LTC3105

HI,

I'm new here. I was planning to ask for a Field application Engineer to come and check this doubt with us, but let me give a try here.

I have a circuit using the LTC3105 similar as last schematic page 14 of datasheet with some differences.

I have a battery 3.7V - 300mA lithium connected on VOUT. My feedback is adjusted to charge with 4.2V, and I have a 5V 30mA solar panel on input.

I'm not using the LDO, so i have just a 4.7uF as cap there and left the pin open. My feedback coming from VOUT is going to FB with voltage divider, and at same time to my uC to measure this voltage on ADC.

My MPPC is set to 3.3V with 330k resistor.

My VOUT supply the power to my circuit. Which peaks 6mA when i'm transmitting. 

I'm using the PGOOD as indication when is day or night, like charging or not, but I'm facing issues with that.

Questions:

1 - Since my battery is direct connected on VOUT, if i remove the battery and i plug a power supply there and simulate battery condition, my FB expected voltage changes, and with that the PGOOD pulls down or not with day condition or charging condition while Vin is constant (solar) is more than 2.5V. Is there a reason for that? I was start thinking i should use a diode in series with my battery to isolate the battery voltage, but I'm really not sure at this point.

2 - I have a battery connected on VOUT. Voltage is 3.8V. I bring my device to sun. It shows charging because PGOOD goes HIGH. That's ok. Now i bring my device to dark. Vin is 0.2V, but PGOOD does not pull down. Is this because my battery is giving a feedback voltage constant there on FB pin ? Sometimes it pulls down, but i can't understand why i doesn't pull all the time as it should.

In other words i'm using PGOOD as my Solar indicator for charging or not on my circuit. PGOOD is connected to the uC with pull up internally. I'm sure it's ok because i can measure the 3.3V or 0.1V when it's low.

Thanks!

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