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Negative voltage supply 7660 voltage drop

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wolf12

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I'm using a 7660 voltage converter to get -5V for a force sensor. The problem is when I'm loading the force sensor the negative voltage starts to drop by 5mV for 50kg. I think 7660 cant handle the current(200uA). I need to measure around 500kg. I think the voltage might drop a lot more for 500kg. I'm using an op-amp for the force sensor. I'm using this circuit with Vt = -5V.

https://www.tekscan.com/sites/default/files/flexiforce-circuit.jpg

Can you suggest me a solution or a different negative voltage converter that can handle the current around 10mA-100mA.

Thank you.
 

You need a stable voltage reference for the minus voltage. For a negative voltage you could use a shunt reference such as one of these. For a -5V supply you would need a lower voltage reference such as 2.5V for operating headroom. You would then need to double the value of Rf to maintain the same output gain.

To avoid generating a negative voltage you could use a positive voltage reference to the force sensor and use a non-inverting op amp configuration to amplify the signal.
 
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    wolf12

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crutschow said:
You need a stable voltage reference for the minus voltage. For a negative voltage you could use a shunt reference such as one of these. For a -5V supply you would need a lower voltage reference such as 2.5V for operating headroom. You would then need to double the value of Rf to maintain the same output gain.

To avoid generating a negative voltage you could use a positive voltage reference to the force sensor and use a non-inverting op amp configuration to amplify the signal.
Click to expand...

Will this one handle the current?
http://www.linear.com/product/LT1634
http://cds.linear.com/docs/en/datasheet/1634fe.pdf

I'm not good at analog electronics. With -5V applied and with the inverting opamp configuration the voltage rises with increasing force. And its nearly 0 when no force is applied. If I connect FSR to a positive voltage reference (+5V), Will the voltage fall with increasing force?
What other changes I should do to the circuit.

Thank you.
 

On further thought, my suggestion to use a non-inverting amp will give a non-linear output, which you likely don't want.

The LT1634-2.5 minus 2.5V output goes to the sensor only and will handle the 2.5mA maximum current.

Below is the schematic.

Reference.gif
 
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crutschow said:
On further thought, my suggestion to use a non-inverting amp will give a non-linear output, which you likely don't want.

The LT1634-2.5 minus 2.5V output goes to the sensor only and will handle the 2.5mA maximum current.

Below is the schematic.

View attachment 93624
Click to expand...

I'm using a 3.7v li-po battery step up to 5v to supply power. With this configuration do I have to use another battery to supply the negative voltage?

Thank you.
 

crutschow said:
I thought you were using a 7660 to get the -5V(?)
Click to expand...

Yes I'm using it. +5v from lipo step up is used to power arduino. -5v from 7660 is given to FSR. So in your schematic instead of the 5v source, should I connect 7660? and the shunt in schematic is LT1634-2.5, should I use LT1634-5.0 for my FSR?

Thank you.
 

Yes, the minus -5V battery shown is the negative output from the 7660. You can't use a LT1634-5.0 since it won't regulate if the 7660 voltage drops below 5V. That's why you need the 2.5V device. You just adjust the op amp feed-back resistor to get the maximum output voltage you want with the maximum load (minimum resistance) on the FSR.
 
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Can you increase your pump capacitor values to obtain
a better load line? Then perhaps add a negative LDO
(with decently low ground current) for supply stability?
Eat some no-load headroom to get consistent bias out
to wherever you stop caring?

Can you up the 7660's switching frequency? Charge
per cycle and cycles per second, those are the two
main determinants of current-throw.

You might consider a negative boost converter IC
instead of the more current limiting cap pump scheme.
 
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dick_freebird said:
Can you increase your pump capacitor values to obtain
a better load line? Then perhaps add a negative LDO
(with decently low ground current) for supply stability?
Eat some no-load headroom to get consistent bias out
to wherever you stop caring?

Can you up the 7660's switching frequency? Charge
per cycle and cycles per second, those are the two
main determinants of current-throw.

You might consider a negative boost converter IC
instead of the more current limiting cap pump scheme.
Click to expand...

Can you suggest me a negative boost converter IC that doesn't require another battery?

crutschow said:
Yes, the minus -5V battery shown is the negative output from the 7660. You can't use a LT1634-5.0 since it won't regulate if the 7660 voltage drops below 5V. That's why you need the 2.5V device. You just adjust the op amp feed-back resistor to get the maximum output voltage you want with the maximum load (minimum resistance) on the FSR.
Click to expand...

Will it be same if I use 2.5V zener doide instead of LT1634-2.5?

Thank you.
 

Instead of keep struggling with negative supply, I would design a single supply circuit for the "force sensor", e.g. based on the concept of a 4 resistor OP differential amplifier.
 
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FvM said:
Instead of keep struggling with negative supply, I would design a single supply circuit for the "force sensor", e.g. based on the concept of a 4 resistor OP differential amplifier.
Click to expand...

I'm not good at analog electronics. Does this change the linearity of force vs voltage output? I'll give it a try, any additional information would be really useful.
Thank you.
 

I have used the MAX1044 which is eseentially the same as the 7660. I had the same problem as you are having with the negative voltage dropping as soon as you load the supply. I contacted Maxim and they pointed me to the graph in the datasheet which shows how the output voltage reduces with output current drawn. If you draw 10mA from the negative supply, your output voltage will drop by 500mV. You can use the graph in the datasheet to work out how much the voltage will drop for your circuit.

The advice I was given by Maxim was to use a more modern part. Newer negative charge-pump ICs will be more efficient. Note, you will probably have to use an inductor with modern ICs. The advantage of the MAX1044/7660 is that it doesn't require an inductor.
 

wolf12 said:
..............................
Will it be same if I use 2.5V zener doide instead of LT1634-2.5?
Click to expand...
It won't be the same. The zener will have much poorer load regulation and temperature stability than the LT1634-2.5. So it depends upon how much stability of the voltage you need, whether to substitute the zener or not.
 



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