50鈩?/div>
GND
*C
L
INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE.
Figure 9. CE Propagation Delay Test Circuit
through a voltage divider, with the power-fail compara-
tor output (PFO) providing the NMI to the 碌P (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 碌P (see
LOWLINE Output
section).
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.
LOWLINE 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
Power-Fail Comparator (MAX793/MAX794)
The MAX793/MAX794鈥檚 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.
UNREGULATED
SUPPLY
REGULATOR
3.0V OR 3.3V
3.0V OR 3.3V
REGULATOR
V
CC
C
HOLD
V
CC
LOWLINE
TO
碌P
NMI
R1
MAX793
MAX794
PFI
PFO
TO
碌P
NMI
C
HOLD
> I
LOAD
x t
SHDN
V
LR
MAX793
MAX794
R2
GND
GND
Figure 10. Using the Power-Fail Comparator to Generate
Power-Fail Warning
Figure 11. Using LOWLINE to Provide Power-Fail Warning
to the 碌P
13
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