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Original question:

UC2845: current limit for ISENSE pin

UC3845: UC3845/ISENSE Overshoot problem

Mandar shitole
Prodigy 10 points
Part Number: UC3845

Hello,

We are currently using one of your DC-DC converter ICs (UC3845) for a 5V/21Watt power supply.

This power supply is primarily employed in a telecommunication system where noise is a significant issue throughout operation.

To be more specific, we have observed noise at the output voltage. This noise is primarily generated during changes in duty cycle,

specifically at the following and rising edges. Despite numerous simulations, we have identified the root cause of this issue to be overshooting at the ISENSE pin.

In our attempts to address this problem, we have added an RC filter and a slope compensation circuit. However, we are still encountering this issue. 

Could you please provide guidance on how we can effectively reduce the spike from the ISENSE pin?

Thank you and looking forward to your response.

  • 0
    TI__Mastermind 32775 points

    Mandar,

    If you have excessive noise spikes on the CS signal that cannot be reduced by RC filtering, you can try introducing LEB (leading edge blanking). An example is shown in Figure 3-1, here. Makes sure you are measuring the CS signal directly across the RC filter capacitor and that the capacitor is placed as close to the CS pin as possible. Make sure you are using a short GND connection on your scope probe and measuring the signal using tip and barrel technique. If you are convinced you have an optimized RC filter, have introduced LEB and are measuring the signal using best practices, the next step might be to investigate reducing the transformer leakage inductance and a careful review of your PCB layout. 

    Regards,

    Steve