TL7702 Datasheet, PDF (16/17 Pages) Texas Instruments – Supply voltage Supervisor

Design Tips
Figure 14. Printed Circuit Layout for the supply voltage supervisor
When using the supply voltage supervisor TL770xB in certain applications a current limiting
resistor R_t (figure 15) in series to the capacitor C_t (pin 3) is required. With this circuit the timing
capacitor will be charged up to the supply voltage V_cc respectively an internal clamping (= 7.1 V),
where the smaller of the these two voltages determines the final voltage at the capacitor. When
the supply voltage drops quickly down to a voltage which is lower than the current voltage at the
capacitor C , a parasitic current path may be turned on which in turn erroneously activates the
t
outputs. This effect is avoided when the mentioned parasitic current I_p stays below 1 mA.
Considering this the resistor R_t is calculated as follows:
R_t
=
≥
V_{Ct max} − V_t
1mA
where V_t = threshold voltage of the SENSE input
V_Ctmax = V_ccmax or 7 V, the lower of these values apply
When using the TL7705B (V_t = 4.55 V) with a maximum supply voltage V_cc = 5.5 V, one gets:
R_t
=
≥
5.5V − 4.55V
1mA
=
950
12
Literature Number: SLVAE04