LMC6953CMX/NOPB National Semiconductor, LMC6953CMX/NOPB Datasheet - Page 9

• Manufacturer Part Number: LMC6953CMX/NOPB
• Description: IC POWER SUP PCI LOCAL BUS 8SOIC
• Manufacturer: National Semiconductor
• Type: Multi-Voltage Supervisorr

Specifications of LMC6953CMX/NOPB

• Number Of Voltages Monitored: 2
• Output: Open Drain or Open Collector
• Reset: Active Low
• Reset Timeout: 100 ms Minimum
• Voltage - Threshold: 3.3V, 5V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LMC6953CMX; *LMC6953CMX/NOPB; LMC6953CMX

Application Note
connected with a constant 3.3 V
connected to a separate power supply that continues to vary
from 3.3V to 6V.
5V AND V
By having the 5V and the V
be used as a back-up power supply in event of a sudden
power supply failure. This circuit is shown in the application
circuit section titled “On Motherboard With Capacitor as a
Back-up Power Supply.” Under normal condition, the diode is
forward-biased and the capacitor is charged up to V
0.7V. If the power supply goes away, the diode becomes
reverse-biased, isolating the 5V and the V
pacitor provides power to the internal comparators for a
short duration for the LMC6953 to operate.
C
The LMC6953 has internal delay circuitry to generate the
reset delay. By choosing different values of capacitor C
reset delay can be programmed to the desired length for the
system to stabilize after a fault condition occurs.
EVALUATING THE LMC6953
To Measure Over-Voltages And Under-Voltages
Connect a 3.3V DC to the 3.3V pin and a 5V DC to the V
and the 5V pins (V
output is high because voltages are within window. These
voltages should be monitored. While keeping the 3.3V con-
stant, increase the 5V DC signal until a RESET low is
detected. The point on the 5V DC signal at which RESET
changes from high to low is the 5V over-voltage. It is typically
5.6V. To detect 5V under-voltage, start the 5V DC signal from
5V and decrease it until a RESET low is detected. The point
on the 5V DC signal at which RESET changes from high to
low is the 5V under-voltage. It is typically 4.4V.
To find 3.3V over-voltage and under-voltage, keep the 5V
DC at 5V and vary the 3.3V DC signal until a RESET low is
detected.
To Measure Timing Specifications
For evaluation purposes only, the V
should have separate signals. It is easier to measure re-
sponse time in this manner. The V
steady 5V DC and the 5V pin is connected to a pulse
generator. To simulate the power supply voltages going out
of window, a pulse generator with disable/enable feature and
rise and fall time adjustment is recommended. To measure
the RESET signal, a oscilloscope is recommended because
of its ability to capture and store a signal.
To measure the 5V under-voltage response time on the
LMC6953, set the pulse generator to trigger mode and pro-
gram the amplitude to have a high value of 5V and a low
value of the 5V under-voltage threshold measured previ-
ously with 50 mV overdrive. For example, if the measured 5V
under-voltage is 4.4V, then a 50 mV overdrive on this signal
is 4.35V. The disable feature on the pulse generator should
be on. Program the fall time of the pulse to be 30 ns and
program the scope to trigger on the falling edge, with trigger
level of 4.5V. Set the scope to 200 ns/division. The probes
should be connected to the 5V pin and the RESET pin. Now
EXT
SETS RESET DELAY IN LINEAR FASHION
DD
PINS
DD
and 5V pins are shorted). RESET
DD
pins separate, a capacitor can
(Continued)
DC
DD
and pins 1 and 2 are
DD
pin is connected to a
and the 5V pins
DD
pins. The ca-
DD
EXT
DD
−
,
9
enable the 5V signal from the pulse generator and trigger the
signal. Be aware that when the signal is enabled, there is
high frequency noise present, and putting a 120 pF capacitor
between the 5V pin and ground suppresses some of the
noise. Response time is measured by taking the 5V under-
voltage threshold on the 5V signal to the point where RESET
goes low. Figure 2 shows a scope photo of 5V under-voltage
waveforms. It is taken with a signal going from 5V to 4.25V at
the 5V pin.
To measure the 100 ms RESET delay, change the scope to
50 ms/division and trigger the 5V signal again. RESET
should stay low for 100 ms after the 5V is recovered and
within window.
Other over-voltages and under-voltages can be measured
by changing the pulse generator to different voltage steps.
Putting a 120 pF capacitor between the 3.3V pin and ground
is recommended in evaluating 3.3V signal.
To measure power-failure response time, set the pulse gen-
erator from 5V to 3V with fall time of the pulse 3 ns and
connect it to the 5V pin. RESET should go low within 90 ns
of power failure. Figure 3 shows a scope photo of power
failure waveforms. It is taken with a signal going from 5V to
3V at the 5V pin.
FIGURE 2. 5V Under-Voltage Waveforms
FIGURE 3. Power Failure Waveforms
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