TI bq24210 Lithium Ion Charging IC Implementation - Page 1
Forum section: electronics / projects

author: Bushougoma
author_rank: Regular Contributor
timestamp: February 22, 2017, 12:07:07 pm
content: A little background I have a low power circuit that I want to power with a lithium ion battery. It will be located outdoors and it would be nice if the battery could be recharged by a solar panel.I've chosen the TI bq24210 charging IC to handle the charging of the cell.Here is the datasheet:http://www.ti.com/lit/ds/symlink/bq24210.pdfThe circuit will be directly connected to the battery at all times so the circuit will run while the battery is being charged. Load mode on this IC (which connects the battery directly to the VBUS (charge input) pin internally when the input voltage (VBUS) is less than battery voltage (BAT) is disabled by tying the /PG (power good) pin which is an open drain MOSFET to the /EN (enable) pin this logic input is tied to battery positive with a 10k pull up resistor which holds the enable pin high when the FET isn't conducting.So essentially when /PG is low /EN is low and the battery is charging. When /PG is high /EN is high the chip is disabled and the battery is disconnected.It's my understanding that the typical blocking diode in series with the solar panel won't be necessary since the battery will be disconnected from the panel by the charge controller when the panel voltage (VBUS) drops below battery voltage (BAT).Here is a crude schematic (required bypass capacitors omitted):In my example I have a 3.7 volt 1000 mAh lithium ion cell. So I'd be aiming for a constant charge current of 500 milliamps using 0.5 * C.Using a gain factor KISET (395) from the datasheet divided by the constant charge current I can calculate the programming resistor value 790 ohms (round up to the next standard value).Here's that equation:However that leads me to my first question the factor KISET varies depending on different conditions as shown here:What conditions is it describing here? The last two factors seem to be describing the precharge current which is 20% of the constant charge current. For example at a 50 ma constant charge current the precharge current would be 10 ma (50 e-3 * 0.20 = 10 ma).Is it saying if your constant charge current is 50 ma or under (<50ma) use 400 if it's greater than 50 ma but under 250 ma (>50ma <250ma) use 395 and if it's greater than 250 ma but under 800 ma (>250ma <800ma) use 390?Another thing I don't completely understand what they call the "Input Voltage Based Dynamic Power Management (VBUS-DPM)".Here is a description and the equation:From what I understand if the input voltage drops below this programmable voltage threshold (set by a resistor) the IC will reduce the charge current (from the programmed value) to prevent the input voltage from dropping. I take it this would happen if the panel wasn't receiving enough light to maintain the set charge current. Therefore if this circuit didn't exist the input voltage would drop and the battery would attempt to discharge itself through the panel?My question is should I program this threshold to just above the fully charged cell voltage of 4.20 volts at say 4.40 volts? Or should I leave the VDPM program pin floating without a resistor which will put it in VTRK (battery tracking) mode in this mode the threshold voltage will vary depending on battery voltage. At a battery voltage of 3.4 volts or lower the threshold will be 3.75V. At a battery voltage above 3.65 volts the threshold will be Battery Voltage + 100mv.I'm leaning towards leaving the VDPM pin floating which will allow a variable threshold based on battery voltage since the DPM threshold is one of the parameters that must be met (input voltage must be higher than Battery Voltage + DPM threshold) for the /PG power good signal to go low which is responsible for enabling the chip via the /EN pin. It is my understanding that once /EN goes low the battery will begin to charge.With the way I have the /PG and /EN pins connected I believe if the input voltage drops below the DPM threshold /PG and therefore /EN will go high effectively disabling the chip, terminating the charge, and disconnecting the panel from the battery. So the DPM input current limiting should never go into effect and the chip should only enable (/PG and /EN low) when it's capable of charging the battery right?Thanks for any help you can offer.