Datasheet

Analog Integrated Circuit Device Data
14 Freescale Semiconductor
34671
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
CHARGE CYCLE
The MC34671 uses the standard charge profile with
trickle, constant-current (CC), and constant-voltage (CV)
charge modes, as shown in
Figure 21. Both the CC and the
CV charge modes are also called fast-charge mode. When
the input voltage rises above an internal power-on-reset
threshold, the
PPR pin outputs a low voltage to indicate the
power supply presence. The charger starts with the trickle-
charge mode until the battery voltage is above 2.7V. The
CHG pin outputs logic low voltage at the beginning of the
trickle-charge mode. If the battery voltage is unable to rise
due to a battery failure, charging will remain in the trickle-
charge mode. When the battery voltage reaches the 2.7V
threshold, the MC34671 softly changes to the CC-mode. The
soft transition minimizes the input voltage drop and reduces
the requirement of the input decoupling capacitance. In the
fast-charge mode, the
FAST pin outputs a low voltage. When
the battery voltage reaches 4.2V, the MC34671 enters the
CV-mode and regulates the output voltage at 4.2V. The
charge current decreases gradually in the CV-mode. When
the current drops to the EOC current threshold, the MC34671
outputs logic high voltage at the
CHG pin to indicate that the
charging is completed.
After the charging is completed, the MC34671 continues to
regulate the output to 4.2V, and monitors the output voltage.
If a load is in parallel with the battery, the charger continues
to output the current to the load even the charge is
completed. If the load current exceeds the programmed CC-
mode current, the battery will supply the additional current to
the load and the battery voltage will decline. Once the battery
voltage drops below the recharge voltage threshold, the
MC34671 returns to the fast-charge mode and indicates a
low voltage at the
CHG pin.
When one of the following three conditions happens, the
MC34671 is disabled.
1. V
IN
> V
OVP
2. V
IN
- V
BAT
< V
OS
3. EN pin is high
where V
OS
is the offset voltage for the comparator that
monitors the input and the battery voltages.
Figure 22 shows the complete charge-cycle state
diagram.
Figure 21. Charge Profile
CHARGE CURRENT SETTING
An external resistor between the ISET pin and the ground
programmes the CC-mode current with the following
equation:
equ. 2
where R
ISET
is the resistor between the ISET pin and ground
in Ω. In addition, the current out of the ISET pin is proportional
to the charge current. The system may measure the ISET pin
I
CHG
Trickle Constant Voltage
I
TRKL
I
EOC
4.2V
TIME
2.7V
CHG
Charge
Current
Charge
Voltage
TIME
FAST
TIME
I
LOAD
>I
CHG
100mV
Constant Current
I
CHG
2667
R
ISET
96+
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