BQ40Z60_16 Datasheet, PDF (31/47 Pages) Texas Instruments – Programmable Battery Management Unit

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bq40z60
SLUSAW3C – DECEMBER 2014 – REVISED JULY 2015
Typical Applications (continued)
10.2.1 Design Requirements
For this design example, use the parameters listed in Table 2 as the input parameters.
Design Parameter
Input Voltage Range
3-Cell Battery Voltage Range
4-Cell Battery Voltage Range
Operating Frequency
Table 2. Design Parameters
Example Value
15–22 V
9 V–12.6 V
12 V–16.8 V
1000 kHz
10.2.2 Detailed Design Procedure
10.2.2.1 Inductor Selection
The bq40z60 has a 1000-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
(1)
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)
(2)
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 3-cell battery pack. For 20-V adaptor voltage, 10-V battery voltage gives
the maximum inductor ripple current. Another example is a 4-cell battery: 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%–40%) maximum charging current as a trade-off between
inductor size and efficiency for a practical design.
The bq40z60 has cycle-by-cycle charge undercurrent protection (UCP) by monitoring the charging-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 charging-current sensing resistor.
10.2.2.2 Input Capacitor
The input capacitor should have enough ripple-current rating to absorb 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 using the following equation:
I_CIN = I_CHG ´ D ´ (1 - D)
(3)
A low-ESR ceramic capacitor such as X7R or X5R is preferred for the input-decoupling capacitor and should be
placed to the drain of the high-side MOSFET and source of the low-side MOSFET as close as possible. The
voltage rating of the capacitor must be higher than the normal input voltage level. A 25-V or higher-rating
capacitor is preferred for 20-V input voltage. 10-µF to 20-µF capacitance is suggested for typical of 3-A to 4-A
charging current.
10.2.2.3 Output Capacitor
Output capacitor also should have enough ripple-current rating to absorb the output switching ripple current. The
output capacitor RMS current I_COUT is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(4)
The output capacitor voltage ripple can be calculated as follows:
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