LMC6953 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – PCI Local Bus Power Supervisor

Application Note (Continued)
Figure 1 is measured by shorting pins 1, 2 and 3 together
when supply voltage is from 0V to 3.3V. Then pin 3 is con-
nected with a constant 3.3 V_DC and pins 1 and 2 are con-
nected to a separate power supply that continues to vary
from 3.3V to 6V.
5V AND V_DD PINS
By having the 5V and the V_DD pins separate, a capacitor can
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_DD −
0.7V. If the power supply goes away, the diode becomes
reverse-biased, isolating the 5V and the V_DD pins. The ca-
pacitor provides power to the internal comparators for a
short duration for the LMC6953 to operate.
C_EXT SETS RESET DELAY IN LINEAR FASHION
The LMC6953 has internal delay circuitry to generate the re-
set delay. By choosing different values of capacitor C_EXT, re-
set 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_DD
and the 5V pins (V_DD and 5V pins are shorted). RESET out-
put is high because voltages are within window. These volt-
ages should be monitored. While keeping the 3.3V constant,
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_DD and the 5V pins
should have separate signals. It is easier to measure re-
sponse time in this manner. The V_DD pin is connected to a
steady 5V DC and the 5V pin is connected to a pulse genera-
tor. To simulate the power supply voltages going out of win-
dow, 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 pro-
gram 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
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 con-
nect 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.
DS012846-21
FIGURE 2. 5V Under-Voltage Waveforms
DS012846-22
FIGURE 3. Power Failure Waveforms
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