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ATL431: Exists a nanoamp shunt regulator to clamp voltage of indoor solar cell?

Part Number: ATL431

Is there a shunt regulator (or other part) that will clamp to 3.6V the voltage from an indoor solar cell  (say Amorton 1815 generating a few or tens of uA in dim light of say 200lux)?

In typical conditions, the solar cell V operating is low enough, but in stronger light the solar cell can generate a max voltage of say 5V (Voc open circuit of 8 cells@0.6V).  That is higher than the Vmax the load (an mcu) can tolerate.

I have looked at the TL431 but the "Imin Minimum cathode current for regulation" of 400uA leads me to believe it would not work.  (But I have no experience with shunt regulators.)

I have searched for a zener but can't find one that has an Ir low reverse leakage of less than a few uA and a sharp knee at Vz or Vr of 3.6V.  I have evaluated a TVS diode (for ESD protection) but it didn't seem to clamp the voltage.   Also, a TVS seems designed for a pulse, not for constant current (but I figured that the package could withstand the say 200uW from the solar cell.)

I have tested the BQ25504 (which will clamp the Vbat output voltage) but I am looking for a simpler solution.  (If I wanted to minimize the area of the solar cell, I would use the BQ25504.  But most of the available amorphous solar panels for indoor light are already larger area and higher voltage than I need, not requiring the boost or MPPT of the BQ25504.  Also, I am using a tantalum capacitor, not a super cap or battery which might require protection functions of the BQ25504.)

My application is nano power, duty-cycled radios.

  • HI,

    The ATL431 behaves the same way as the TL431, but requires a minimum cathode current of 35uA (max) for regulation. As long as your cathode current is above 35uA and below 100mA the output voltage will stay very close to the set value.

    Below is a link for an application note that goes over how to set the output voltage of this type of shunt regulator.


    Below is another application note that goes over specific details for the ATL431 stability.


    Regards, Diego Lewis
  • In reply to Diego Lewis:

    The app note seems to say "not guaranteed to regulate at Ika less than 35uA" but the solar cells produce less than 10uA.

    Also, as a solar cell sees an increasing voltage, the current goes down (see "maximum power point") so even less likely enough current for the shunt regulator to work?

    I suppose I should just experiment. Or use a different solar cell such as the new AM1606 whose Voc is 3.6V.

    In the application, an IoT radio such as CC2650 or MP430 whose Vmax is 3.6V, powered by a solar cell without a battery, the mcu can clamp its own Vcc by shedding power to say an LED. But I am looking for a fallback, non-software solution (something simpler than the BQ25504.) I also use the TPS3839, which uses nano amp power, and hoping you had a nano power shunt regulator.