BQ24040_16 Datasheet, PDF (22/38 Pages) Texas Instruments – Single Cell Li-Ion and Li-Pol Battery Charger With Auto Start

bq24040, bq24041, bq24045
SLUS941F – SEPTEMBER 2009 – REVISED MARCH 2015
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Typical Applications (continued)
9.2.1.2 Detailed Design Procedures
9.2.1.2.1 Calculations
9.2.1.2.1.1 Program the Fast Charge Current, ISET:
R_ISET = [K_(ISET) / I_(OUT)]
(3)
From the Electrical Characteristics table:
• K_(SET) = 540AΩ
• R_ISET = [540AΩ/0.54A] = 1.0 kΩ
Selecting the closest standard value, use a 1.0 kΩ resistor between ISET (terminal 16) and Vss.
9.2.1.2.1.2 Program the Termination Current Threshold, ITERM:
R_PRE-TERM = K_(TERM) × %_IOUT-FC
(4)
R_PRE-TERM = 200Ω/% × 10% = 2kΩ
(5)
Selecting the closest standard value, use a 2 kΩ resistor between ITERM (terminal 15) and Vss.
One can arrive at the same value by using 20% for a pre-charge value (factor of 2 difference).
R_PRE-TERM = K_(PRE-CHG) × %_IOUT-FC
(6)
R_PRE-TERM = 100Ω/% × 20%= 2kΩ
(7)
9.2.1.2.1.3 TS Function (bq24040)
Use a 10k NTC thermistor in the battery pack (103AT).
To Disable the temp sense function, use a fixed 10k resistor between the TS (terminal 1) and Vss.
9.2.1.2.1.4 CHG and PG
LED Status: connect a 1.5k resistor in series with a LED between the OUT terminal and the CHG terminal.
Connect a 1.5k resistor in series with a LED between the OUT terminal and the and PG terminal.
Processor Monitoring: Connect a pull-up resistor between the processor’s power rail and the CHG terminal.
Connect a pull-up resistor between the processor’s power rail and the PG terminal.
9.2.1.2.2 Selecting In and Out Terminal Capacitors
In most applications, all that is needed is a high-frequency decoupling capacitor (ceramic) on the power terminal,
input and output terminals. Using the values shown on the application diagram, is recommended. After
evaluation of these voltage signals with real system operational conditions, one can determine if capacitance
values can be adjusted toward the minimum recommended values (DC load application) or higher values for fast
high amplitude pulsed load applications. Note if designed for high input voltage sources (bad adaptors or wrong
adaptors), the capacitor needs to be rated appropriately. Ceramic capacitors are tested to 2x their rated values
so a 16V capacitor may be adequate for a 30V transient (verify tested rating with capacitor manufacturer).
9.2.1.2.3 Thermal Package
The bq2404x family is packaged in a thermally enhanced MLP package. The package includes a thermal pad to
provide an effective thermal contact between the IC and the printed circuit board (PCB). The power pad should
be directly connected to the VSS terminal. Full PCB design guidelines for this package are provided in the
application note entitled: QFN/SON PCB Attachment Application Note (SLUA271). The most common measure
of package thermal performance is thermal impedance (θ_JA ) measured (or modeled) from the chip junction to the
air surrounding the package surface (ambient). The mathematical expression for θ_JA is:
θ_JA = (T_J – T) / P
(8)
Where:
• T_J = chip junction temperature
• T = ambient temperature
• P = device power dissipation
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