BQ24085 Datasheet, PDF (19/34 Pages) Texas Instruments – 750 mA SINGLE-CHIP Li-Ion/Li-Pol CHARGE MANAGEMENT IC WITH THERMAL REGULATION

bq24085, bq24086
bq24087, bq24088
www.ti.com .................................................................................................................................................... SLUS784B – DECEMBER 2007 – REVISED MAY 2008
DYNAMIC TIMER FUNCTION
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 bq24085/6/7/8 activates the dynamic timer
control, an internal circuit that slows down the safety timer's clock frequency. 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 bq24085/6/7/8 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 2²⁴ 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 16 and Figure 17.
The device deglitch timers are set by the same oscillators as the safety and precharge timers. In thermal
regulation, the timers are scaled appropriately, see Figure 2.
5
4
3
2
1
0
0
1
2
3
4
5
V_(SET)/V_(ISET) - V
Figure 16. Safety Timer Linearity
Internal Clock Period Multiplication Factor
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