TCM809/TCM810
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 VDD pin of the TCM809/TCM810.
3.0
3.1
APPLICATIONS INFORMATION
V
Transient Rejection
DD
The TCM809/TCM810 provides accurate VDD 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.
3.2
RESET Signal Integrity During
Power-Down
The TCM809 RESET output is valid to VDD = 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 VDD = 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 kΩ will be suitable for most
applications). Similarly, a pull-up resistor to VDD is
required for the TCM810 to ensure a valid high RESET
for VDD below 1.0V.
VDD
VTH
Overdrive
Duration
400
TA = +25°C
320
240
160
VDD
VDD
TCM8XXL/M/J (SOT-23)
80
TCM809
TCM8XXZ/R/S/T
(SOT-23)
RESET
0
1
5
1000
100
Reset Comparator Overdrive
[VTH - VDD] (mv)
R1
100 kΩ
GND
130
120
110
100
90
TCM8XXL/M/J (SC-70)
FIGURE 3-2:
RESET output of the TCM809 ensures that the
RESET output is valid to VDD = 0V.
The addition of R1 at the
80
70
60
50
TCM8XXZ/R/S/T (SC-70)
40
30
1
10
100
1000
Reset Comparator Overdrive (mV)
[VTH - VDD] (mv)
FIGURE 3-1:
Maximum Transient
Duration vs. Overdrive for Glitch Rejection at
+25°C.
2004 Microchip Technology Inc.
DS21661C-page 5
MICROCHIP [ MICROCHIP ]
