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REG103: Foldback current

Danilo A.
Mastermind 26260 points
Part Number: REG103

Hi Team,

We have received this inquiry from our customer.

I need support on the REG103UA-3.3. I need details on how the foldback current works. I have an application that I had to add the 5K boost resistor and need to understand why. I have the LDO driving a GPS chipset (UBlox) and an external active GPS antenna. Depending on the cable length to the external antenna and number of adapters used, we get into a situation where the LDO does not start up. When I add the 5K ohm boost resistor from input to output, it functions normally. I need to be able to explain why this fix works and why it did not work in the original configuration. So, I need to know how the foldback protection works. If there are any application notes please include them.

Regards,

Danilo

  • 0
    TI__Genius 13265 points

    Hi Danilo, 

    Foldback current limit reduces the allowable current when the output voltage is very low, this helps reduce power dissipation and the amount of current drawn from the input supply during a fault condition. However, some older devices like REG103 allow the output current to be pulled down to 0A if the output voltage is pulled to a negative value (see below diagram showing the concept using a simplified Vout vs Iout curve). 

    So in your case it seems like the output voltage is being pulled negative in certain scenarios. You mentioned an external antenna and various adapters, which makes me think that the inductance of the current path and antenna may cause the voltage to go negative during shutdown which could charge any capacitance on that node to a negative value. This is why adding a 5k between Vin and Vout then allows the device to startup, since that provides a discharge path for that negative voltage to be dissipated.