BQ24080-Q1 Datasheet, PDF (16/28 Pages) Texas Instruments – Li-Ion and Li-Pol Automotive Charger

bq24080-Q1, bq24081-Q1
SLUSCB6 – MAY 2015
www.ti.com
9.2.2 Detailed Design Procedure
9.2.2.1 Calculations
Program the charge current for 750 mA:
R_ISET = [V_(SET) × K_(SET) / I_(OUT)]
(5)
From Electrical Characteristics table, V_(SET) = 2.5 V.
From Electrical Characteristics table, K_(SET) = 322.
R_ISET = [2.5 V × 322 / 0.75 A] = 1.073 kΩ
(6)
Selecting the closest standard value, use a 1.07-kΩ resistor connected between ISET (pin 6) and ground.
9.2.2.2 Battery Temperature Sense (bq24081-Q1):
Use a Semitec 103AT-4 NTC thermistor connected between TS (pin 9) and ground.
R_THERM-cold = [V_(TS1) / I_(TS) ] = 2.5V / 100 μA = 25 kΩ
(7)
R_THERM-hot = [V_(TS2) / I_(TS) ] = 0.5V / 100 μA = 5 kΩ
(8)
Look up the corresponding temperature value in the manufacturer's resistance-temperature table for the
thermistor selected. For a 103AT-4 Semitec thermistor:
5 kΩ = 44.5°C
25 kΩ = 2°C
9.2.2.3 STAT Pins (All Devices) and PG Pin (bq24080-Q1):
Status pins Monitored by Processor:
Select a pullup resistor that can source more than the input bias (leakage) current of both the processor and
status pins and still provide a logic high.
R_PULLUP ≤ [V_(cc-pullup) – V_{(logic hi-min)} / (I_{(µP-monitor)} + I_{(STAT-OpenDrain)}) ] = (3.3 V – 1.9 V) / (1 μA + 1 μA) ≤ 700 kΩ;
(9)
Connect a 100-kΩ pullup between each status pin and the V_CC of the processor. Connect each status pin to
a μP monitor pin.
Status viewed by LED:
Select an LED with a current rating less than 10 mA and select a resistor to place in series with the LED to
limit the current to the desired current value (brightness).
R_LED = [(V_(IN) – V_(LED-on)) / I_(LED)] = (5 V – 2 V) / 1.5 mA = 2 kΩ.
(10)
Place an LED and resistor in series between the input and each status pin.
9.2.2.4 Selecting Input and Output Capacitors
In most applications, all that is needed is a high-frequency decoupling capacitor on the input power pin. A 0.1-μF
ceramic capacitor, placed in close proximity to the IN pin and GND pad works well. In some applications, it may
be necessary to protect against a hot plug input voltage overshoot. This is done in three ways:
1. The best way is to add an input zener, 6.2 V, between the IN pin and V_SS.
2. A low-power zener is adequate for the single event transient. Increasing the input capacitance lowers the
characteristic impedance which makes the input resistance move effective at damping the overshoot, but
risks damaging the input contacts by the high inrush current.
3. Placing a resistor in series with the input dampens the overshoot, but causes excess power dissipation.
The device only requires a small capacitor for loop stability. A 0.1-μF ceramic capacitor placed between the OUT
and GND pad is typically sufficient.
16
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