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BQ24707 参数 Datasheet PDF下载

BQ24707图片预览
型号: BQ24707
PDF下载: 下载PDF文件 查看货源
内容描述: 1-4节锂电池SMBus充电控制器具有独立的比较器和先进的电路保护 [1-4 Cell Li Battery SMBus Charge Controller With Independent Comparator and Advanced Circuit Protection]
分类和应用: 电池比较器电路保护控制器
文件页数/大小: 36 页 / 1269 K
品牌: TI [ TEXAS INSTRUMENTS ]
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bq24707  
bq24707A  
SLUSA78B JULY 2010REVISED MARCH 2011  
www.ti.com  
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 IC has charge under current protection (UCP) by monitoring charging current sensing resistor cycle-by-cycle.  
The typical cycle-by-cycle UCP threshold is 5mV falling edge corresponding to 0.5A falling edge for a 10mΩ  
charging current sensing resistor. When the average charging current is less than 125mA for a 10mΩ charging  
current sensing resistor, the low side MOSFET is off until BTST capacitor voltage needs to refresh charge. As a  
result, the converter relies on low side MOSFET body diode for the inductor freewheeling current.  
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 Equation 6:  
ICIN = ICHG  
´
D × (1 - D)  
(6)  
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.  
Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is  
applied across a ceramic capacitor, as on the input capacitor of a charger. The effect may lead to a significant  
capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's data  
sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage  
rating or nominal capacitance value in order to get the required value at the operating point.  
Output Capacitor  
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The  
output capacitor RMS current is given:  
IRIPPLE  
ICOUT  
=
» 0.29 ´ IRIPPLE  
2 ´  
3
(7)  
The IC 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 20 kHz. The preferred ceramic capacitor is 25V  
X7R or X5R for output capacitor. 10-20μF capacitance is suggested for typical of 3-4A charging current. Place  
capacitors after charging current sensing resistor to get the best charge current regulation accuracy.  
Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is  
applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a significant  
capacitance drop, especially for high output voltages and small capacitor packages. See the manufacturer's data  
sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage  
rating or nominal capacitance value in order to get the required value at the operating point.  
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 19-20V input voltage.  
Figure-of-merit (FOM) is usually used for selecting proper MOSFET based on a tradeoff between the conduction  
loss and switching loss. For top side MOSFET, FOM is defined as the product of a MOSFET's on-resistance,  
RDS(ON), and the gate-to-drain charge, QGD. For bottom side MOSFET, FOM is defined as the product of the  
MOSFET's on-resistance, RDS(ON), and the total gate charge, QG.  
FOMtop = RDS(on) x QGD; FOMbottom = RDS(on) x QG  
(8)  
24  
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© 20102011, Texas Instruments Incorporated  
Product Folder Link(s): bq24707 bq24707A  
 
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