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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.


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.


  • Hi,

    1. The first condition when power supply is plugged in instead of the battery , power supply is feeding the output and also changing the feedback voltage. A schottky diode between the output and power supply should help.
    2. In the second condition, PGOOD doesn’t pull down since battery is connected to output which is feeding Vout and keeping vout regulated. PGOOD will remain high until Vout is in regulation.  Probably depending upon charge in the battery sometimes PGOOD gets pulled down as the battery discharges. Connecting a schottky diode in series between the output and battery should prevent battery to back feed the output in case of no input supply.

    Please note it is recommended to use external battery protection circuit such as LTC4070 or any other battery protection circuit depending upon your battery chemistry and configuration. Also LTC3105 is a boost converter so it should not be operate in higher operating voltages of the solar cell (which you mentioned is 5V) more than the output voltage. You can also try another part LTC3106 which might be more suited for your application as it is a buck-boost converter and has a battery trickle charger.


  • I have a very similar situation, except my solar cell MPP is at 4V.  Anisha states that the boost converter should not be operated in higher operating voltages of the solar cell (which you mentioned is 5V) more than the output voltage. 

    I did not see that particular consideration in the data sheet for the 3105.  Can you please clarify and give guidance?  Will the 3106 be a better match for MPP of 4V?

    I will be charging a 3.7V Li-Ion, 2200mAh.  My solar panel measure 5.8V open circuit, and 30-34mA short circuit.

  • Hi,

    The datasheet mentions that LTC3105 is a step up converter, however if Vin is higher than Vout, the part will not fail. Vout will be able to maintain regulation although it will be highly inefficient. So in that respect it is not recommended to operate in that region. Also if Vin operates within absolute max rating but beyond the specified input voltage range in that case the part might survive from failing however its performance or behavior cannot be guaranteed or relied upon.


  • Hello All,

    Thanks for your answer Anisha.

    I had so many things to besides this project that i had to put aside. Now coming back I'm going to:

    1 - First test the diode suggestion in series with battery;

    2 - I'll make sure solar in any condition won't be above 5V, even if I need to replace that;

    3 - Circuit is behaving well, but eventually I'll replace with LTC3106;

    I'll return this answers here.



  • Hello,


    1 - Circuit works well with diode in series on the battery. I also adjusted the voltage feedback to be 0.3V above the dropping voltage on diode. So I can charge the batteries with 4.2V. It works well! Even though I have the diode, I can't use PGOOD, and I decided to keep measuring the LDO voltage instead (2.2V). Since my circuit has a peak of 10mA when i shoot to TX, PGOOD oscillates and turn OFF, but LDO keeps (2.2V). I believe my solar is not capable to delivery that current, so it drops voltage, but it returns back when the current goes back to uA. 

    2 - Solar does not reach 5V;

    3 - I'll think about that.

    So far the logic is working for me and circuit works well.

    Thanks All!!!