TLE4678 Datasheet, PDF (22/31 Pages) Infineon Technologies AG – Low Drop Out Linear Voltage Regulator 5 V Fixed Output Voltage

8
Watchdog Function
TLE4678
Watchdog Function
8.1
Description
The TLE4678 features a load dependent watchdog function with a programmable activating threshold as well as
a programmable watchdog timing.
The watchdog function monitors a microcontroller, including time base failures. In case of a missing rising edge
within a certain pulse repetition time, the watchdog output is set to ‘low’. The programming of the expected
watchdog pulse repetition time can be easily done by an external reset delay capacitor.
The watchdog output “WO” is separated from the reset output “RO”. Hence, the watchdog output might be used
as an interrupt signal for the microcontroller independent from the reset signal. It is possible to interconnect pin
“WO” and pin “RO” in order to establish a wire-or function with a dominant low signal.
Programmable Watchdog Activation Threshold and Hysteresis
In case a microcontroller is set to sleep mode or to low power mode, its current consumption is very low and the
controller might not be able to send any watchdog pulses to the regulators watchdog input “WI”. In order to avoid
unwanted wake-up signals due to missing edges at pin “WI”, the TLE4678 watchdog function can be activated
dependent on the regulator’s output current. The TLE4678 comprises a default watchdog activating threshold
I_Q,WDact,th with a small hysteresis I_Q,WDact,hy. The thresholds can be increased by connecting an external resistor
R_WADJ,ext to pin “WADJ”. For using the default watchdog activating threshold, leave pin “WADJ” open.
The following equation calculates the external resisistor R_WADJ,ext that is needed at pin “WADJ” for activating the
watchdog at a desired output current I_Q,WDact,th:
R_WADJ,ext
=
F R _WDact,th × _WADJ,int
R( _WADJ,int × I_Q,WDact,th) - F_WDact,th
for I_Q,WDact,th larger than the default value given in Item 8.2.1. (5)
At decreasing output current, the deactivation threshold then would be:
I F ^{RR RR} _Q,WDdeact,th = _WDdeact,th ×
WADJ,int ⁺
WADJ,int ^×
WADJ,ext
WADJ,ext
(6)
The watchdog activating threshold hysteresis I_Q,WDact,hy calculates:
I F ^RR _{RR Q,WDact,hy} = _WDact,hy ×
WADJ,int ⁺
WADJ,int ^×
WADJ,ext
WADJ,ext
(7)
with:
• I_Q,WDact,th : Desired “Watchdog Activating Threshold”
• R_WADJ,int : Internal Watchdog Adjust Resistor
• R_WADJ,ext : External Watchdog Adjust Resistor
• F_WDact,th : Activating Threshold Factor
• F_WDdeact,th : Deactivating Threshold Factor
• F_WDact,hy : Activating Threshold Factor Hysteresis
Datasheet
22
Rev. 1.1, 2009-08-27