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V -InputVoltage-V
I
BatteryCurrent-mA
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50
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175
T
-JunctionT
emperature-°C
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IBAT
T JunctionTemperature
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ThermalRegulation
NormalOperation
ThermalShutdown
DYNAMIC TIMER FUNCTION
bq2406x
SLUS689A – JUNE 2006 – REVISED OCTOBER 2006
Figure 19. Thermal Regulation Loop Performance and Thermal Shutdown
The charge and pre-charge safety timers are programmed by the user to detect a fault condition if the charge
cycle duration exceeds the total time expected under normal conditions. The expected charge time is usually
calculated based on the fast charge current rate.
When the thermal loop is activated the charge current is reduced, and bq2406x activates the dynamic timer
control, an internal circuit that slows down the safety timer's clock frequnency. The dynamic timer control circuit
effectively extends the safety time duration for either the precharge or fast charge timer modes. This minimizes
the chance of a safety timer fault due to thermal regulation.
The bq2406x dynamic timer control (DTC) monitors the voltage at pin ISET during pre-charge and fast charge,
and if in thermal regulation slows the clock frequency proportionately to the change in charge current. The time
duration is based on a 2
24
ripple counter, so slowing the clock frequency is a real time correction. The DTC
circuit changes the safety timers clock period based on the V
(SET)
/V
(ISET)
ratio (fast charge) or V
(PRECHG)
/V
(SET)
ratio (pre-charge). Typical safety timer multiplier values relative to the V
(SET)
/V
(ISET)
ratio is shown in Figure 20
and Figure 21 .
19
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