Datasheet

MAX793/MAX794/MAX795
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
______________________________________________________________________________________ 13
10). If there is no easy access to the unregulated sup-
ply, the LOWLINE output can be used to generate an
NMI to the µP (see LOWLINE Output section).
LLOOWWLLIINNEE
Output (MAX793/MAX794)
The low-line comparator monitors V
CC
with a threshold
voltage typically 45mV above the reset threshold (10mV
of hysteresis) for the MAX793, and 15mV above RESET
IN (4mV of hysteresis) for the MAX794. For normal
operation (V
CC
above the reset threshold), LOWLINE is
pulled to V
CC
. Use LOWLINE to provide an NMI to the
µP when power begins to fall.
In most battery-operated portable systems, reserve
energy in the battery provides ample time to complete
the shutdown routine once the low-line warning is
encountered and before reset asserts. If the system
must also contend with a more rapid V
CC
fall time, such
as when the main battery is disconnected or a high-
side switch is opened during normal operation, use
capacitance on the V
CC
line to provide time to execute
the shutdown routine (Figure 11).
First, calculate the worst-case time required for the sys-
tem to perform its shutdown routine. Then, with the worst-
case shutdown time, the worst-case load current, and the
minimum low-line to reset threshold (V
LR
min), calculate
the amount of capacitance required to allow the shut-
down routine to complete before reset is asserted:
C
HOLD
> I
LOAD
x t
SHDN
/ V
LR
where I
LOAD
is the current being drained from the
capacitor, V
LR
is the low-line to reset threshold differ-
ence (V
LL
- V
RST
), and t
SHDN
is the time required for
the system to complete an orderly shutdown routine.
Power-Fail Comparator (MAX793/MAX794)
The MAX793/MAX794’s PFI input is compared to an
internal reference. If PFI is less than the power-fail
threshold (V
PFT
), PFO goes low. The power-fail com-
parator is intended for use as an undervoltage detector
to signal a failing power supply (Figure 12). However,
the comparator does not need to be dedicated to this
function because it is completely separate from the rest
of the circuitry.
V
CC
GND
V
CC
50pF
C
L
*
CE IN
*C
L
INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE.
50Ω
3.6V
25Ω EQUIVALENT
SOURCE IMPEDANCE
50Ω
50Ω CABLE
BATT
CE OUT
MAX793
MAX794
MAX795
Figure 9. CE Propagation Delay Test Circuit
V
CC
GND
PFI
TO µP NMI
R1
UNREGULATED
SUPPLY
3.0V OR 3.3V
R2
PFO
MAX793
MAX794
REGULATOR
Figure 10. Using the Power-Fail Comparator to Generate
Power-Fail Warning
GND
V
CC
TO µP NMI
C
HOLD
C
HOLD
> I
LOAD
x t
SHDN
V
LR
3.0V OR 3.3V
LOWLINE
MAX793
MAX794
REGULATOR
Figure 11. Using LOWLINE to Provide Power-Fail Warning
to the µP