LTC2904 データシートの表示（PDF） - Linear Technology
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LTC2904 Datasheet PDF : 16 Pages
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LTC2904/LTC2905
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
high. VCC of at least 1V guarantees a logic low of 0.4V at
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
high output of the tri-state buffer has to satisfy the VIL and
VIH of the 3-state pin listed in the Electrical Characteristics
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) or 15% below 5V, requiring the monitored system
to work over a much wider operating voltage range.
29045fd
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