MAX821 Maxim, MAX821 Datasheet - Page 5

• Manufacturer Part Number: MAX821
• Description: 4-Pin P Voltage Monitors with Pin-Selectable Power-On Reset Timeout Delay
• Manufacturer: Maxim

A microprocessor’s (µP’s) reset input starts the µP in a
known state. These P supervisory circuits assert reset
to prevent code-execution errors during power-up,
power-down, or brownout conditions. They also provide
a reset timeout delay that is pin programmable to 1ms
(max), 20ms (min), or 100ms (min). This feature allows
flexibility in designing bar-code scanners, hand-held
devices, and other applications that require quick or
nonstandard power-up times.
The MAX821’s RESET output is guaranteed to be a
logic low for V
threshold, an internal timer keeps RESET low for the
reset timeout period, as determined by the Set Reset
Timeout (SRT) input. See the Setting the Reset Timeout
Delay section.
If a brownout condition occurs (V
reset threshold), RESET goes low. Any time V
below the reset threshold, the internal timer resets to
zero, and RESET goes low. The internal timer begins
counting after V
and RESET remains low for the reset timeout period.
The MAX822 has an active-high RESET output that is
the inverse of the MAX821’s RESET output.
Use the three-level Set Reset Timeout (SRT) input to set
the reset timeout delay. Connect SRT to GND for a 1ms
(max) delay; connect it to V
or leave it unconnected for a 100ms (min) delay.
If you choose to drive the SRT pin with an external sig-
nal, make sure the signal source can charge/discharge
the capacitance on SRT quickly enough (<500µs) to
avert an unintended reset timeout delay.
To ensure proper operation when selecting the 100ms
timeout (SRT = unconnected), minimize capacitive
loading on the SRT pin (< 200pF). Excessive capacitive
loading can select an unintended faster timeout mode.
The MAX821/MAX822 are designed to meet their worst-
case specifications over their entire operating tempera-
ture range. Choose a reset threshold guaranteed to
assert at a voltage below the power supply’s regulation
range and above the minimum specified operating volt-
age range for the system’s ICs.
_______________Detailed Description
4-Pin µP Voltage Monitors with Pin-Selectable
Setting the Reset Timeout Delay
CC
CC
_______________________________________________________________________________________
> 1V. Once V
returns above the reset threshold,
Reset Threshold Accuracy
CC
for a 20ms (min) delay;
CC
CC
exceeds the reset
Reset Output
dips below the
Power-On Reset Timeout Delay
CC
goes
While designed to issue a reset to the microprocessor
(µP) during power-up, power-down, and brownout con-
ditions, the MAX821/MAX822 are relatively immune to
short-duration, negative-going V
Figure 1 shows the maximum transient duration vs. reset
comparator overdrive for which the MAX821/MAX822
typically do not generate a reset pulse. This graph was
generated using a negative-going pulse applied to V
starting above the actual reset threshold and ending
below it by the magnitude indicated (reset comparator
overdrive). The graph indicates the typical maximum
pulse width a negative-going V
without causing a reset pulse to be issued. As the mag-
nitude of the transient increases (goes farther below the
reset threshold), the maximum allowable pulse width
decreases. Typically, for the MAX821/MAX822, a V
transient that goes 100mV below the reset threshold and
lasts 12µs or less will not cause a reset pulse to be
issued. A 0.1µF capacitor mounted as close as possible
to V
desired.
Figure 1. Maximum Transient Duration Without Causing a
Reset Pulse vs. Comparator Overdrive
__________Applications Information
CC
can provide additional transient immunity, if
300
250
200
150
100
50
0
RESET COMPARATOR OVERDRIVE, V
1
Negative-Going V
10
CC
100
CC
T
transients (glitches).
A
transient may have
TH
= +25°C
CC
-V
CC
1000
(mV)
Transients
CC
CC
5
,