TC1274 Datasheet, PDF (6/12 Pages) Microchip Technology – 3-Pin Reset Monitors for 5V Systems

TC1272/TC1273/TC1274
4.2 V_CC Transient Rejection
The TC1272/TC1273/TC1274 provides accurate V_CC
monitoring and reset timing during power-up, power-
down and brownout/sag conditions, and rejects
negative-going transients (glitches) on the power
supply line. Figure 4-5 shows the maximum transient
duration vs. maximum negative excursion (overdrive)
for glitch rejection. Any combination of duration and
overdrive that lays under the curve will not generate a
reset signal. Combinations above the curve are
detected as a brownout or power-down condition.
Transient immunity can be improved by adding a
capacitor in close proximity to the V_CC pin of the
TC1272/TC1273/TC1274.
4.3 RESET Signal Integrity During
Power-Down
The TC1272 RESET output is valid to V_CC = 1.2V.
Below this voltage the output becomes an "open
circuit" and does not sink current. This means CMOS
logic inputs to the µP will be floating at an undeter-
mined voltage. Most digital systems are completely
shut down well above this voltage. However, in situa-
tions where RESET must be maintained valid to
V_CC = 0V, a pull-down resistor must be connected from
RESET to ground to discharge stray capacitances and
hold the output low (Figure 4-6). 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). Simi-
larly, a pull-up resistor to V_CC is required for the
TC1274 to ensure a valid high RESET for V_CC below
1.8V.
V_CC
V_TH
Overdrive
Duration
500
T_A = +25°C
400
300
200
100
TC1272/3/4
0
1
10
100
1000
RESET COMPARATOR OVERDRIVE,
[V_CCTP-V_CC] (mV)
FIGURE 4-5:
Maximum Transient
Duration vs. Overdrive For Glitch Rejection At
+25°C.
V_CC
V_CC
TC1272
RESET
GND
R₁
100 kΩ
FIGURE 4-6:
V_cc = 0V.
Ensuring Reset Valid To
DS21382B-page 6
 2003 Microchip Technology Inc.