I think I've messed up again with the N-Mosfet.

The idea is to disconnect the stepup regulator (U3 = LTC3525D) when the output of the op-amp (U2 = MAX9015) is low, and viceversa.

The problem is that the LTC3525 V_OUT is a strange 0.98V instead of 3.3V, when the V_IN is 2.2V and the op-amp out is high. Between GND pin 2 of LTC3525 and GND of C4 (supercap) I measure 1.9V.

I am not sure if I connected the N-mosfet properly. Do you see any mistake from the circuit in the picture?

Thank you very much for your help.

Sincerelyenter image description here

  • 1
    \$\begingroup\$ Your schematic is VERY hard to read and doesn't follow good circuit design practices. Voltage nets should go on top, GND bottom, and logic flows left to right. \$\endgroup\$ – Jim Mar 20 '18 at 23:39
  • \$\begingroup\$ I can't follow the circuit either. \$\endgroup\$ – Henry Crun Mar 21 '18 at 0:11
  • \$\begingroup\$ Just use PMOS. It will be so much easier. There are always sneaky pathways to GND. At this voltage and power level PMOS works great. \$\endgroup\$ – mkeith Mar 21 '18 at 2:28

I can't follow the circuit very well either,

but I guess you are trying to disconnect the ground of the power switch using the N-Ch.

R8,R9 are dividing the gate voltage, but you probably need 6V on the gate to turn the N-ch on properly. i.e max9015 needs 24V supply to get enough Vgs

I can't really understand the circuit. Your opamp seems to be powered by the output of the switchmode it is turning off. You are using a fet for some reason, when the LTC has a shutdown pin.

Your question really needs a lot of editing to make sense....

  • \$\begingroup\$ Sorry guys for the unclear explanation and schematic. Your comments have helped me anyway. \$\endgroup\$ – haku15 Mar 21 '18 at 23:00
  • \$\begingroup\$ That voltage divider with R8 and R9 was unintentional. I wanted R8 to limit the current to the gate and R9 as pulldown, but I connected R9 after R8 instead of before. This means, I had 0.53V V_GS at the mosfet and it was off. I've removed R9 and now V_GS is 2.1V, as it should be, but that does not seem enough. At 2.1V V_GS, I_Drain is about 0.1A and probably not enough to start the LTC3525 properly. I'll have to test with other mosfet, but have none now. In case I will still have problems, I'll create new question. Thank you very much to you all. Sincerely \$\endgroup\$ – haku15 Mar 21 '18 at 23:04

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