BQ24075-Q1 Datasheet, PDF (28/36 Pages) Texas Instruments – 1.5A USB-FRIENDLY Li-Ion BATTERY CHARGER AND POWER-PATH MANAGEMENT IC

bq24075-Q1
SLUSAU3B – FEBRUARY 2012 – REVISED MARCH 2012
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CHG and PGOOD
LED Status: connect a 1.5kΩ resistor in series with a LED between OUT and CHG to indicate charging status.
Connect a 1.5kΩ resistor in series with a LED between OUT and PGOOD to indicate when a valid input source is
connected.
Processor Monitoring Status: connect a pullup resistor (on the order of 100 kΩ) between the processor’s power
rail and CHG and PGOOD
System ON/OFF (SYSOFF)
Connect SYSOFF high to disconnect the battery from the system load. Connect SYSOFF low for normal
operation
SELECTING IN, OUT AND BAT pin CAPACITORS
In most applications, all that is needed is a high-frequency decoupling capacitor (ceramic) on the power pin,
input, output and battery pins. 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 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).
THERMAL PACKAGE
The bq24075 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 V_SS pin. 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
Where:
T_J = chip junction temperature
T = ambient temperature
P = device power dissipation
Factors that can influence the measurement and calculation of θ_JA include:
1. Whether or not the device is board mounted
2. Trace size, composition, thickness, and geometry
3. Orientation of the device (horizontal or vertical)
4. Volume of the ambient air surrounding the device under test and airflow
5. Whether other surfaces are in close proximity to the device being tested
Due to the charge profile of Li-Ion batteries the maximum power dissipation is typically seen at the beginning of
the charge cycle when the battery voltage is at its lowest. Typically after fast charge begins the pack voltage
increases to ≉3.4V within the first 2 minutes. The thermal time constant of the assembly typically takes a few
minutes to heat up so when doing maximum power dissipation calculations, 3.4V is a good minimum voltage to
use. This is verified, with the system and a fully discharged battery, by plotting temperature on the bottom of the
PCB under the IC (pad should have multiple vias), the charge current and the battery voltage as a function of
time. The fast charge current will start to taper off if the part goes into thermal regulation.
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