Hello there,
I bought an STEF01 eval board STEVAL-EFUSE01 to test the IC out, as it is a very good fit for my upcoming project.
I am having some issues starting the device into higher loads. It looks like the current limiter kicks in a little bit too early. Before going into details, let me present my configuration and setup.
- Supply voltage is 24V
- I have added a 47uF 35V electrolytic capacitor to both the input and output
- Resistor Rlim is 100 ohms. This gives a current limit of approx. 1.8A according to datasheet. This limit has been verified.
- I have tried different values on Cdv/dt
- Load used for testing is a purely resistive load
After doing some testing, i verified that the current limit of 1.8A behaves as expected, so no problem there. The problems start when i try to start the device with a resistive load that gives an output current of greater than 950mA. In this case the device limits the current to 950mA, and the output voltage varies depending on what load resistance is connected.
For the pictures below, each color represents the following signal during a startup with a load greater than 950mA
- GREEN = Vin (24V supply)
- YELLOW = Vout
- BLUE = Iout, measured using a 360mOhm series resistor
- PURPLE = EN/FAULT signal
Cdv/dt = not mounted
Cdv/dt = 10nF
Cdv/dt = 47nF
I have also tested with Cdv/dt = 100nF, same result.
So i guess here comes my question; why is the circuit limiting the current to 950mA with a Rlim resistor of 100 ohms?
- According to the datasheet, the current limit should be approx 1.8A with an Rlim resistor of 100 ohms
- It cannot be the Maximum dissipated power protection feature according to datasheet section 5.2.2, in this case the internal power mosfet should be turned off and the EN/FAULT signal should be pulled low, which is not happening. This is also why different values of Cdv/dt was tested.
- The output rise time is correct according to the datasheet and the Cdv/dt value that is mounted. My assumption was that the foldback current limit activates, but this can not be the case because of the different values tested of Cdv/dt and foldback current with Rlim = 100ohms is approx 400mA based on datasheet.
Any suggestion to why the circuit is behaving this way?
