2001-2012 Microchip Technology Inc. DS21661E-page 5
TCM809/TCM810
4.0 APPLICATIONS INFORMATION
4.1 V
DD
Transient Rejection
The TCM809/TCM810 provides accurate V
DD
monitor-
ing and reset timing during power-up, power-down and
brown-out/sag conditions. These devices also reject
negative-going transients (glitches) on the power
supply line. Figure 4-1 shows the maximum transient
duration vs. maximum negative excursion (overdrive)
for glitch rejection. Any combination of duration and
overdrive that lies under the curve will not generate a
reset signal.
FIGURE 4-1: Maximum Transient
Duration vs. Overdrive for Glitch Rejection at
+25°C.
Combinations above the curve are detected as a
brown-out or power-down condition. Transient
immunity can be improved by adding a capacitor in
close proximity to the V
DD
pin of the TCM809/TCM810.
4.2 RESET Signal Integrity During
Power-Down
The TCM809 RESET output is valid to V
DD
= 1.0V.
Below this voltage the output becomes an "open cir-
cuit" and does not sink current. This means CMOS
logic inputs to the microcontroller will be floating at an
undetermined voltage. Most digital systems are
completely shut down well above this voltage.
However, in situations where RESET
must be main-
tained valid to V
DD
= 0V, a pull-down resistor must be
connected from RESET
to ground to discharge stray
capacitances and hold the output low (Figure 4-2). This
resistor value, though not critical, should be chosen
such that it does not appreciably load RESET
under
normal operation (100 k will be suitable for most
applications). Similarly, a pull-up resistor to V
DD
is
required for the TCM810 to ensure a valid high RESET
for V
DD
below 1.0V.
FIGURE 4-2: The addition of R
1
at the
RESET
output of the TCM809 ensures that the
RESET
output is valid to V
DD
= 0V.
Reset Comparator Overdrive
[V
TH
– V
DD
] (mv)
400
240
160
320
80
0
1
5
100
1000
T
A
= +25°C
V
TH
Duration
Overdrive
V
DD
TCM8XXL/M/J (SOT-23)
Maximum Transient Duration (µsec)
TCM8XXZ/R/S/T
(SOT-23)
[V
TH
– V
DD
] (mv)
30
40
50
60
70
80
90
100
110
120
130
1 10 100 1000
Reset Comparator Overdrive (mV)
V
DD
to Reset Delay (µsec)
TCM8XXL/M/J (SC-70)
TCM8XXZ/R/S/T (SC-70)
TCM809
V
DD
V
DD
R
1
100 k
RESET
GND
