BQ24262_15 Datasheet, PDF (39/56 Pages) Texas Instruments – bq2426x 3-A, 30-V, Host-Controlled Single-Input, Single-Cell Switched-Mode Li-Ion Battery Charger With Power-Path Management and USB-OTG Support

Not Recommended for New Designs : bq24260, bq24261
www.ti.com
bq24260, bq24261, bq24261M, bq24262
SLUSBU4D – DECEMBER 2013 – REVISED APRIL 2015
Typical Application (continued)
10.2.1 Design Requirements
For this example, use the parameters listed in Table 9.
Table 9. Design Requirements
DESIGN PARAMATER
Input Voltage Range
Input Current Limit
Input DPM Threshold
Fast Charge Current
Battery Charge Voltage
Termination Current
EXAMPLE VALUE
4.75 V to 5.25 V nominal, withstand 28 V
2500 mA
4.25 V
3000 mA
4.2 V
50 mA
10.2.2 Detailed Design Procedure
Following the guidance in the next section, the capacitors on IN, PMID, SYS, BAT and BOOT are the minimum
recommended values of 4.7 µF, 1 µF, 10 µF, 1 µF, and 0.033 µF, respectively. It is assumed that at least 10 µF
of additional capacitance is on the SYS rail. To minimize footprint, a 1.5-µH inductor with at least 3.5-A saturation
current is selected. The optional FET, with gate connected at BGATE, only turns on in high-impedance mode (for
example, no input power/battery only) to reduce the losses across the internal battery FET of the IC . See the
bq24261EVM for exact part numbers. Pullup resistors for STAT and INT of 1.5 kΩ were selected per the current
requirements of the LED. The values for the resistor divider on TS were found using Equation 1 and Equation 2,
where RHOT is the resistance of the NTC thermistor at the hot temperature, RCOLD is the resistance of the
thermistor at cold temperature, VDRV = 5 V, VHOT = 0.3 × VDRV and V_COLD = 0.6 x V_DRV.
Many parameters configurable by the I²C registers can be changed by using the EVM software.
10.2.2.1 Output Inductor and Capacitor Selection Guidelines
When selecting an inductor, several attributes must be examined to find the right part for the application. First,
the inductance value should be selected. The bq2426x is designed to work with 1.5-µH to 2.2-µH inductors. The
chosen value will have an effect on efficiency and package size. Due to the smaller current ripple, some
efficiency gain is reached using the 2.2-µH inductor. However, due to the physical size of the inductor, this option
may not be viable. The 1.5-µH inductor provides a good tradeoff between size and efficiency.
Once the inductance has been selected, the peak current must be calculated in order to choose the current
rating of the inductor. Use Equation 5 to calculate the peak current.
I_PEAK
=
I_LOAD(MAX)
´
æ
çè¹
+
%_RIPPPLE
2
ö
÷ø
(5)
The inductor selected must have a saturation current rating greater than or equal to the calculated I_PEAK. Due to
the high currents possible with the bq24260/1/1M/2, a thermal analysis must also be done for the inductor. Many
inductors have 40°C temperature rise rating. This is the DC current that will cause a 40°C temperature rise
above the ambient temperature in the inductor. For this analysis, the typical load current may be used adjusted
for the duty cycle of the load transients. For example, if the application requires a 1.5-A DC load with peaks at
2.5 A 20% of the time, a Δ40°C temperature rise current must be greater than 1.7 A:
I_TEMPRISE = I_LOAD + D × (I_PEAK – I_LOAD) = 1.5 A + 0.2 × (2.5 A – 1.5 A) = 1.7 A
(6)
The internal loop compensation of the bq24260/1/1M/2 is designed to be stable with 10 µF to 150 µF of local
capacitance but requires at least 20 µF total capacitance on the SYS rail (10 µF local + ≥ 10 µF distributed). The
capacitance on the SYS rail can be higher than 150 µF if distributed amongst the rail. To reduce the output
voltage ripple, a ceramic capacitor with the capacitance between 10 µF and 47 µF is recommended for local
bypass to SYS. If greater than 100 µF effective capacitance is on the SYS rail, place at least 10-µF bypass on
the BAT terminal. Pay special attention to the DC bias characteristics of ceramic capacitors. For small case
sizes, the capacitance can be derated as high as 70% at workable voltages. All capacitances specified in this
data sheet are effective capacitance, not capacitor value.
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