ꢀ ꢁꢂ ꢃ ꢄ ꢅ ꢂ ꢆ ꢀꢁ ꢂꢃ ꢄꢅ ꢇ
SLUS553D − MAY 2003 − REVISED JULY 2005
COMPONENT SELECTION
MOSFET Selection
MOSFET selection depends on several factors, namely, gate-source voltage, input voltage, and input current.
The MOSFET must be a P-channel device capable of handling at least 15-V gate-to-source with a drain-source
breakdown of V ~ V +1 V. The average input current can be approximated by:
BV
IN
(
)
I
avg + D Ichg
A
IN
D = Duty cycle
Ichg = Charge current
(4)
(5)
The RMS current through the MOSFET is defined as:
Ǹ
(
)
I
RMS + Ichg
D A
IN
RMS
The rise/fall times for pin PWM for the selected MOSFET should be greater than 40 nsec.
Schottky Rectifier (Freewheeling)
The freewheeling Schottky rectifier must also be selected to withstand the input voltage, V . The average
IN
current can be approximated from:
(
)
(
)
I
avg + Ichg 1 * D A
D1
(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.
D ǒVIN
Ǔ
* V
BAT
Ichg Ripple
L +
F
S
Ripple = % Ripple allowed (Ex.: 0,2 for 20% ripple)
(7)
Too large an inductor value results in the current waveform of Q1 and D1 in Figure 2 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:
ǒVIN
Ǔ
* V
D
BAT
L
(
)
I
RMS +
A
c
RMS
F
S
(8)
21
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