MP5042 current limit and behaviour

We are experiencing unexpected behaviour from the MP5042. The current limit is around 40 % lower than expected. Our board has two outputs with one MP5042 on each output. Rlimit is 3.6k which should result in a current limit of 148 mA. On the one board we’ve tested so far, we’re getting a current limit of 97 mA on one output (called P10) and 85 mA on the other output (called P20).

We get the same result on P10 with a 114 ohm load, an 82 ohm load and during start-up with a real load with a switched power supply. With a 114 ohm load, the current shouldn’t even be limited since 15 V would give only 132 mA which is below 148 mA minus the room temperature tolerance of 7 %.

Conditions:

  • VIN is 15 V (rising from 0 V during start-up)
  • C(dVdT) is 1 uF
  • EN is tied to VIN via a 330k resistor
  • There is a 10 uF capacitor at each output
  • Testing was done at room temperature



A second question I have is about the hiccup mode described in the datasheet. It says the MP5042 will turn off its output for 90 ms if it experiences 2 ms of overload. But what is overload? In the tests we’ve done it doesn’t turn itself off until it’s been in current limit for much longer, 0.3 to 0.4 seconds in the attached images. This is a good thing for us but we would like to understand the behaviour so that we can rely on it.

Hello @niklas.angare,

Could you try decreasing the resistance until you see roughly the right current limit? could you also try testing how accurate the current limit is at 250mA with Rlimit floated and at a higher current like 500mA. This will help me figure out if this accuracy issue is only happening at small currents.

As for the hiccup mode, it can be triggered by either by a current that exceeds Ilimit, or by overtemperature. However, during startup Ilimit is used as the setpoint for the control loop to regulate the internal FETs and thus the output current/voltage. This is because it is expected that there will be capacitance that needs to be charged up at the output, so this is used to limit the inrush and charge up the caps slowly. Once the chip sees that the output is charged up then the normal Ilimit behavior begins. Try testing using load after powering on the E fuse and charging up any capacitance and see if that has any effect on the Ilimit value.

let me know what the results of testing are.

Best regards,
-Kerr