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
bq24620
SLUS893A –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
APPLICATION INFORMATION
Inductor Selection
The bq24620 has a 300-kHz 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
):
(10)
The inductor ripple current depends on input voltage (V
IN
), duty cycle (D = V
OUT
/V
IN
), switching frequency (f
S
) and
inductance (L):
(11)
The maximum inductor ripple current happens with D = 0.5. For example, the battery-charging voltage range is
from 2.8 V to 14.4 V for a four-cell battery pack. For 20-V adapter voltage, 10-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 bq24620 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 input switching-ripple current. The worst
case rms ripple current is half of the charging current when 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:
(12)
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 20-µF capacitor is suggested for typical of 3-A to 4-A charging
current.
Output Capacitor
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:
(13)
The output-capacitor voltage ripple can be calculated as follows:
(14)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24620 has an internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 10 kHz and 15 kHz. The preferred ceramic capacitor
is 25 V, X7R or X5R for 4-cell applications.
24 Submit Documentation Feedback Copyright © 2010–2011, Texas Instruments Incorporated
Product Folder Link(s): bq24620