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2004 Microchip Technology Inc.
DS21661C-page 5
TCM809/TCM810
3.0
APPLICATIONS INFORMATION
3.1
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 sup-
ply line. Figure 3-1 shows the maximum transient dura-
tion vs. maximum negative excursion (overdrive) for
glitch rejection. Any combination of duration and over-
drive that lies under the curve will not generate a reset
signal.
FIGURE 3-1:
Duration vs. Overdrive for Glitch Rejection at
+25°C.
Maximum Transient
Combinations above the curve are detected as a
brown-out or power-down condition. Transient immu-
nity can be improved by adding a capacitor in close
proximity to the V
DD
pin of the TCM809/TCM810.
3.2
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 3-2). 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 3-2:
RESET output of the TCM809 ensures that the
RESET output is valid to
V
DD
= 0V.
The addition of R
1
at the
Reset Comparator Overdrive
[V
TH
- V
DD
] (mv)
400
240
160
320
80
0
1
5
100
1000
T
A
= +25°C
V
TH
Overdrive
Duration
V
DD
TCM8XX
L/M/J
(SOT-23)
M
TCM8XX
Z/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
D
TCM8XX
L/M/J (SC-70)
TCM8XX
Z/R/S/T (SC-70)
TCM809
V
DD
V
DD
R
1
100 k
RESET
GND