SM3320-BATT-EV-NOPB Datasheet, PDF (16/21 Pages) Texas Instruments – AN-2121 SolarMagic™ SM3320-BATT-EV Charge Controller Reference Design

Charging a Li-ion Battery
www.ti.com
• R103 and R104 set the voltage at the input of the microcontroller. The voltage at the input of the
microcontroller is:
R₁₀₃
V_A12 = V_BAT x _{R103 + R104}
(2)
• R103 and R104 should be chosen so that the maximum expected battery voltage creates a voltage
close to 5V to maximize resolution (but less than 5V to avoid saturating the measure).
• R51, R52 and R53 are for the voltage measurement of the SM72442 and should be modified in the
same way:
R₅₃
V_AVOUT = V_BAT x _{R51 + R52 + R53}
(3)
• R21 needs to be set to zero ohm (short).
• Once the values are picked, the proper threshold needs to be programmed through I2C. The
maximum level (0x3FF) is now V_AVOUT = 5V at the input of the SM72442.
• Finally, the overvoltage protection should be adjusted to:
R₇₂
V_{HARD_OVP} = V_BAT x _{R71 + R72}
(4)
• The OVP level is set at V_{HARD_OVP} = 5v.
2. The proper voltage setpoints and charging curve need to be programmed in the microcontroller. The
initial voltage limit is set by R28 and R38. Voltage limit setpoint is AVOUT = A0. Once overridden
through I2C, the voltage at A0 is not used anymore. Hence, there is the option of setting the value
through resistors R28 and R38 or by programming it from the microcontroller into SM72442 through
I2C each time the SM72442 is reset/powered.
3. Proper current limits also need to be set if required by the battery model. The current limit value is set
when the voltage at pin 3 of U11A equals the voltage at pin 2. Hence, R111 and R112 will need to be
adjusted accordingly.
4. The software needs to be changed to follow the Li-ion charge control profile: battery voltage is set
either by hardware as stated above, which requires no action from the software, or it is set from the
microcontroller through the I2C interface similar to the Lead Acid battery.
5. Finally, the software needs to include the full State-Of-Charge cut-off: When the battery reaches its full
voltage and current has dropped below 500mA (can vary depending on battery), charge is cut-off and
the battery is considered fully charged (no trickle charge of Li-ion batteries should be done). It is
important to remember that current can drop below 500mA during the charge when solar power
becomes unavailable (low light intensity). Therefore the charge cut-off needs to be programmed to
occur only when the battery voltage is at the limit AND current has dropped below the required
threshold.
Figure 19 shows the typical charging profile for a Li-ion battery.
Voltage Limit
Voltage
Current
Charge
Cut-Off
time
Figure 19. Li-ion Charge Profile
16
AN-2121 SolarMagic™ SM3320-BATT-EV Charge Controller Reference
SNOSB76C – December 2010 – Revised May 2013
Design
Submit Documentation Feedback
Copyright © 2010–2013, Texas Instruments Incorporated