L99VR01STR datasheet(22/47 Pages) STMICROELECTRONICS

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Note:

A diode (0.25 V < VF < 0.75 V) connected in series with EN pin is requested only if the regulator is directly

supplied by car battery.

4.5

Reset

The reset circuit supervises the output voltage VO. If the output voltage falls below VO_th then RST is pulled low

with a reaction time Trr. When the output voltage rises above VO_th + VO_th_hyst then RST is pulled high with a

delay time Trd. The delay is generated by an internal circuit. The reset circuit is active when En is high. Being

RST an open-drain output an external resistance (Rrst) is needed between the RST pin and the Vo pin. The

external resistance value can be in a range between 4.7 KΩ and 20 KΩ. Leave the RST pin floating if not used.

Be aware that the current flowing through the RST pin drawn from Vo when the RST pin is pulled low may affect

the watchdog activation/deactivation based on the regulator output current consumption monitoring.

Figure 37. Reset timing diagram

4.6

Autonomous watchdog

A supplied microcontroller is monitored by the watchdog input Wi (L99VR01J version only). If pulses are missing,

the RST output pin is set low. The watchdog timeout can be set within a wide range with the external capacitor,

Ctw. The watchdog circuit discharges the capacitor Ctw, with the constant current ICWd. If the lower threshold Vwlth

is reached, a watchdog reset is generated. To prevent this from happening the microcontroller must generate a

positive edge during the discharge of the capacitor before the voltage reaches the threshold Vwlth. To calculate the

sawtooth period “Twop”, taking care that the microcontroller triggers the positive edge during the discharge phase

of Ctw (Td), the following equations can be used:

(Vwhth-Vwlth) x Ctw = ICWd x Td

(Vwhth-Vwlth) x Ctw = ICWc x Twol

Twop = Td + Twol

Every Wi positive edge switches the current source from discharging to charging. The same happens when the

lower threshold is reached. When the voltage reaches the upper threshold, Vwhth, the current switches from

charging to discharging. The result is a saw-tooth voltage at the watchdog timer capacitor Ctw. If a microcontroller

operates in low power mode it will not be able to generate any pulse to refresh the voltage regulator watchdog,

triggering so the microcontroller reset. In such a case, to avoid generating the microcontroller reset, the watchdog

functionality will be automatically deactivated any time the microcontroller current consumption falls under the

Iw_off threshold. On the other hand, when the current consumption rises above the Iw_on threshold the watchdog

functionality will be once again activated. Once the regulator is enabled for the first time, if Io < Iw_off the

watchdog will not be activated, while if Io > Iw_off the watchdog will be activated.

L99VR01

Reset

DS13649 - Rev 3

page 22/47

L99VR01STR bảng dữ liệu(PDF) 22 Page - STMicroelectronics