Datasheet

MCP102/103/121/131
DS20001906D-page 14  2004-2014 Microchip Technology Inc.
4.2 Negative Going V
DD
Transients
The minimum pulse width (time) required to cause a
reset may be an important criterion in the
implementation 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 dependent 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 versus
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 of the
transient pulse goes, the shorter the duration of the
pulse required to cause a reset gets. 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’s 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