current sensing amplifier

Sam,

Most of our products should be able to handle this application, so this comes down to accuracy needs and feature sets:

- do you need an analog output, or is a digital output preferred? Note that with our digital devices you can also perform measurements such as power monitoring, Coulomb counting, and more. 

- what level of accuracy do you need here over the measurement?

- do you need any integrated features, such as an integrated comparator for OCP, etc?

Off the cuff, I would recommend checking out the following devices for analog and digital approaches. If neither of these hit the mark, some additional context to my above questions would be helpful.

Analog: INA240 or INA190

Digital: INA228 or INA229 (these are I2C and SPI, respectively)

Sam,

Based on the above, I would check out the devices I mentioned in the first post for analog out: INA190 and INA240. 

The shunt selection is going to be based on your optimization between power and signal integrity. Given the smaller nature of your current load here, I would probably recommend maximizing the shunt for best case performance here. As an example, using the A1 variant of the INA240, the shunt choice would be 

RSHUNT,max = VOUT,max/(ILOAD,max*GAIN) = (5V-0.2V)/(900mA*20V/V) = 267mΩ

Therefore I would choose a something like a 250mΩ shunt for standard values. With a 250mΩ shunt, the performance of the INA240 from the calculator tool located on the product page looks like this:

For a common mode voltage of 5V, the iNA240 should provide a <0.5% total error measurement for the totality of your measurement range. Keep in mind this does not take into account any external sources of error, such as the shunt tolerance. 

The worst case power dissipation in the shunt would be (900mA)^2*250mΩ = 203mW. You can always reduce the shunt value to consume less power here at the expense of error.