Datasheet

ending below it by the magnitude indicated (reset threshold
overdrive). As the magnitude of the transient increases
(farther below the reset threshold), the maximum allowable
pulse width decreases. Typically, a V
CC
transient that goes
100mV below the reset threshold and lasts 50μs or less
does not cause a reset pulse to be issued.
Interfacing to Other Voltages for
Logic Compatibility
The open-drain RESET output can be used to interface
to a μP with other logic levels. As shown in Figure 6, the
open-drain output can be connected to voltages from 0
to 6V.
Generally, the pullup resistor connected to RESET
connects to the supply voltage that is being monitored at
the IC’s V
CC
pin. However, some systems can use the
open-drain output to level-shift from the monitored supply
to reset circuitry powered by some other supply. Keep
in mind that as the supervisor’s V
CC
decreases towards
1V, so does the IC’s ability to sink current at RESET.
Also, with any pullup resistor, RESET is pulled high as
V
CC
decays toward zero. The voltage where this occurs
depends on the pullup resistor value and the voltage to
which it is connected.
Ensuring a Valid RESET Down to V
CC
= 0V
(Push-Pull RESET)
When V
CC
falls below 1V, RESET current sinking capabil-
ities decline drastically. The high-impedance CMOS logic
inputs connected to RESET can drift to undetermined
voltages. This presents no problems in most applications,
since most μPs and other circuitry do not operate with
V
CC
below 1V.
In those applications where RESET must be valid down to
0V, add a pulldown resistor between RESET and GND for
the MAX6746/MAX6748/MAX6750/MAX6752 push/pull
outputs. The resistor sinks any stray leakage currents,
holding RESET low (Figure 7). The value of the pulldown
resistor is not critical; 100kΩ is large enough not to load
RESET and small enough to pull RESET to ground. The
external pulldown cannot be used with the open-drain
reset outputs.
Figure 7. Ensuring RESET Valid to V
CC
= 0V
MAX6746
MAX6748
MAX6450
MAX6752
GND
RESET
V
CC
V
CC
100kΩ
MAX6746–MAX6753 μP Reset Circuits with Capacitor-Adjustable
Reset/Watchdog Timeout Delay
www.maximintegrated.com
Maxim Integrated │ 11