TL7700 Datasheet, PDF (10/11 Pages) STMicroelectronics – SUPPLY VOLTAGE SUPERVISORS

TL7700
SUPPLY-VOLTAGE SUPERVISOR
SLVS220A – JULY 1999 – REVISED NOVEMBER 1999
sense-voltage hysteresis setting
If the sense voltage, V_s, does not have hysteresis in it and the voltage on the sensing line contains ripples, the
resetting of TL7700 will be unstable. Hysteresis is added to the sense voltage to prevent such problems. As
shown in Figure 20, the hysteresis, V_hys, is added, and the value is determined as:
V_hys = I_s × R1
Where:
I_s = 2.5 µA, typically at T_A = 25°C
At room temperature, I_s has variations of 2.5 µA ± 0.5 µA. Therefore, in the circuit shown in Figure 19, V_hys has
variations of (±0.5 × R1) µV. In circuit design, it is necessary to consider the voltage-dividing resistor tolerance
and temperature coefficient in addition to variations in V_s and V_hys.
VCC _Vhys
Vs
1.5 V
T
RESET
tpo
tpo
T
Figure 20. V_CC-RESET Timing Chart
output pulse-duration setting
Constant-current charging starts on the timing capacitor when the sensing-line voltage reaches the TL7700
sense voltage. When the capacitor voltage exceeds the threshold level of the output drive comparator, RESET
changes from a low to a high level. The output pulse duration is the time between the point when the sense-pin
voltage exceeds the threshold level and the point when the RESET output changes from a low level to a high
level. When the TL7700 is used for system power-on reset, the output pulse duration, t_po, must be set longer
than the power rise time. The value of t_po is:
t_po = C_t × 10⁵ seconds
Where:
C_t is the timing capacitor in farads
There is a limit on the device response speed. Even if C_t = 0, t_po is not 0, but approximately 5 µs to 10 µs.
Therefore, when the TL7700 is used as a comparator with hysteresis, without connecting C_t, switching speeds
(t_r/t_f, t_po/t_pd, etc.) must be considered.
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