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FUNCTIONAL DESCRIPTION
FUNCTIONAL DEVICE OPERATION
current needs to stay below the EOC threshold for more than
0.5 seconds. The charger is turned off and the LED indication
is green when charge completes.
If the total fast charge time limit is exceeded, the charger
also enters the charge completion state.
RECHARGE MODE
If the battery voltage drops below the recharge threshold
after charge completion, the charger will try to recharge the
battery to 4.2 V. Because the battery voltage drop can also
be caused by the removal of the battery, before starting
recharge, the charger tries to verify if the battery is still
present. If the battery is not found, then the connection fault
is issued again. If the battery is still connected, the charger
restarts charging to bring the battery to a full state. The LED
indication remains green in this mode.
The recharge mode has the same total charge time limit as
the fast charge mode. For any reason the battery voltage falls
below the trickle-charge threshold in the recharge mode, the
charger will enter the battery connection verification state
again, as shown in Figure 23 .
TEMPERATURE AND OVER-VOLTAGE FAULT
The NTC interface block offers an interface to an external
NTC thermistor circuit to monitor the battery temperature.
When the battery temperature is out of a user-programmable
window, the charger is disabled and a fault condition is issued
with a yellow LED indication. When the fault conditions are
removed, the charger enters the battery connection
verification state. More detailed description on the NTC
interface is offered later in this datasheet.
The charger has an 11 V (typ.) input OVP threshold. When
the input voltage is higher than this threshold, the charging is
stopped and a fault condition is issued with a yellow LED
indication. When the input voltage falls below the OVP
threshold, the charger restarts charging and resets the
internal digital logic control block.
TIMEOUT FAULT
The TIMEOUT fault can only occur when the charger stays
in the trickle-charge mode for a period longer than the time
limit. The charger is turned off and a yellow LED indication is
issued when this fault occurs. The only path to exit this fault
is by toggling the EN input or by recycling the power input.
DETAILED FUNCTIONAL DEVICE OPERATION
----------------------------------------- = K x
NTC INTERFACE
The MC34674 offers an interface to an external NTC
thermistor to monitor the battery temperature.
R NTC + R S
R NTC + R S + R U
equ. 1
The low and high temperature thresholds in the Table 4
allow users to set a temperature window (such as 0°C to
50°C), within which the charging is allowed. If the battery
temperature is out of such a window, a temperature fault is
issued and the LED indicates a yellow color.
Figure 19 shows the internal equivalent circuit for the NTC
interface and the external NTC thermistor circuit. An internal
resistor divider that is powered by the VREF pin voltage,
where R NTC is the thermistor resistance at the given
temperature, and K X is the ratio of the internal divider at the
given triggering points (see Table 4 ). R U and R S represent a
pull-up resistor and a series resistor in the external resistor
divider respectively.
The resistance selection of R U and R S can be figured out
by the following two equations:
----------------------------------------- = K HOT
--------------------------------------------- = K COLD
V VREF , creates two reference voltages, 1/3 V VREF and 2/3
V VREF . An external resistor divider also powered by V VREF
generates the voltage V TEMP to represent the battery
temperature. Because the resistance of the NTC thermistor,
R NTC , decreases as temperature rises, as shown in
Figure 20 , V TEMP decreases as the battery temperature
R HOT + R S
R HOT + R S + R U
R COLD + R S
R COLD + R S + R U
equ. 2
equ. 3
increases. Assume T COLD and T HOT are the two temperature
thresholds, such as 0°C and 50°C. When the battery
temperature falls below T COLD , V TEMP rises above 2/3 V VREF
and an under-temperature fault is issued. Similarly, when the
battery temperature rises above T HOT , V TEMP falls below 1/3
V VREF , so an over-temperature fault is issued. The
relationship between the internal and the external divider
voltages at the triggering points can be expressed as the
where K HOT and K COLD are the resistor divider ratios for the
temperature thresholds T HOT and T COLD respectively; R HOT
and R COLD are the NTC thermistor resistance at T HOT and
T COLD respectively. The typical values for K HOT and K COLD
are 1/3 and 2/3 respectively, as given in Table 4 .
Refer to the Application Information section for more
details regarding the R U and R S selection.
following:
34674
Analog Integrated Circuit Device Data
16
Freescale Semiconductor
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