LTC2904/LTC2905

10

29045fd

APPLICATIONS INFORMATION

Power-Down

On power-down, once either V1 or V2 inputs drops below

its threshold, RST asserts logic low and RST weakly pulls

RST.

Programming Pins

The three 3-state input pins: S1, S2 and TOL should be

connected to GND, V1 or left unconnected during normal

operation. Note that when left unconnected, the maximum

leakage current allowable from the pin to either GND or

V1 is 10μA.

In margining applications, all the 3-state input pins can be

driven using a tri-state buffer. Note however the low and

Table. Moreover, when the tri-state buffer is in the high

impedance state, the maximum leakage current allowed

from the pin to either GND or V1 is 10μA.

Monitor Programming

Connecting S1 and S2 to GND, V1 or leaving them open

selects the LTC2904/LTC2905 input voltage combina-

tions. Table 1 shows the nine possible combinations of

nominal input voltages and their corresponding S1, S2

connections.

Table 1. Voltage Threshold Programming

V1

V2

S1

S2

5.0

3.3

V1

V1

3.3

2.5

Open

GND

3.3

1.8

V1

Open

3.3

1.5

Open

V1

3.3

1.2

Open

Open

2.5

1.8

GND

GND

2.5

1.5

GND

Open

2.5

1.2

GND

V1

2.5

1.0

V1

GND

Note: Open = open circuit or driven by a three state buffer in high

impedance state with leakage current less than 10μA.

Tolerance Programming

The three-state input pin, TOL programs the common

supply tolerance for both V1 and V2 input voltages (5%,

7.5% or 10%). The larger the tolerance the lower the trip

threshold. Table 2 shows the tolerances selection corre-

sponding to a particular connection at the TOL pin.

Table 2. Tolerance Programming

Tolerance

TOL

5%

V1

7.5%

Open

10%

GND

Threshold Accuracy

Reset threshold accuracy is of the utmost importance in a

supply sensitive system. Ideally such a system should not

reset while supply voltages are within a specified margin

below the rated nominal level. Both of the LTC2904/LTC2905

inputs have the same relative threshold accuracy. The

specification for LTC2904/LTC2905 is

±1.5% of the pro-

grammed nominal input voltage (over the full operating

temperature range).

For example, when the LTC2904/LTC2905 are programmed

to handle a 5V input with 10% tolerance (S1 = S2 = V1 and

TOL = GND, refer to Table 1 and Table 2), it does not issue

a reset command when V1 is above 4.5V. The typical 10%

trip threshold is at 11.5% below the nominal input voltage

level. Therefore, the typical trip threshold for the 5V input

is 4.425V. With

±1.5% accuracy, the trip threshold range is

4.425V

±75mV over temperature (i.e. 10% to 13% below

5V). This implies that the monitored system must operate

reliably down to 4.35V over temperature.

The same system using a supervisor with only

±2.5%

accuracy needs to work reliably down to 4.25V (4.375V

±125mV)or15%below5V,requiringthemonitoredsystem

to work over a much wider operating voltage range.