DS1231SLASHS datasheet(2/9 Pages) MAXIM

2 / 9 page

DS1231/S

022698 2/9

OPERATION

The DS1231 Power Monitor detects out-of-tolerance

power supply conditions and warns a processor-based

system of impending power failure. The main elements

of the DS1231 are illustrated in Figure 1. As shown, the

DS1231 actually has two comparators, one for monitor-

ing the input (Pin 1) and one for monitoring VCC (Pin 8).

The VCC comparator outputs the signals RST (Pin 5)

and RST (Pin 6) when VCC falls below a preset trip level

as defined by TOL (Pin 3).

When TOL is connected to ground, the RST and RST

signals will become active as VCC goes below 4.75

volts. When TOL is connected to VCC, the RST and RST

signals become active as VCC goes below 4.5 volts. The

RST and RST signals are excellent control signals for a

microprocessor, as processing is stopped at the last

possible moments of valid VCC. On power-up, RST and

RST are kept active for a minimum of 150 ms to allow the

power supply to stabilize (see Figure 2).

The comparator monitoring the input pin produces the

NMI signal (Pin 7) when the input threshold voltage

(VTP) falls to a level as determined by Mode (Pin 2).

When the Mode pin is connected to VCC, detection oc-

curs at VTP-. In this mode Pin 1 is an extremely high im-

pedance input allowing for a simple resistor voltage di-

vider network to interface with high voltage signals.

When the Mode pin is connected to ground, detection

occurs at VTP+. In this mode Pin 1 sources 30 µA of cur-

rent allowing for connection to switched inputs, such as

a UL-approved opto-isolator. The flexibility of the input

pin allows for detection of power loss at the earliest point

in a power supply system, maximizing the amount of

time allotted between NMI and RST. On power-up, NMI

is released as soon as the input threshold voltage (VTP)

is achieved and VCC is within nominal limits. In both

modes of operation the input pin has hysteresis for

noise immunity (Figure 3).

APPLICATION – MODE PIN

CONNECTED TO VCC

When the Mode pin is connected to VCC, pin 1 is a high

impedance input. The voltage sense point and the level

of voltage at the sense point are dependent upon the

application (Figure 4). The sense point may be devel-

oped from the AC power line by rectifying and filtering

the AC. Alternatively, a DC voltage level may be se-

lected which is closer to the AC power input than the

regulated +5-volt supply, so that ample time is provided

for warning before regulation is lost.

Proper operation of the DS1231 requires a maximum

voltage of 5 volts at the input (Pin 1), which must be

derived from the maximum voltage at the sense point.

This is accomplished with a simple voltage divider net-

work of R1 and R2. Since the IN trip point VTP- is 2.3

volts (using the -20 device), and the maximum allowable

voltage on pin 1 is 5 volts, the dynamic range of voltage

at the sense point is set by the ratio of 2.3/5.0=.46 min.

This ratio determines the maximum deviation between

the maximum voltage at the sense point and the actual

voltage which will generate NMI.

Having established the desired ratio, and confirming

that the ratio is greater than .46 and less than 1, the

proper values for R1 and R2 can be determined by the

equation as shown in Figure 4. A simple approach to

solving this equation is to select a value for R2 which is

high enough impedance to keep power consumption

low, and solve for R1. Figure 5 illustrates how the

DS1231 can be interfaced to the AC power line when

the mode pin is connected to VCC.

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