I installed the Lithium cell charger chip and it’s associated components. While I was soldering the components, I noticed that R5 wasn’t soldered correctly. This is the current limiting resistor for the voltage boost circuit. So I need to retest the boost circuit.
I then attached the cell and the radio led blinked once.(with no connection, this was expected) I connected the USB from my computer into the circuit and D2 (Red) lit up. This is STAT1 signal from the charger IC. From Table 5 in the AAT3672 datasheet STAT1 on by itself indicates the system is fast charging the lithium cell. I disconnected the Cell with the USB still connected, both D3 and D2 blinked until I reconnected the lithium cell and the system then went back to fast charge.
I grabbed my DMM and checked some voltages:
From USB: 4.65V (A Little low, but I have connected the Uprogrammer to a long USB cable for convenience)
Output to Board: 4.65 V (Matches input voltage)
Lithium Cell: 3.96 V (Good range for Fast Charge)
These voltages make sense, I am very happy with these results. I waited a while to check the results again. While I was waiting, I started doing some testing of the Voltage Boost Circuit. With Just the boost circuit turned on, I measured 4.64 Volts on Vpp. I checked the Duty and prescaler and they were set to 0 and 4 respectively.
I then set the duty cycle and measured the voltage at Vpp
20% : 19.6 Volts
30% : 20.8 Volts
40%: 21.8 Volts
50%: 22.6 Volts
I played with the prescaler and the highest voltage I got was 24 Volts at 50% duty cycle and prescaler set to 9. This is beyond design specification and has the potential to cause damage to the circuit, I don’t expect to do this in the future. I am happy to know that I have some margin in the design to if I need 20 Volts. I took the following image from my oscilloscope with a Prescaler of 4 and a duty cycle of 10% (25).
I am not happy with the large steps setting the output voltage of the boost circuit. I decided to load the circuit with a 10 K Resistor to see how it affected the output voltage. I soldered a 1206 10K Resistor on top of C20. This lowered the output voltage for 10% with a prescaler of 4 down to 12.8 Volts But the best voltage I could get out of the system was 18 Volts. This also made Vpp a lot noisier(See scope image below), I want to add some more filtering.
I added a resistor to the schematic parallel to C20 and also a place for another capacitor. The resistor I set the value to 20K as a starting point and the capacitor I set to 1 uF. The 20 K resistor will draw less current and that should reduce the noise. The capacitor will also reduce the noise and provide a larger reservoir for current changes when programming a target device.
After doing all this testing the lithium cell voltage was at 4.12 V. This is near a complete charge, I expected the system to go to complete charge very soon. This is indicated by the Green LED being on alone.
After it switched to charge complete, I grabbed my DMM and checked some voltages:
From USB: 5.05V (Minimal current draw so no Voltage drop through the USB cable)
Output to Board: 5.05 V (Still matches input voltage)
Lithium Cell: 4.19 V (4.2 Volts is maximum charge voltage for individual LiPo Cells)
The charging circuit is working as expected. I disconnected the USB and the LEDs turned off. I re-connected the USB and the Red LED came on for a few minutes and then it went back to only the green LED on.
I have uploaded the updated schematic to GitHub, click the hardware link in the right hand column to go get it.
Do you have a circuit you want to test before layout? Do you have a design you are tinkering with?