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LM66100: 2 stage power supply selector, but only one works??

Part Number: LM66100


I have an application with an energy harvester (U_GEN) delivering in best case about 4.1 VDC. With this voltage a Supercap is charged; the SuperCap's voltage is then transformed by a DC/DC switcher TPS61099YFF to 3.5VDC (U_CAP). Both voltages are connected to a first wiring OR stage (double LM66100); this part of the electronics works well. The output delivers the expected voltage (U_GEN-CAP) which is connected to the input of a second Wiring OR stage (also double LM66100); here the voltage to compare comes from a coin cell CR2032 (max. 3.15V). The load at the output of the second stage is between 0.05...10 mA.

Now I see 2 problems:

1. When no battery is in place the voltage U_GEN-CAP is not fed to the output of the second Wiring OR stage (+U_F1)

2. When no harvester or SuperCap energy is available the battery voltage does not appear at the second Wiring OR stage (+U_F1)

I can't find the problem as the second stage is designed identical to the first one which works well. Do you have any idea? Are the pull-ups R5 / R12 correctly chosen?

Thanks in advance.

Best regards,


  • Hi Matthias,

    I think the issue here may be due to CE left floating inadvertently on the second stage devices (D3 and D4).

    Is it possible to add a large pulldown resistor of 500kOhm or maybe even 1Mohm  on the inputs of these devices and retest this? 

    Another option may be to test D3 and D4 separate from the D1 and D2 inputs by using TP7 to supply some voltage.