TLC770X Datasheet, PDF (8/27 Pages) Texas Instruments – BiCMOS SUPPLY VOLTAGE SUPERVISORS

2.2 Battery Operated Power Supplies
Battery operated power supplies are playing an ever increasing role, thanks to the continuing trend towards portable
systems.
In comparison to line dependent power supplies, battery supplies have the advantage that the voltage at switch on
immediately rises to its final value. This does not however take place quite as quickly as one might wish, because
the blocking and filter capacitor C_F must also be charged via the internal resistance R_i of the battery. To a first
approximation, the following values can be assumed:
R_i = 0.1Ω - 10 Ω ; C_F = 10 µF − 1000 µF
The time period needed by the voltage to reach 95% of its nominal level can be calculated as follows:
t
−
V_DD (1− e ^RiCF ) = 0.95 V_DD
⇒ t = − R_i C_F ⋅ log 0.05
This results in values for the rise time of between 0.03 and 30 ms.
An additional factor which influences the switching on time of the battery voltage is the bouncing of the contacts of
the mechanical switch (Figure 4.).
Figure 4.
supply.
The effect of bouncing switch contacts on the switch-on time of a battery-operated power
The Reset circuit must also in this case remain active until the supply voltage has stabilized.
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