CN3703 Three-cell lithium battery series step-down charging circuit

Most charging chips on the market use single-cell charging, and multiple cells need to be connected in parallel. However, many portable power devices, such as juicers, electric drills, etc., use 12v voltage supply, that is, 3-cell 3.7v 18650 The batteries are connected in series to 12v, and there are many solutions for charging them. The CN3703 charging management chip I use here.

CN3703 is a PWM step-down mode three-cell lithium battery charging management integrated circuit that independently and automatically manages the charging of three-cell lithium batteries.

Detailed parameters and functions can be found in the data sheet. I will talk about the main functions based on my PCB here.

Temperature detection and enablement: R5 is a negative temperature coefficient SMD 0805 thermistor. The resistance is 10kΩ at normal temperature and 3.5kΩ at 50 degrees. This way the chip has high temperature protection.

If you are careful, you will find that if the resistance drops, charging will stop. If the TEMP pin is directly connected to GND, charging will stop. Then you can connect an N-channel MOS tube to R5 in parallel. Control the start and stop of charging. But I didn't do this function.

Selection of inductor and current sensing resistor: There is also a choice regarding the inductor. It is recommended to use power inductor L2 and the package is CDRH74. L1 is a chip inductor, not a power inductor. L1 is used in testing. In actual installation, L1 does not need to be installed with an inductor.

R1 is the current sensing resistor, which is based on this formula:

For example, here, I want the charging current to be 2A, and my charging voltage is 15V, then the L2 inductor is 20uH, and the current sensing resistor is 0.2/2=0.1Ω.

Indicator light:

LED1 is the charging indicator light, it is recommended to install red

LED2 is a full power indicator light, it is recommended to install a green

When the battery is not installed or the charging voltage is insufficient, the two charging indicators will flash.

Q1: Finally Q1 is a P-channel MOS tube, I use IRF9Z34N

Note: The charging voltage must be greater than the battery voltage, otherwise it cannot be charged, and the two charging indicators will flash. The one I use is 15V. It is recommended to use a power supply greater than 15V for charging.

Advantages: fewer peripheral components, charging current up to 5A, battery temperature monitoring function

Disadvantages: The biggest disadvantage is that it needs to use a voltage above 12.6V for charging (however, the manual actually says 7.5V to 28V), because the IC is charged at a reduced voltage.

The picture above shows the twin LED lights flashing when the voltage is insufficient.

The picture above is a picture of charging completed when the voltage is 12.6V, and the charging completed indicator light is on.

So is there any solution that can charge 3 lithium batteries using only 5V? Of course there is, just add a boost! Of course, it is not possible to add a separate boost circuit, which would cause a bit of loss. I will share it with you again if I have time.