Datasheet

Data Sheet 17 Rev. 1.1, 2008-10-09
TLE42794
Block Description and Electrical Characteristics
5.6 Reset Function
The reset function provides several features:
Output Undervoltage Reset:
An output undervoltage condition is indicated by setting the Reset Output RO to “low”. This signal might be used
to reset a microcontroller during low supply voltage.
Power-On Reset Delay Time:
The power-on reset delay time
t
rd
allows a microcontoller and oscillator to start up. This delay time is the time frame
from exceeding the reset switching threshold
V
RT
until the reset is released by switching the reset output “RO” from
“low” to “high”. The power-on reset delay time
t
rd
is defined by an external delay capacitor C
D
connected to pin D
charged by the delay capacitor charge current
I
D,ch
starting from V
D
=0V.
If the application needs a power-on reset delay time
t
rd
different from the value given in Item 5.6.7, the delay
capacitor’s value can be derived from the specified values in Item 5.6.7 and the desired power-on delay time:
with
•
C
D
: capacitance of the delay capacitor to be chosen
•
t
rd,new
: desired power-on reset delay time
•
t
rd
: power-on reset delay time specified in this datasheet
For a precise calculation also take the delay capacitor’s tolerance into consideration.
Reset Reaction Time:
The reset reaction time avoids that short undervoltage spikes trigger an unwanted reset “low” signal. The reset
reaction rime
t
rr
considers the internal reaction time t
rr,int
and the discharge time t
rr,d
defined by the external delay
capacitor
C
D
(see typical performance graph for details). Hence, the total reset reaction time becomes:
with
•
t
rr
: reset reaction time
•
t
rr,int
: internal reset reaction time
•
t
rr,d
: reset discharge
C
D
t
rd new,
t
rd
----------------
47nF×=
t
rr
t
rd int,
t
rr d,
+=