Current sensing and overcurrent protection circuitry with LT1999

[Kryptychon]

Hi,

I'm trying to design a current sensing and overcurrent protection circuit based in the LT1999-20 (20V/V gain)

The requirements are:

1. Must be able to sense current in both directions
2. Overcurrent shutdown threshold must be adjustable and it must pull the EN1/EN2 lines to GND as fast as possible (should protect MOSFETS on a h-bridge by disabling the driver IC)
3. Provide a scalable output signal for further processing with an external MCU's ADC to realize current depending acceleration limiting etc. Scaling the output seems desirable so the supply voltage of the LT1999 can be idependent from the external ADCs ref voltage)

The circuit I got so far measures two voltages from two shunt resistors with two LT1999.
The following circuit should amplify / attenuate the LT1999's output signal for further processing and it should pull the EN1/EN2 lines to GND if the overcurrent threshold is reached.

I've a couple of questions:

1. Is my approach to realize an adjustable gain via an OpAmp okay?
2. The LM358 seems unsuitable to me because of it's slow slew rate and the low GBW (compared to the LT1999). Am I right?
3. What's you opinion regarding the "external processing"? Should I integrate the ADC directly into the current sensing circuit and use e.g. I²C for communication with the MCU? I imagine transmitting digital signals is less error prone than doing so with analog signals.

Besides my concrete questions, I really appreciate any feedback from you.

Thanks!

Kryp

[Terry Bites]

INA300?
see page 24. You can set the trip point with a resistor or with a control voltage on the LIMIT pin.

[Kryptychon]

Hi,

thanks for your reply.

The INA300 looks quite nice as over current protection device. First I thought it cannot be used in a bi-directional current flow setup, but reading the data sheet shows this application is possible with two oINA300 which is nice.

To clarify my requirements: I want this current sensing and over current protection board to protect a double h-bridge (See my other post here: https://www.eevblog.com/forum/projects/double-brushed-motor-controller-feedback-appreciated/).

So, with the INA300, the over current protection part is much easier to implement than on my original design. Thanks for that!

Regarding the LT1999 output signal scaling, I just realized, that digitizing it directly on this board might be the better approach. As the LT1999 and the ADC share the same Vcc/Vref, the output of the ADC is always ratiometric. Ensuring that with a "distributed" setup might be a little harder.

To sum it up:

- I'll ditch the LM339 in favour of the INA300 for the protection part.
- I'll use a ADC to digitize the LT1999's outputs and do the communication between ADC and MCU via a bus

What would be a appropriate ADC for this purpose?

Thanks in advance!

Regards
Kryp