LMC6953 Datasheet, PDF (10/19 Pages) National Semiconductor (TI) – PCI Local Bus Power Supervisor

LMC6953
SNVS132D – APRIL 1998 – REVISED APRIL 2013
EVALUATING THE LMC6953
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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 output is high because voltages are within window. These voltages 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
response time in this manner. The V_DD pin is connected to a 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
program the amplitude to have a high value of 5V and a low value of the 5V under-voltage threshold measured
previously 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 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 19 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 generator 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 20 shows a scope photo
of power failure waveforms. It is taken with a signal going from 5V to 3V at the 5V pin.
Figure 19. 5V Under-Voltage Waveforms
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