BQ24620 Datasheet, PDF (24/30 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Lithium Phosphate Battery Charger with Low Iq

bq24620
SLUS893 – MARCH 2010
APPLICATION INFORMATION
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Inductor Selection
The bq24620 has 300kHz 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):
I_SAT ³ I_CHG + (1/2) 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):
I_RIPPLE =
V_IN
´D
f_S
´ (1
´L
-
D)
(11)
The maximum inductor ripple current happens with D = 0.5. For example, the battery charging voltage range is
from 2.8V to 14.4V for 4-cell battery pack. For 20V adapter voltage, 10V 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 bq24620 has cycle-by-cycle charge under current protection (UCP) by monitoring charging current sensing
resistor to prevent negative inductor current. The Typical UCP threshold is 5mV falling edge corresponding to
0.5A falling edge for a 10mΩ charging current sensing resistor.
Input Capacitor
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:
I_CIN = I_CHG ´ D ´ (1 - D)
(12)
Low ESR ceramic capacitor such as X7R or X5R is preferred for 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. Voltage
rating of the capacitor must be higher than normal input voltage level. 25V rating or higher capacitor is preferred
for 20V input voltage. 20µF capacitance is suggested for typical of 3-4A charging current.
Output Capacitor
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current I_COUT is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(13)
The output capacitor voltage ripple can be calculated as follows:
DV_o
=
V_OUT
8LCf_s2
æç_1-
è
V_OUT
V_IN
ö
÷
ø
(14)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24620 has 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 25V, X7R or X5R for 4-cell application.
Power MOSFETs Selection
Two external N-channel MOSFETs are used for a synchronous switching battery charger. The gate drivers are
internally integrated into the IC with 6V of gate drive voltage. 30V or higher voltage rating MOSFETs are
preferred for 20V input voltage and 40V MOSFETs are preferred for 20-28V input voltage.
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