Choosing opamp of 500 mA output

Hello Vijeendra,

I suspect the issue is the output swing limits of the OPA547. Your input voltage on the low end is +0.4 V, when gained by +2 V/V, that places the output +0.8 V above the 0 V negative supply rail. The minimum negative swing is rated at (V-) + 1.6 V for -0.5 A, and (V-) + 1.2 V for -0.1 A. The output might not be able get down to the required (V-) + 0.8 V needed by your application even at lesser output current levels. Indeed the fuel gauge would hit some minimum setting represented by the maximum negative output swing of the amplifier.

All of the power operational amplifiers that I reviewed will have similar output swing limits. The limit comes about because of the output transistor's drain-source, or collector-emitter on resistance. When the output current flows through that on resistance a voltage drop is developed preventing the output from swinging all the way to the supply rail.

The best approach in this case to overcome this is to provide a negative supply level for V-; then the OPA547 would be completely appropriate for this application. There are buck/boost regulator ICs that are perfectly suited for converting a positive supply input, to a negative output voltage. They offer a great deal of flexibility in terms of input voltage, and output voltage settings. One device that I have used, can supply up to 1.5 Amps of current and works well is the MC33063A. Here's the link to the data sheet:

http://www.ti.com/lit/ds/symlink/mc33063a.pdf

Figure 10 shows the MC33063A as a voltage-inverting converter. The Applications Information table on page 10 provides the equations needed to design an inverting step-down regulator. My thought is you would start with the existing +12 V supply as the input and then set the output to a convenient, standard level such as -3.3 V, or -5 V, to be used as the OPA547 V- supply.

Also, there is another device that could work to provide a negative supply; the TPS54350. Figure 27 in its data sheet shows it being applied as a voltage-inverting  converter. The data sheet can be seen at:

http://www.ti.com/lit/ds/symlink/tps54350.pdf

Please note that these two Power Management products are supported by another organization within TI. If you have any questions about applying them, that group should be able to assist you on their E2E forum.

Regards, Thomas

PA - Linear Applications Engineering

Hello Thomas,

Thanks for clarification.  We are planning to try with Buck/Boost regulator or op-amp driving FET in active region.

A question on Op-amp supply current :

OPA547 data sheet says Quiescent current is +/-15 mA(Max) and my load current is 500 mA.  That means my busk /boost regulator should supply minimum of 500+30=530mA to the op-amp?

Thanks.

Hello Vijeendra,

Good question! Driving the OPA547 non-inverting input more positive relative to the negative supply rail will cause the output to source current to a grounded load. That load current, plus the positive operating current, would be supplied by the V+ supply. Only if the output were required to sink current through the load would V- be required to supply high current. Therefore, it looks like the buck/boost regulator need only supply enough current to keep the negative operating current supplied.

I would select a regulator that has some current capacity margin. That would help keep the V- voltage more constant, and could help supply any extra current that might be needed for transient response.

Regards, Thomas

PA - Linear Applications Engineering