STLED325 datasheet(29/62 Pages) STMICROELECTRONICS

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STLED325

Functional description

Doc ID 17576 Rev 1

29/62

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 = VCC – VF – VCCmin

where VF is approximately 0.5 V. Therefore,

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 = C

ΔV/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.

3.17.1

Switchover

During the period the VCC falls, in order for the battery switchover circuit to work reliably, the

fall time of VCC from 5V to 0V should be at least 100µs. This is to allow the comparator to

trigger and switch from VCC to VBAT mode should there be a need. During the VCC rise

period from 0V to 5V, the rise time of VCC is not critical. This is indicated by the Figure 12.

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 VCC to VBAT switchover happens. From VBAT to VCC switching, the current

STLED325 Datasheet(PDF) 29 Page - STMicroelectronics