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The uc2845a has the VCC pin rated at 30V, but there is an internal 34V zener at this pin. Is the VCC input tolerant to more than the 30V rating due to the internal 34V zener? What is the purpose of the zener for design purposes?

Other Parts Discussed in Thread: UC2845A

The uc2845a has the VCC pin rated at 30V, but there is an internal 34V zener at this pin. Is the VCC input tolerant to more than the 30V rating due to the internal 34V zener? What is the purpose of the zener for design purposes?

  • Hi Dave,

    The abs max rating for the Vcc pin is 30V from a low impedance. Above 30V there will be a risk that the internal 34V zener, at the low end of its tolerance range, could start to conduct current and dissipated power causing the device temperature to rise if Vcc is fed from a low impedance source.

    In the simplified diagram on the first page of the datasheet you can see that the Vcc pin is connected to Vin. When Vin is applied the cap on the Vcc pin is charged through this resistor and the internal zener will clamp the voltage on Vcc. Figure 10 in the datasheet shows some typical values for these components.

    Regards

    Peter