Datasheet

LTC694-3.3/LTC695-3.3
12
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CE IN can be derived from the microprocessor’s address
decoder output. Figure 6 shows a typical nonvolatile CMOS
RAM application.
Memory protection can also be achieved with the LTC694-
3.3 by using RESET as shown in Figure 7.
Power-Fail Warning
The LTC694-3.3/LTC695-3.3 generate a Power Failure Out-
put (PFO) for early warning of failure in the microprocessor’s
power supply. This is accomplished by comparing the
power failure input (PFI) with an internal 1.3V reference.
PFO goes low when the voltage at the PFI pin is less than
1.3V. Typically PFI is driven by an external voltage divider
(R1 and R2 in Figures 8 and 9) which senses either an
unregulated DC input or a regulated 3.3V output. The
voltage divider ratio can be chosen such that the voltage
at the PFI pin falls below 1.3V several milliseconds before
the 3.3V supply falls below the maximum reset voltage
threshold 3.0V. PFO is normally used to interrupt the
microprocessor to execute shutdown procedure between
PFO and RESET or RESET.
The power-fail comparator, C3, does not have hysteresis.
Hysteresis can be added however, by connecting a resistor
between the PFO output and the noninverting PFI input
pin as shown in Figures 8 and 9. The upper and lower trip
points in the comparator are established as follows:
When PFO output is low, R3 sinks current from the sum-
ming junction at the PFI pin.
V
H
=1.3V 1+
R1
R2
+
R1
R3
⎛
⎝
⎜
⎞
⎠
⎟
When PFO output is high, the series combination of R3
and R4 source current into the PFI summing junction.
V
L
=1.3V 1+
R1
R2
–
(3.3V ±1.3V)R1
1.3V(R3 +R4)
⎛
⎝
⎜
⎞
⎠
⎟
Assuming R4 << R3, V
HYSTERESIS
= 3.3V
R1
R3
APPLICATIONS INFORMATION
Figure 6. A Typical Nonvolatile CMOS RAM Application
3.3V
2.4V
0.1µF
10µF
V
BATT
V
CC
LTC695-3.3
V
OUT
GND
694/5-3.3 F06
V
CC
RESET
CE IN
CE OUT
RESET
0.1µF
TO µP
FROM DECODER
CS
30ns PROPAGATION DELAY
62512
RAM
GND
+
3.3V
2.4V
0.1µF
10µF
V
BATT
V
CC
V
OUT
GND
694/5-3.3 F07
V
CC
RESET
0.1µF
CS
62128
RAM
CS1
CS2
GND
LTC694-3.3
+
Figure 7. Write Protect for RAM with LTC694-3.3
100µF
694/5-3.3 F08
V
CC
0.1µF
10µF
TO µP
PFO
GND
V
IN
≥ 5V
R4
10k
PFI
LTC694-3.3
LTC695-3.3
R1
51k
R2
16k
R3
200k
3.3V
V
IN
V
OUT
LT1129-3.3
SHDN
OUT SENSE
ADJ
+ +
Figure 8. Monitoring Unregulated DC Supply with the
LTC694-3.3/LTC695-3.3’s Power-Fail Comparator
Figure 9. Monitoring Regulated DC Supply with the
LTC694-3.3/LTC695-3.3’s Power-Fail Comparator
10µF
694/5-3.3 F09
0.1µF
TO µP
V
IN
r 6.5V
10µF
R3
2.7M
3.3V
R1
27k
R2
16k
R5
5k
V
CC
GND
PFO
PFI
LTC694-3.3
LTC695-3.3
R4
10k
V
IN
V
OUT
LT1129-3.3
SHDN
OUT SENSE
ADJ
+ +