AN1336

Functional description

1

Functional description

An independent bandgap reference comparator is used to monitor the unregulated supply

voltage by connecting this supply to the power-fail input pin. The RC time constant of the

typical power supply will provide several milliseconds of operating voltage before decaying

below a usable value. The power-fail input is constantly compared with an internal voltage

reference of 1.25 V (see Figure 4: "Power-fail comparator circuit"). If the input voltage falls

below 1.25 V, the power-fail output goes low. When it later goes above 1.25 V, the output

returns high.

Adding two external resistors (see Figure 5: "PFI/PFO in a typical system" ) as a voltage

divider circuit allows the comparator to supervise any voltage above 1.25 V. The formula to

Figure 4: Power-fail comparator circuit

The sum of both resistors should be about 1 Mohm to minimize power consumption and to

ensure the current in the PFI pin can be neglected compared with the current through the

resistor network. The suggested resistor values are shown below (see Table 1: "Look-up

table for different trip points"). The tolerance of the resistors should not exceed 1% to

ensure the sensed voltage does not vary too much.

Table 1: Look-up table for different trip points

R1 (kOhms)

R2 (kOhms)

Vtrip (V)

750

130

8.5

910

130

10.0

820

100

11.5

820

91

12.5

1100

100

15.0

+

+

–

–

1.25V

PFI

PFO

AI04221

DocID007594 Rev 2

3/10

where VPFI = 1.25 V

V TRIP

VPFI

R1

R2

+

(

)

R2

--------------------

=