What is the simplest way to add low-side overcurrent protection to L6206?

[Baz Huffington]

Posted on January 28, 2017 at 23:20

I am designing a board with L6206 drivers for solenoids.  My board is using connectors with a 3-amp per pin maximum, so I chose the 6206 for its ability to set a conservative 3 amp shutdown limit on the high side using an external resistor.  Unfortunately these parts do not have over-current or short protection on the low side.  What is the best, simplest way to augment the typical application circuit for protection on the low side?  I am using the part initially as four separate half-bridges for driving solenoids (so the high-side current limit is split between two solenoids on each bridge), and would like to protect the chip, by deactivating the low-side switches, if the output gets shorted to a 12V supply.

Here are a couple ideas:

1. A comparator on a low-side sense resistor pulling the enable pin low alongside the high-side overcurrent shutdown pin;

2. An NPN transistor hanging off the low-side sense resistor, pulling the enable pin low

3. A chip-based solution such as the L6506

I looked at the app node

http://www.st.com/resource/en/application_note/cd00003789.pdf

  where the author suggested something similar to option &sharp2, but using an SCR, and then a one-shot.  These solutions probably stem from the lack of Schmitt triggers on the L6201-3.  Since the L6206 has Schmitt triggers on the enable pins, I'm assuming a simple NPN tugging on the enable pin, with sense resistor chosen for 0.7V at the overcurrent level, would do the trick.  I'm planning to add small inductance (10uH) to each half-bridge output to guarantee enough time for this protection to act.  Is this necessary?#l6206 #current-limiting #short-circuit-protection

[Enrico Poli]

Posted on February 01, 2017 at 15:27

I think the solution using the sense resistor and a comparator is the better one because allows you to use smaller resistor value. In fact the 0.7 V threshold imposed by the NPN solution implies a resistor of about 233 mOhm --> 2 W of power dissipation when the load approaches the 3 A limit.

Please consider also this point: when you are protecting against the short to ground the current will recirculate on the sense resistor generating a

negative voltage

. The higher the shunt resistor value, the higher the negative voltage. 

Whatever the solution you select, take care of the robustness on the comparator/NPN to the negative voltages.

Adding a small induction in series to to output is a good idea, in particular to make more robust the short circuit protection.