BQ24266_15 Datasheet, PDF (27/41 Pages) Texas Instruments – bq24266 3A, 30V Standalone Single-Input, Single-Cell Switchmode Li-Ion Battery Charger

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Typical Applications (continued)
8.2.1.1 Design Requirements
bq24266
SLUSBY5F – JUNE 2014 – REVISED AUGUST 2015
Table 4. 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
4.2 V (Externally Set)
3000 mA
4.2 V
300 mA
8.2.1.2 Detailed Design Procedure
The parameters are configurable using the EVM jumper options as described in the Users Manual. The typical
application for the bq24266EVM is shown in Figure 14. The default IUSB settings are for 2.5A input current limit
and external V_INDPM threshold, which is IUSB3 = 1, IUSB = 2 = 1, IUSB1 = 0. The VDPM resistors were
selected using Equation 1. The charge current, I_CHARGE, was set to be 3A using Equation 2.
The typical application circuit shows the minimum capacitance requirements for each pin. Options for sizing the
inductor outside the 1.5 μH recommended value and additional SYS pin capacitance are explained in the next
section. The resistors on PG and CHG are sized per each LED's current requirements. The TS resistor divider
for configuring the TS function to work with the battery's specific thermistor can be computed from Equation 3
and Equation 4. The external battery FET is optional.
8.2.1.2.1 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 bq24266 is designed to work with 1.5µH to 2.2µH 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 2.2µH inductor, however, due to the physical size of the inductor, this may
not be a viable option. The 1.5µH 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 7 to calculate the peak current.
I_PEAK
=
I_LOAD(MAX)
´
æ
çè¹
+
%_RIPPPLE
2
ö
÷ø
(7)
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 bq24266, 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 1.5A DC load with peaks at
2.5A 20% of the time, a Δ40°C temperature rise current must be greater than 1.7A:
I_TEMPRISE = I_LOAD + D × (I_PEAK – I_LOAD) = 1.5 A + 0.2 × (2.5 A – 1.5 A) = 1.7 A
(8)
The internal loop compensation of the bq24266 is designed to be stable with 10µF to 150µF of local capacitance
but requires at least 20µF total capacitance on the SYS rail (10µF local + ≥ 10µF distributed). The capacitance
on the SYS rail can be higher than 150µF if distributed amongst the rail. To reduce the output voltage ripple, a
ceramic capacitor with the capacitance between 10µF and 47µF is recommended for local bypass to SYS. If
greater than 100µF effective capacitance is on the SYS rail, place at least 10µF bypass on the BAT pin. 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 datasheet are effective
capacitance, not capacitor value.
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