MAX793 Maxim, MAX793 Datasheet - Page 12

• Manufacturer Part Number: MAX793
• Description: 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
• Manufacturer: Maxim

The CE transmission gate remains disabled and CE IN
remains high impedance (regardless of CE IN activity)
for the first half of the reset timeout period (t
time a reset is generated. While disabled, CE IN is
high impedance. When the CE transmission gate is
enabled, the impedance of CE IN appears as a 46Ω
resistor in series with the load at CE OUT.
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
Figure 7. Chip-Enable Transmission Gate
Figure 8. Chip-Enable Timing
12
CE IN
V
RESET PULLED UP TO V
______________________________________________________________________________________
V
CE OUT
RESET
(PULLED TO V
CE IN
BATT
CC
CHIP-ENABLE
GENERATOR
= 3.6V
MAX793
MAX794
MAX795
CONTROL
OUTPUT
V
RESET
BATT
CC
)
t
V
RP
RST
/
CC
2
t
RP
P
N
OUT
RP
P
/ 2), any
CE OUT
V
RST
V
10 s
SW
V
BATT
The propagation delay through the CE transmission
gate depends on V
drive connected to CE IN, and the loading on CE OUT
(see the Chip-Enable Propagation Delay vs. CE OUT
Load Capacitance graph in the Typical Operating
Characteristics ). The CE propagation delay is produc-
tion tested from the 50% point on CE IN to the 50%
point on CE OUT using a 50Ω driver and 50pF of load
capacitance (Figure 9). For minimum propagation
delay, minimize the capacitive load at CE OUT, and
use a low-output-impedance driver.
When the CE transmission gate is enabled, the imped-
ance of CE OUT is equivalent to a 46Ω resistor in series
with the source driving CE IN. In the disabled mode,
the transmission gate is off and an active pull-up con-
nects CE OUT to OUT (Figure 8). This pull-up turns off
when the transmission gate is enabled.
Critical systems often require an early warning indicat-
ing that power is failing. This warning provides time for
the µP to store vital data and take care of any additional
“housekeeping” functions, before the power supply
gets too far out of tolerance for the µP to operate reli-
ably. The MAX793/MAX794 offer two methods of
achieving this early warning. If access to the unregu-
lated supply is feasible, the power-fail comparator input
(PFI) can be connected to the unregulated supply
V
RST
V
CC
CC
Early Power-Fail Warning
, the source impedance of the
(MAX793/MAX794)
Chip-Enable Output
V
RST
V
SW