Datasheet

SAT CHG RIPPLE
I I + (1/2) I³
IN
RIPPLE
S
V D (1 D)
I =
f L
´ ´ -
´
CIN CHG
I = I D (1 D)´ ´ -
RIPPLE
COUT RIPPLE
I
I = 0.29 I
2 3
» ´
´
æ ö
D = -
ç ÷
ç ÷
è ø
2
BAT
o BAT
2
IN
s
V
1
V V
V
8LCf
bq24600
www.ti.com SLUS891A –FEBRUARY 2010– REVISED OCTOBER 2011
APPLICATION INFORMATION
Inductor Selection
The bq24600 has a 1.2-MHz switching frequency to allow the use of small inductor and capacitor values.
Inductor saturation current should be higher than the charging current (I
CHARGE
) plus half the ripple current
(I
RIPPLE
):
(11)
The inductor ripple current depends on input voltage (V
IN
), duty cycle (D = V
OUT
/V
IN
), switching frequency (f
s
) and
inductance (L):
(12)
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 a 20-V adapter voltage, 10-V battery voltage
gives the maximum inductor ripple current. Another example is a 4-cell battery, for which 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% of maximum charging current as a trade-off
between inductor size and efficiency for a practical design.
The bq24600 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.
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:
(13)
Output Capacitor
A low-ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be
placed near the drain of the high-side MOSFET and the 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 rating or higher
capacitor is preferred for a 20-V input voltage. A 10-µF to 20-µF capacitance is suggested for typical of 3-A to
4-A charging current.
The 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:
(14)
The output capacitor voltage ripple can be calculated as follows:
(15)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24600 has an internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 17 kHz and 25 kHz. The preferred ceramic capacitor
for a 4-cell application has a 25-V or higher rating and X7R or X5R dielectric.
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