TCM809_05 datasheet(5/16 Pages) MICROCHIP

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DS21661D-page 5

TCM809/TCM810

4.0

APPLICATIONS INFORMATION

4.1

VDD Transient Rejection

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

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

power-down

condition.

Transient

immunity can be improved by adding a capacitor in

close proximity to the VDD pin of the TCM809/TCM810.

4.2

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 4-2). This

resistor value, though not critical, should be chosen

such that it does not appreciably load RESET under

normal operation (100 k

Ω 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.

FIGURE 4-2:

The addition of R1 at the

RESET output of the TCM809 ensures that the

RESET output is valid to VDD = 0V.

Reset Comparator Overdrive

TH – VDD] (mv)

400

240

160

320

100

1000

A = +25°C

Duration

Overdrive

TCM8XXL/M/J (SOT-23)

TCM8XXZ/R/S/T

(SOT-23)

TH – VDD] (mv)

100

110

120

130

100

1000

Reset Comparator Overdrive (mV)

TCM8XXL/M/J (SC-70)

TCM8XXZ/R/S/T (SC-70)

TCM809

100 k

RESET

GND

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