I've be thankful if somebody can help me with my energy harvesting board problem.
I've created an energy harvesting board based on the step-ups Linear Technology LTC3105 and LTC3525-3.3.
Below please find the schematic and board design.
A small 3V solar panel charges via the LTC3105 a 2.7V 3F Supercap up to max 2.66V.
The LTC3525 steps up the voltage from the supercap to 3.3V that goes to the load.
An SPDT slide switch is between the supercap and the LTC3525, to turn off the board (no energy consumption from supercap).
I want to use the Shutdown feature of the LTC3525 and disconnect the output when the supercap voltage is less than 0.9V is and turn on again when it is 2.3V.
So I connected the SHDN pin to the supercap via a voltage divider with 100K and 75K resistors.
I've tested the board using a 100Ohm Resistor as load and also with a 3mm white LED after a 100Ohm Resistor.
I've charged the supercap to 2.6V. I've connected the load and I see 3.3V on the ouput until the supercap reaches 1.47V and the SHDN on LTC3525 is at 0.62V, then LTC3535 disconnects the output (0V).
Then I wanted to test the output reconnect. So I've discharged the supercapacitor down to 0.7V and then charged it, while monitoring its voltage and the output of the LTC3525. The LTC3525 reconnected the output at 1.47V.
This is not as it should be, because the LTC3525 should disconnect the output when V_SHDN is less then 0.4V and reconnects when more than 1V.
I have an other board but with SHDN of the LTC3525 connected to its V_IN (shutdown feature disabled) and the board can provide 3.3V on the output until the supercap reaches 0.7V (same load). This means the problems is not caused by too low voltage of V_IN of LTC3525.
This also means the Shutdown feature on the other board works, but for some reason it reacts to the wrong thresholds.
Does anybody have an idea why?
LTC3525 Datasheet LTC3525 Datasheet
Thank you very much and happy new year!
Sincerely
Best Answer
Wrong assumptions and interpretation of datasheet specs.
This means it switches On OR Off at some threshold of 0.6V with a worst case tolerance of +0.4/-0.2V. ( Disabled active low or negative logic)
When they define a logic level, for safety margin from noise they define a "1" above some level and a "0" below some level. "1">=1.0V and "0"<=0.4V but it has some real threshold that is variable in between.
So SHDN should not be used as an accurate Ultracap charge comparator.
It does not mean what you said... "This is not as it should be, because the LTC3525 should disconnect the output when V_SHDN is less then 0.4V and reconnects when more than 1V."
The other board switches OFF and ON @ 0.7V so this is close to typical.
You need a better comparator to control SHDN.