BQ24618_15 Datasheet, PDF (28/43 Pages) Texas Instruments – System Power Selector and Low

bq24618
SLUSA55B – OCTOBER 2010 – REVISED APRIL 2015
www.ti.com
Typical Application (continued)
9.2.1 Design Requirements
For this design example, use the parameters listed in Table 3 as the input parameters.
Table 3. Design Parameters
DESIGN PARAMETER
AC adapter voltage (VIN)
AC adapter current limit
Battery charge voltage (number of cells in series)
Battery charge current (during constant current phase)
Precharge and termination current
Safety timer
VALUE
19 V
4A
12.6 V (3 cells)
3A
0.3 A
5 hours
9.2.2 Detailed Design Procedure
9.2.2.1 Inductor Selection
The bq2461x has 600-kHz switching frequency to allow the use of small inductor and capacitor values. Inductor
saturation current should be higher than the charging current (I_CHG) plus half the ripple current (I_RIPPLE):
I_SAT ³ I_CHG + (1/2) I_RIPPLE
(12)
The inductor ripple current depends on input voltage (V_IN), duty cycle (D = V_OUT/V_IN), switching frequency (f_S) and
inductance (L):
I_RIPPLE =
V_IN
´D
f_S
´ (1
´L
-
D)
(13)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9 V to 12.6 V for a three-cell battery pack. For 20-V adapter voltage, 10-V battery voltage
gives the maximum inductor ripple current. Another example is a four-cell battery, where the battery voltage
range is from 12 V to 16.8 V, and 12-V battery voltage gives the maximum inductor ripple current.
Usually inductor ripple is designed in the range of 20% to 40% of maximum charging current as a trade-off
between inductor size and efficiency for a practical design.
The bq24618 has cycle-by-cycle charge undercurrent protection (UCP) by monitoring the charge current-sensing
resistor to prevent negative inductor current. The typical UCP threshold is 5 mV falling edge, corresponding to
0.5 A falling edge for a 10-mΩ charge current-sensing resistor.
9.2.2.2 Input Capacitor
The input capacitor should have enough ripple current rating to absorb the input switching ripple current. The
worst-case RMS ripple current is half of the charging current when the duty cycle is 0.5. If the converter does not
operate at 50% duty cycle, then the worst-case capacitor RMS current I_CIN occurs where the duty cycle is closest
to 50% and can be estimated by the following equation:
I_CIN = I_CHG ´ D ´ (1 - D)
(14)
A low-ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be
placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET. The
voltage rating of the capacitor must be higher than the normal input voltage level. A 25-V rating or higher
capacitor is preferred for 20-V input voltage. A 10-µF to 20-µF capacitor is suggested for typical 3-A to 4-A
charging current.
9.2.2.3 Output Capacitor
The output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current I_COUT is given:
28
Submit Documentation Feedback
Product Folder Links: bq24618
Copyright © 2010–2015, Texas Instruments Incorporated