AN137 Silicon_Laboratories, AN137 Datasheet - Page 3

• Manufacturer Part Number: AN137
• Description: Lithium ION Battery Charger Using C8051f300
• Manufacturer: Silicon_Laboratories

within the load limits of the circuit. See Figure 3
for details.
Tapered Charger Using the F30x
Figure 3 illustrates an example buck converter
using the ‘F30x. The pass switch is controlled via
the on-chip 8-bit PWM (Pulse Width Modulator)
output of the PCA. When the switch is on, current
will flow like in Figure 3A. The capacitor is
charged from the input through the inductor. The
inductor is also charged. When the switch is
opened (Figure 3B), the inductor will try to main-
tain its current flow by inducing a voltage as the
current through an inductor can’t change instanta-
neously. The current then flows through the diode
and the inductor charges the capacitor. Then the
cycle repeats itself. On a larger scale, if the duty
cycle is decreased (shorter “on” time), the average
Source
Power
(A)
Pass Switch On
Shottky
Diode
Conditioning
Phase
Figure 2. Lithium Ion Charge Profile.
Capacitor
Charge Current
Current regulation
Inductor
Figure 3. Buck Converter.
Battery
Rev. 1.2
Charge Voltage
Voltage regulation
Source
Power
voltage decreases and vice versa. Therefore, con-
trolling the duty cycles allows one to regulate the
voltage or the current to within desired limits.
Selecting the Buck Converter Inductor
To size the inductor in the buck converter, one first
assumes a 50 percent duty cycle, as this is where
the converter operates most efficiently.
Duty cycle is given by
period of the PWM (in our example T = 10.5µS).
(B)
Pass Switch Off
Shottky
Diode
Equation 1. Duty Cycle.
DutyCycle
Time
Capacitor
Equation
Inductor
=
ton
-------- -
T
1, where T is the
Battery
AN137
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