the input might not trigger and the circuit could get 'stuck.'

It would be extremely beneficial to get scope shots of this so we could actually troubleshoot and stop guessing.