BQ24742 Datasheet, PDF (23/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

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bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
APPLICATION INFORMATION
Inductor Selection
The bq24741/2 can program the switching frequency between 300k and 800kHz for different applications. Higher
switching frequency allows the use of smaller 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
(6)
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)
(7)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9V to 12.6V for 3-cell battery pack. For 20V adapter voltage, 10V battery voltage gives the
maximum inductor ripple current. Another example is 4-cell battery, the battery voltage range is from 12V to
16.8V, and 12V 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 bq24741/2 has charge under current protection (UCP) by monitoring charging current sensing resistor. The
Typical UCP threshold is 30mV falling edge and 40mV rising edge corresponding to 1.5A falling edge and 2A
rising edge for a 20mΩ charging current sensing resistor. To prevent negative inductor current, the inductance
must be high enough so that peak to peak ripple current is less than 3A (for a 20mΩ charging current sensing
resistor) when charging current tapers down. Considering UCP threshold tolerance for worst case, peak to peak
ripple current less than 2.5A for a 20mΩ charging current sensing resistor is preferred.
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 occurs where the duty cycle is closest to 50% and
can be estimated by the following equation:
I_CIN = I_CHG ´ D ´ (1 - D)
(8)
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 19-20V input voltage. 10-20µF 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 is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(9)
The bq24741/2 has internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 8 kHz and 12.5 kHz.
The preferred ceramic capacitor is 25V, X7R or X5R for output capacitor. 10-20µF capacitance is suggested for
practical application. Two capacitors, one capacitor is located before and another one after charging current
sensing resistor to get the best average charge current regulation accuracy.
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 5.9V of gate drive voltage. 30V or higher voltage rating MOSFETs are
preferred for 19-20V input voltage.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s) :bq24741 bq24742
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