unknown op-amp current leak

Question

I've created a dual source energy harvesting board to store energy from small energy sources, like a 3V 15mA solar panel, into a 3F 2.7V supercap and to power a small wireless node (3.3V, 30mA), but I am having trouble finding the cause of a current leak. Schematic is below:

The voltage regulator LTC3125 (U1) gets the energy from the solar panel and steps the voltage up to 2.65V to charge the 3F 2.7V supercap.

The supercap is the input for the 3.3V voltage stepup regulator LTC3525D-3.3 (U3) and its output (3.3V) goes to the load (wireless node).

This regulator get shut down when the voltage of the supercap goes below 0.9V and starts if the supercap goes above 2.5V. This is done by the non-inverting op-amp with additional hystheresis, the MAX9015 (U2), that drives the shutdown pin of the LTC3525.

The op-amp is powered by two sources: if the sun shines, it is powered by the 2.1V regulated voltage from the LTC3105 (LDO, pin2). If no sun and supercap >0.9V, the power comes from the LTC3525D-3.3 output . If no sun and supercap <=0.9, then the opamp is off and so is the LTC3525. Usually the supercap is >0.9V. Min VCC of op-amp is 1.8V.

There is also a connection from supercap to VCC of the op-amp through diode D9, but I did this only to stay inside the "input command mode voltage range" of the op-amp (VCC+0.2V). Without this, I had a current leak, when the sun was not shining because of op-amp latchup.

Everything works fine except for a current leak that happens when:

• 1.24V < V_Supercap < 2.5V

• Solar input voltage < (V_Supercap + (0.2V or more))

• LTC3525 not started (no 3.3V output)

In this situation, the supercap discharges circa (44uA-60uA) instead of charging (0.01mA or more).

If I disconnect the solar panel, there is no current leak. If its voltage is similar or higher than supercap voltage, the supercap charges.

I suspect the cause is something in the op-amp, as its internal V_REF = 1.24V and I use this directly connected to the IN- of the op-amp. The V_REF output impedance is 35kOhm, and the current leaks is exactly V_Supercap / 35kOhm. The IN- and V_Ref are at 1.24V as soon as the supercap >= 1.24V.

What could be the cause of this and how can I solve it? Or maybe is the problem somewhere else?

Thank you very much for your help.

U1 (LTC3105)

  AUX  =  1.49V   
  V_OUT = 1.49V  
  SW    = 1.07V   
  V_IN  = 1.07V   
  FB    = 0.53V
  LDO   = 1.49V   
  MPPC  = 1.39V

U2 (MAX9015)

  REF=IN- = 1.23V
  IN+     = 0.72V
  VCC     = 1.44V
  OUT     = 0V 

U3 (LTC3525)

  SHDN    = 0V
  V_IN    = 1.49V
  SW      = 1.49V
  V_OUT   = 0V

PSU (to simulate solar cell):

 1.10V 30mA 

PSU current draw = 0mA.

Supercap:

 1.49V discharging at 44uA
 If I turn PSU off, supercap discharges with 12uA.

Answer 1

U1 (LTC3105)

. FB    = 0.53V      AUX  =  1.49V   
  LDO   = 1.49V      V_OUT = 1.49V  
  FBLDO =    0V      PGOOD = open
  SHDN* = open       SW    = 1.07V   
  MPPC  = 1.39V      V_IN  = 1.07V 

If PSU off , V_IN on U3 is open and C4=3F/2.7V
- V_OUT/(R4+R5//R6)=1.49/(300k+560k//620k)=2.48uA
- V_OUT/(R1+R2)=1.49/(1M+1.65M) = 0.56uA
- above = 3uA total
- supercap discharges with 12uA.
- D9 leakage = ? with Vf= 1.49-1.44=0.05V

• REMOVE D9 to isolate then remove any other paths for causes of leakage

U2 (MAX9015)

. REF   = 1.23V      n.c.
  IN-   = 1.23V      Vcc   = 1.44V  
  IN+   = 0.72V      OUT   = 0V 
  Vee   =    0V      n.c.

U3 (LTC3525)

  SHDN    = 0V       SW    = 1.49V
  GND     = 0V       GND   = 0V  
  V_IN    = 1.49V    V_OUT = 0V

PSU (to simulate solar cell):

 1.10V 30mA Supercap: 
 1.49V discharging at 44uA