Datasheet

 2004-2013 Microchip Technology Inc. DS21906C-page 13
MCP102/103/121/131
4.2 Negative Going V
DD
Transients
The minimum pulse width (time) required to cause a
reset may be an important criteria in the implementa-
tion of a Power-on Reset (POR) circuit. This time is
referred to as transient duration, defined as the amount
of time needed for these supervisory devices to
respond to a drop in V
DD
. The transient duration time is
dependant on the magnitude of V
TRIP
– V
DD
. Generally
speaking, the transient duration decreases with
increases in V
TRIP
– V
DD
.
Figure 4-3 shows a typical transient duration vs. reset
comparator overdrive, for which the
MCP102/103/121/131 will not generate a reset pulse. It
shows that the farther below the trip point the transient
pulse goes, the duration of the pulse required to cause
a reset gets shorter. Figure 2-32 shows the transient
response characteristics for the MCP102/103/121/131.
A 0.1 µF bypass capacitor, mounted as close as
possible to the V
DD
pin, provides additional transient
immunity (refer to Figure 4-1).
FIGURE 4-3: Example of Typical
Transient Duration Waveform.
4.3 Reset Power-up Timer (t
RPU
)
Figure 4-4 illustrates the device current states. While
the system is powering down, the device has a low
current. This current is dependent on the device V
DD
and trip point. When the device V
DD
rises through the
voltage trip point (V
TRIP
), an internal timer starts. This
timer consumes additional current until the RST
pin is
driven (or released) high. This time is known as the
Reset Power-up Time (t
RPU
). Figure 4-4 shows when
t
RPU
is active (device consuming additional current).
FIGURE 4-4: Reset Power-up Timer
Waveform.
4.3.1 EFFECT OF TEMPERATURE ON
RESET POWER-UP TIMER (T
RPU
)
The Reset Power-up timer time-out period (t
RPU
)
determines how long the device remains in the reset
condition. This is affected by both V
DD
and temperature.
Typical responses for different V
DD
values and
temperatures are shown in Figures 2-26, 2-27 and 2-28.
Time (µs)
0V
Supply Voltage
5V
V
TRIP(MIN)
- V
DD
t
TRANS
V
TRIP(MAX)
V
TRIP(MIN)
V
TRIP
V
DD
RST
t
RPU
Reset Power-up
Timer Inactive
Reset
Power-up
Timer
Inactive
Reset Power-up Timer Active
See Figures 2-1,
2-2 and 2-3
See Figures 2-4,
2-5 and 2-6
See Figures 2-1,
2-2 and 2-3