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# LMH6321: Problem in negative feedback loop

Part Number: LMH6321
Other Parts Discussed in Thread: INA818,

Hi TI Technical Support Team,

I have designed discrete constant current charge discharge circuit. I am adjusting current between 1mA and 10mA using DAC (output voltage corresponding to current). My aim is as follows: I would like to set charge current between 1mA - 10mA by setting DAC output  between 2.6V and 3.5V. And to adjust discharge current between 1mA and 10mA, I am setting DAC output between 2.4V and 1.5V.

When I am using 1k ohm resistor instead of battery to test circuit, the circuit works as expected. For example, my aim is to achieve 1mA charge current, then I am measuring 100mV across 100 ohm current sense resistor and 1V in the output of INA818. When I am setting -1mA current by setting DAC out to 2.4V, I am again measurinv  -1V at the output of INA818 and -100mV across 100 ohm current sense resistor. But when using battery instead of resistor, I cannot achieve my target value. For example to achieve 1mA current value I am setting DAC out to 2.6V, but I am measuring 0.170mV across current sense resistor (or 2.670V at the inverting input of the opamp U1A). I also applied different set points using DAC (between 2.4V and 1.5V and also between 2.6V and 3.5V) and it worked as expected when using resistor to test the lopp instead of battery. But when I am connecting battery (3.6V rechargable Li-ion coin cell), the loop fails and opamps noninverting (DAC input U1A) and inverting (current feedback U1A) input do not become equal. I am guessing maybe the loop is not negative (but then why it works with 1k resistor value?)

Could you please help me to identify the reason why the circuit fails to work with battery and also if possible, suggest me some solutions.

Ahmet

NegativeFeedbackProblem.pdf

• Hi Ahmet,

like this?

ahmet_lmh6321.TSC

You still should carry out a phase stability analysis, though.

Kai

• Hi Kai,

Thank you for your response. Yes, the circuit works in the simulation as I expect, but for example in practice, if I connect coin cell battery instead of V9, the loop fails. The inverting and noninverting inputs of U2 opamp donot become equal. I am not scanning the VG1 input, I only set the input once and measure output. Maybe there is component issue. I will replace LMH6321 and opamps with new ones and check if it works.

Regards,

Ahmet

• Hi Ahmet,

Can you provide some measurement details from the failed circuit? How far apart are the inputs of U2?

Best,
Sam

• Hi Sam,

With battery connected, I am setting U2 noninverting input to 2.6V and measuring inverting input as 2.670V. There is 70mV difference between opamp inputs. Actually, before there was 380mV difference between U2 inputs (set point was again 2.6V and inverting input was 2.980V). But I replaced the buffer (labeled as U3 in simulation file) and voltage adder (labeled as U4 in the simulation file) opamps with new ones, and then the difference dropped to 70mV. But in all cases connecting resistor (1k ohm) instead of battery made the circuit work. The U2 inputs became equal.

Regards,

Ahmet

• Hi Ahmet,

why are you using this fast 110MHz OPAmp LMH6321 in this rather slow circuit? This can cause stability issues.

Kai

• Hi Kai,

My final goal is to convert this circuit to device that will be used to characterize the battery. For example I will sweep the DC voltage of battery between initial and final voltage, and while sweeping I will measure the resulting current. I am expecting very low current (micro level) outside battery plateu voltage, and high currents (depending on battery chemistry and scan speed it can be 50mA) in plateu region. Opamp output current may not be sufficient for me sometimes, so I thought to add buffer to the opamp output that has simple connections and current limit like LMH6321.

I have not made any stability simulations yet. I am reading some TI notes and trying to figure out how to achieve this. Because in my final circuit there will be some switches (mostly reed relays) that will be used for current ranging and I do not know how to include them in the spice simulation.

Today, I replaced opamps in my circuit with new ones and the circuit worked. It seems one of the opamps was damaged or faulty.

Regards,

Ahmet

• Hi Ahmet,

what is the maximum current the buffer has to provide? And is it important to have a current limit?

A reed relay can easily be modelled by assuming a certain "on" resistance in the mOhm range in the "on" state and some pF between the contacts in the "off" state. The datasheet of reed relay will give the details.

Kai

• Hi Kai,

The maximum current can be at most ±100mA (charge, discharge). Current limit is not important, because as long as I am selecting correct sweep rate and correct initial and final voltage, the current can not exceed this value.

Thank you for your reed relay suggestion. I will use it.

Regards,

Ahmet

• Hi Ahmet,

Just want to confirm, the circuit is working after replacing the amplifiers?

Best,
Sam

• Hi Sam,

Yes, the circuit worked as expected after replacing the opamps.

Regards,

Ahmet