BQ24700_14 Datasheet, PDF (20/32 Pages) Texas Instruments – NOTEBOOK PC BATTERY CHARGE CONTROLLER

bq24700, bq24701
ą
NOTEBOOK PC BATTERY CHARGE CONTROLLER
AND SELECTOR WITH DPM
SLUS452B – APRIL 2001 – REVISED NOVEMBER 2002
APPLICATION INFORMATION
The RMS current through the MOSFET is defined as:
Ǹ _{IIN(RMS) + IIN(avg)}
1
D
ARMS
(5)
Schottky rectifier (freewheeling)
The freewheeling Schottky rectifier must also be selected to withstand the input voltage, V_IN. The average
current can be approximated from:
ID1(avg) + IO (1 * D) A
(6)
choosing an inductance
Low inductance values result in a steep current ramp or slope. Steeper current slopes result in the converter
operating in the discontinuous mode at a higher power level. Steeper current slopes also result in higher output
ripple current, which may require a higher number, or more expensive capacitors to filter the higher ripple
current.
In addition, the higher ripple current results in an error in the sensed battery current particularly at lower charging
currents. It is recommended that the ripple current not exceed 20% to 30% of full scale dc current.
ǒ Ǔ _{L +}
VIN * VBAT
fs 0.2 IFS
VBAT
VIN
(7)
Too large an inductor value results in the current waveform of Q1 and D1 in Figure 8 approximating a
squarewave with an almost flat current slope on the step. In this case, the inductor is usually much larger than
necessary, which may result in an efficiency loss (higher DCR) and an area penalty.
selecting an output capacitor
For this application the output capacitor is used primarily to shunt the output ripple current away from the battery.
The output capacitor should be sized to handle the full output ripple current as defined as:
ǒ Ǔ _{Ic (RMS) +}
VIN * VBAT
fs L ^Ǹ12
D
(8)
selecting an input capacitor
The input capacitor is used to shunt the converter ripple current on the input lines. The capacitor(s) must have
a ripple curent (RMS) rating of:
Ǹ ǒ Ǔ IRMS + IIN(avg)
VIN
VO
VIN
VO
1
*
VIN
VO
ARMS
(9)
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