bq24702, bq24703

SLUS553D − MAY 2003 − REVISED JULY 2005

21

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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

I

IN

(avg)

+ D

Ichg

A

D = Duty cycle

Ichg = Charge current

The RMS current through the MOSFET is defined as:

I

IN

(RMS)

+ Ichg

D

A

RMS

The rise/fall times for pin PWM for the selected MOSFET should be greater than 40 nsec.

Schottky Rectifier (Freewheeling)

current can be approximated from:

I

D1

(avg)

+ Ichg

(1

* D) A

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.

L

+

D

V

IN *

V

BAT

F

S

Ichg

Ripple

Ripple = % Ripple allowed (Ex.: 0,2 for 20% ripple)

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:

Ic (RMS) +

V

IN *

V

BAT

D

F

S

L

A

RMS

(4)

(5)

(6)

(7)

(8)