stled325 STMicroelectronics, stled325 Datasheet - Page 29

• Manufacturer Part Number: stled325
• Description: I?c Interfaced, Advanced Led Controller/driver With Keyscan, Standby Power Management And Real Time Clock Rtc
• Manufacturer: STMicroelectronics

STLED325
3.17.1
Figure 11. Circuit
The minimum operating voltage of STLED325 is 2.5 V with a typical VBAT voltage of VCC-
VF (diode). Therefore, the typical delta voltage swing across the capacitor is
ΔV = V
where V
ΔV = 5 – 0.5 – 2.5 = 2 V
Since the typical battery current (IBAT) is limited to 7 µA, the capacitance and duration of
power-out time can be calculated using the formula:
I = CΔV/Δt
Where I = 7µA, ΔV=2V, C= capacitance in Farads and Δt is power-out time in seconds.
Using a 0.1F super-cap, for example, the equation would be:
7µA = 0.1F x 2V/Δt
Solving for Δt, the typical power-down time is about 28,571 seconds = 8 hours.
Switchover
During the period the V
fall time of V
trigger and switch from V
period from 0V to 5V, the rise time of V
Figure 12. Battery switchover waveform
Also note that for battery operation, there will be a current spike of 5mA in 10us into the
battery when V
CC
F
is approximately 0.5 V. Therefore,
– V
CC
F
CC
– V
from 5V to 0V should be at least 100µs. This is to allow the comparator to
to V
CCmin
BAT
CC
CC
falls, in order for the battery switchover circuit to work reliably, the
switchover happens. From V
to V
Doc ID 17576 Rev 1
BAT
mode should there be a need. During the V
CC
is not critical. This is indicated by the
BAT
to V
CC
switching, the current
Functional description
Figure 12.
CC
rise
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