LiPo charger MCP73831 indication LED - Page 1
Forum section: electronics / beginners

author: khiworld
author_rank: Newbie
timestamp: September 23, 2019, 04:24:40 am
content: Hi.I am using the MCP73831 Battery charger for my design. I decided to add 2.7k ohm resistor at the PROG pin. So, Ireg is set to 370mA.LTST-C195KG, dual color chip led is connnected between the VDD and STAT pins for status indicator.Pin2 and 4 - REDPin1 and 3 - GreenWhen testing the circuit, I got three different cases..1. The red LED lights when charging and green when finished.2. The red LED is on when charging and the red light wont turn green.3. Red while charging, both Red and green when charged.Any thoughts?

author: Peabody
author_rank: Super Contributor
timestamp: September 23, 2019, 05:11:39 am
content: What is the function of the resistor labeled "21" which appears to be a load resistor? You've used the MCP73831, which has no load sharing circuitry. So you can't have any load current while the battery is charging - it can confuse the charger if the load current is more than the cutoff current to terminate charging, so it would never stop charging. That could be case #2. I don't know what #3 is all about.The MCP73871 has load sharing, which permits running the device while charging the battery. It might be a better choice.You can't have the charger not terminate charging. That's a safety issue because it can lead to explosions.

author: khiworld
author_rank: Newbie
timestamp: September 23, 2019, 05:38:12 am
content: Hey Peabody,Thanks for the reply. The labeled "21" is not a load resistor. It is the symbol for the Li-ion battery. I am sorry to confuse you.

author: Peabody
author_rank: Super Contributor
timestamp: September 23, 2019, 01:58:34 pm
content: Sorry I was confused.According to the datasheet, scenario #1 is the normal result when charging is successful. Scenario #2 would indicate that charging has not completed. Scenario #3 occurs in shutdown mode, which is when the supply voltage is too low. See Section 4.

author: khiworld
author_rank: Newbie
timestamp: September 24, 2019, 12:30:19 am
content: Yes, I agree with you opinion. but, I would like to figure out what causes case #2.While charging, LDO(MIC5504) is working..It means that the current is drained from LDO so the charge never ends.. As you can see the attached file, the input current of LDO is sum of I_load and I_GND.The amount of current drained from LDO cannot be ignored....this is what I am guessing..If I am right, the solution would be adding pmos between J2 and C3.Please correct me.

author: Peabody
author_rank: Super Contributor
timestamp: September 24, 2019, 03:38:28 am
content: I think this goes back to my first reply. The charger terminates charging when the charging current drops to 10% of the normal charging current set by the PROG resistor. If there is a load on the battery, and hence on the charger, which exceeds that level, charging will never end. Lithium batteries cannot be continuously charged like that. The solution is a "load sharing" circuit which allows the 5V supply to directly power the LDO so the battery will continue to charge in isolation until termination. That circuit is normally made up of a p-channel mosfet, a resistor, and a diode. That circuit is built into the MCP73871, but not your73831. This app note shows the circuit:http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdfAny time the supply voltage is connected, it supplies both the charger and the load, independently. Only when that supply is disconnected does the battery power the load.