MAX795 MAXIM [Maxim Integrated Products], MAX795 Datasheet - Page 13

• Manufacturer Part Number: MAX795
• Description: 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
• Manufacturer: MAXIM [Maxim Integrated Products]

through a voltage divider, with the power-fail compara-
tor output (PFO) providing the NMI to the µP (Figure
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).
The low-line comparator monitors V
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
Figure 9. CE Propagation Delay Test Circuit
Figure 10. Using the Power-Fail Comparator to Generate
Power-Fail Warning
*C
UNREGULATED
SUPPLY
L
INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE.
50
R1
R2
CC
REGULATOR
50 CABLE
above the reset threshold), LOWLINE is
25 EQUIVALENT
SOURCE IMPEDANCE
LOWLINE Output (MAX793/MAX794)
______________________________________________________________________________________
3.6V
50
3.0V OR 3.3V
PFI
3.0V/3.3V Adjustable Microprocessor
MAX793
MAX794
GND
V
BATT
CE IN
CC
MAX793
MAX794
MAX795
PFO
CC
GND
V
V
CC
CC
CE OUT
with a threshold
TO P NMI
50pF
C
L
*
pulled to V
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
as when the main battery is disconnected or a high-
side switch is opened during normal operation, use
capacitance on the V
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
the amount of capacitance required to allow the shut-
down routine to complete before reset is asserted:
where I
capacitor, V
ence (V
the system to complete an orderly shutdown routine.
The MAX793/MAX794’s PFI input is compared to an
internal reference. If PFI is less than the power-fail
threshold (V
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.
Figure 11. Using LOWLINE to Provide Power-Fail Warning
to the µP
Supervisory Circuits
REGULATOR
C
LL
HOLD
LOAD
C
CC
Power-Fail Comparator (MAX793/MAX794)
- V
> I
HOLD
LR
PFT
LOAD
. Use LOWLINE to provide an NMI to the
RST
C
is the current being drained from the
HOLD
3.0V OR 3.3V
is the low-line to reset threshold differ-
), PFO goes low. The power-fail com-
V
x t
LR
> I
SHDN
), and t
LOAD
CC
line to provide time to execute
SHDN
V
x t
CC
SHDN
MAX793
MAX794
GND
is the time required for
LOWLINE
/ V
LR
LR
CC
min), calculate
fall time, such
TO P NMI
13