LTC2919C View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'LTC2919C' PDF : 20 Pages View PDF
LTC2919
APPLICATIONS INFORMATION
threshold range is 4.425V ±75mV over temperature (i.e.,
10% to 13% below 5V). The monitored system must thus
operate reliably down to 4.35V or 13% below 5V over
temperature.
The above discussion is concerned only with the DC value
of the monitored supply. Real supplies also have relatively
high frequency variations from sources such as load tran-
sients, noise and pickup.
The LTC2919 uses two techniques to combat spurious
outputs toggling from high frequency variation. First,
the timeout period helps prevent high frequency varia-
tion whose frequency is above 1/ tRST from appearing at
the RST output. Second, the propagation delay versus
overdrive function filters short glitches before the OUT1,
OUT2 toggling or RST pulling low.
TMR pin to VCC). If hysteresis is desired in other modes,
it may be added externally. See 48V Telecom UV/OV with
Hysteresis Applications on page 14 for an example.
Selecting External Resistors
In a typical positive supply monitoring application, the
ADJx pin connects to a tap point on an external resistive
divider between a positive voltage being monitored and
ground, as shown in Figure 1.
When monitoring a negative supply, the ADJx pin con-
nects to a tap point on a resistive divider between the
negative voltage being monitored and the buffered refer-
ence (REF), as shown in Figure 2.
VMON
When an ADJ becomes invalid, the corresponding OUT
and RST pin assert low. When the supply recovers past
the valid threshold, the reset timer starts (assuming it is
not disabled) and RST does not go high until it finishes.
If the supply becomes invalid any time during the timeout
period, the timer resets and starts fresh when the supply
next becomes valid.
To reduce sensitivity of short glitches from toggling the
output pins, the comparator outputs go through a low-
pass filter before triggering the output logic. Any transient
at the input of a comparator needs to be of sufficient
magnitude and duration to pass the filter before it can
change the monitor state.
RP2
ADJx
+
RP1
–
0.5V +–
2919 F01
Figure 1. Setting Positive Supply Trip Point
REF
RN1
ADJx
+
The combination of the reset timeout and comparator
filtering prevents spurious changes in the output state
without sacrificing threshold accuracy. If further supply
glitch immunity is needed, the user may place an external
capacitor from the ADJ input to ground. The resultant RC
lowpass filter with the resistor divider will further reject
high frequency components of the supply, at the cost of
slowing the monitor’s response to fault conditions.
A common solution to the problem of spurious reset is
to introduce hysteresis around the nominal threshold.
However, this hysteresis degrades the effective accuracy
of the monitor and increases the range over which the
system must operate. The LTC2919 therefore does not
have hysteresis, except in comparator mode (by tying
RN2
VMON
–
0.5V +–
2919 F02
Figure 2. Setting Negative Supply Trip Point
Normally the user will select a desired trip voltage based
on their supply and acceptable tolerances, and a value of
RN1 or RP1 based on current draw. Current used by the
resistive divider will be approximately:
0.5V
0.5V
I = .OR =
RP1
R N1
Rev. A
12
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