TC1272A Datasheet, PDF (4/14 Pages) Microchip Technology – 3-Pin Reset Monitor

TC1272A
3.0 APPLICATIONS INFORMATION
3.1 V_DD Transient Rejection
The TC1272A provides accurate V_DD monitoring 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 3-1 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.
V_DD
V_TH
Overdrive
Duration
400
T_A = +25°C
320
240
160
TC1272A/L/M/J
80
0 ^{TC1272A/Z/R/S/T}
1
5
100
1000
Reset Comparator Overdrive
[V_TH - V_DD] (mv)
130
120
110
100
90
80
70
60
50
40
30
1
TC1272A/L/M/J
TC1272A/Z/R/S/T
10
100
Reset Comparator Overdrive (mV)
[V_TH - V_DD] (mv)
1000
FIGURE 3-1:
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 immu-
nity can be improved by adding a capacitor in close
proximity to the V_DD pin of the TC1272A.
3.2 RESET Signal Integrity During
Power-Down
The TC1272A 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 com-
pletely shut down well above this voltage. However, in
situations where RESET must be maintained 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).
V_DD
V_DD
TC1272A
RESET
GND
R₁
100 k
FIGURE 3-2:
The addition of R₁ at the
RESET output of the TC1272A ensures that the
RESET output is valid to V_DD = 0V.
DS21877B-page 4
 2004-2012 Microchip Technology Inc.