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

nected 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 re-

set delay. By choosing different values of capacitor C

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

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

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 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

9

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