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 Rt (figure 15) in series to the capacitor Ct (pin 3) is required. With this circuit the timing
capacitor will be charged up to the supply voltage Vcc 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 Ct, a parasitic current path may be turned on which in turn erroneously activates the
outputs. This effect is avoided when the mentioned parasitic current Ip stays below 1 mA.
Considering this the resistor Rt is calculated as follows:
VCt max - V
t
R = ³
t
1mA
where Vt = threshold voltage of the SENSE input
VCtmax = Vccmax or 7 V, the lower of these values apply
When using the TL7705B (Vt = 4.55 V) with a maximum supply voltage Vcc = 5.5 V, one gets:
5.5V - 4.55V
R = ³
= 950
t
1mA
12
Literature Number: SLVAE04
TI [ TEXAS INSTRUMENTS ]
