Datasheet

TLE42694
Block Description and Electrical Characteristics
Data Sheet 17 Rev. 1.2, 2010-11-26
5.3 Reset Function
The reset function provides several features:
Output Undervoltage Reset:
An output undervoltage condition is in
dicated 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
= 0 V.
If the application needs a power-on reset delay time
t
rd
different from the value given in Item 5.3.8, the delay
capacitor’s value can be derived fro
m the specified values in Item 5.3.8 and the desired power-on delay time:
C
D
t
rd new,
t
rd
----------------
100nF×=
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 capa
citor’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:
t
rr
t
rd int,
t
rr d,
+=
with
•
t
rr
: reset reaction time
•
t
rr,int
: internal reset reaction time
•
t
rr,d
: reset discharge
Optional Reset Output Pull-Up Resistor
R
RO,ext
:
The Reset Output RO is an open collecto
r output with an integrated pull-up resistor. To improve the EMC
behaviour of the component, an external pull-up resistor to the output
V
Q
can be added. In Table “Electrical
Characteristics Reset Function” on Page 21 a minimum value for the external resistor
R
RO,ext
is given.