BQ25120A Datasheet, PDF (48/67 Pages) Texas Instruments – Low IQ Highly Integrated Battery Charge Management Solution for Wearables and IoT

bq25120A
SLUSD08 – MAY 2017
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Typical Application (continued)
10.2.1 Design Requirements
This application is for a low power system that has varying loads from less than 10 mA up to 300 mA. It must
work with a valid adaptor or USB power input. Below are some of the key components that are needed in normal
operation. For this example, the fast charge current is 50 mA, input current limit is 400 mA and the pre-charge
and termination current is 10% of the fast charge current.
• Supply voltage = 3.4 V to 20 V
• Fast charge current is default to 10 mA with ISET pin shorted to ground. To program the fast charge current,
connect an external resistor from ISET to ground.
• Input current limit is default to 100 mA with ILIM pin shorted to ground. To program the input current limit,
connect an external resistor from ILIM to ground.
• Termination current threshold is default to 2 mA with IPRETERM pin shorted to ground. To program the input
current limit, connect an external resistor from IPRETERM to ground.
• A 2.2-µH inductor is needed between SW pin and SYS pin for PWM output.
• TS- Battery temperature sense needs a NTC connected on TS pin.
10.2.2 Detailed Design Procedure
See Figure 37 for an example of the application diagram.
10.2.2.1 Default Settings
• • Connect ISET, ILIM and IPRETERM pins to ground to program fast charge current to 10mA, input current
limit to 100mA and pre-charge/termination current to 2 mA.
• BAT_UVLO = 3 V.
• VSYS = 1.8 V
• LS/LDO is LS
• VBREG = 4.2 V
• VIN_DPM is enabled and VIN_DPM Threshold = 4.6 V.
• Safety Timer = 3 hr
• If the function is not needed, connect TS to the center tab of the resistor divider between V_IN and the ground.
(pull up resistor = 14 kΩ, pull down resistor = 14.3 kΩ)
10.2.2.2 Choose the Correct Inductance and Capacitance
Refer to the Buck (PWM) Output section for the detailed procedure to determine the optimal inductance and
capacitance for the buck output.
10.2.2.3 Calculations
10.2.2.3.1 Program the Fast Charge Current (ISET)
R_ISET = K_ISET/ICHG
(10)
K_ISET = 200 AΩ from the Specifications table
R_ISET = 200 AΩ / 0.05A = 4 kΩ
(11)
Select the closest standard value, which in this case is 4.99 kΩ. Connect this resistor between ISET pin and
GND.
10.2.2.3.2 Program the Input Current Limit (ILIM)
R_ILIM = K_ILIM/I_{I_MAX}
(12)
K_ILIM = 200 AΩ from the Specifications table
R_ILIM = 200 AΩ / 0.4A = 500 Ω
(13)
Select the closest standard value, which in this case is 499 Ω. Connect this resistor between ILIM pin and GND.
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