V

=

VB

V

+ 0.6 V

STC

t

on

V

DDon

V

BUS

V

STC

V

VS

t

off

V

DDoff

V

VDD

V

PF

typical threshold voltage

for power fail PF

STC

VDDoff

off

STC

VDD

CI1

V

– V

t

= C

I

+ I

TSS521

SLAS980 – SEPTEMBER 2013

www.ti.com

Power On and Off

Figure 5. Power On/Off Timing

Power Fail Function

Because of the rectifier bridge BR at the input, BUSL1, and BUSL2, the TSS521 is polarity independent. The pin

VB to ground (GND) delivers the bus voltage VVB less the voltage drop over the rectifier BR. The voltage

comparator TC2 monitors the bus voltage. If the voltage VVB > VSTC + 0.6 V, then the output PF = 1. The output

level PF = 0 (power fail) provides a warning of a critical voltage drop to the microcontroller to save the data

immediately.

6

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