[MAX14914A]: Short-Circuit and Overload Protection

As it states in the datasheet, we define a short-circuit current as "over 3x higher than the set current limit..." In case of short-circuit, the output will be active for about 4ms and then turned off. However, the current limit is not a short-circuit current. It still takes place whenever the load current exceeds the current limit. There is some delay when the circuitry compares the load current with the current limit settings before the output driver actually limits the current. And you are right, the output driver (MOSFET) is in active state and heats up. It can be in the current limit conditions for much longer than 4ms, depends on the internal and ambient temperature. There is another protection mechanism implemented: if the MOSFET temperature exceeds the limit, it will be completely shut off.

Best Regards

Dear YuriyK1,

Thank you for your answer.

I performed more tests but now with loads exceeding the 3x current limit: 30 and 20 ohms (corresponding to 0.8 A and 1.2 A respectively). The current limit of the EV kit is always 200 mA. You can see in the images below the input and output voltages for CH1 output voltage (DOI); CH2 input voltage (TP10). The behavior is the same as before: the MOSFET is always in the active region whether the load current is less than 3x or greater than 3x.

Part of your answer was: "... In case of short-circuit, the output will be active for about 4ms and then turned off." I didn't find that in datasheet (datasheet says that the rise and fall time are modified, but never that turned-off). After 4 ms of being turned on, the MOSFET does not switch off, I didn't measure that.

1) Could you clarify this point, please?

Additionally, during the current limit phase, the voltage value is somehow chaotic, like if the current's control loop is unstable

2) Do you have any explanation about that?

3) In any case, what is the risk to leave a permanent over-load/short-circuit? The MOSFET could remain at a high temperature without reaching the thermal protection. Is there a life time reduction?

Thank you and best regards

JFR

Waveforms for 30 Ohm and zoom at turn-on

Waveforms for 30 Ohm detail of current limit zone

Waveforms for 20 Ohm and zoom at turn-on

Dear JFR,

1) In the data sheet we have the following description of the short circuit protection, "Short circuits at the DOI output generates high transient current until the active current limiting kicks in. In order to protect the MAX14914_ against high currents that can be seen over an extended time, especially if the output is switching at a high rate into a short circuit, the MAX14914_ enters a protect mode. When the MAX14914_ detects that the DOI current is over 3x higher than the set current limit, the driver is switched to protect mode with reduced turn-on slew rate of the rising and falling edges for a duration of 4ms. The FAULT signal does not become active and the chip operates normally, but with reduced slew rate. If the cause for the short circuit is not removed, the protect mode will remain for an additional 4ms until the short circuit is removed."
If the driver temperature exceeds the thermal shutdown threshold of 170degC, that will lead to turning off the DOI output. Thus, DOI driver is always active except thermal shutdown.

2) That "chaotic" output behavior in the current limit phase is the result of combination of thermal noise and auto-zeroing (AZ) circuitry. The AZ structure helps to cut down offset and low frequency noise (e.g. 1/f). 

3) The MAX14914 can be under current limit conditions for weeks if the junction temperature does not exceed 170degC. Of course, high temperature reduces life of the chip. You can refer to the Reliability report here max14914.pdf (analog.com).