L99VR02XP datasheet(26/50 Pages) STMICROELECTRONICS

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4.5

Reset

Two independently reset circuits supervise the output voltages VO_LDO1,2. If at least one output voltage falls below

Vo_th_LDO1,2 then RST_LDO1,2 is pulled low with a reaction time Trr.

When the output voltage rises above Vo_th_LDO1,2 + Vo_th_hyst_LDO1,2 then RST_LDO1,2 is pulled high with a

delay time Trd. The delay is generated by an internal circuit.

The reset circuit is active when EN_LDO1,2 is high. Being RST_LDO1,2 an open-drain output an external

resistance (Rrst) is needed between the RST_LDO1,2 pin and the VO_LDO1,2 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_LDO1,2 when the RST pin is pulled low may affect the watchdog

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

Trd is set by an external capacitor in Vcr_LDO1,2 pins, following the below rule:

Trd= 2.2V

Icr_LDO1,2Ctr

(1)

Where:

Trd = 16 µs if Vcr_LDO1,2 pins are floating.

Figure 44. Reset timing diagram

4.6

Autonomous watchdog

Up to two supplied microcontrollers are monitored by the watchdog inputs Wi_LDO1,2. If pulses are missing, the

relative RST_LDO1,2 output pins are set to low. The watchdog timeout can be set within a wide range with the

external capacitor, Ctw. The watchdog circuits discharge the capacitor Ctwx, with the constant current ICWd_LDO1,2.

The value of the watchdog ignore time changes according to the value of the capacity used, according to the

following formula:

If the lower threshold Vwlth_LDO1,2 is reached, a watchdog reset is generated. To prevent this from happening the

microcontrollers must generate a positive edge during the discharge of the capacitors before the voltage reaches

the threshold Vwlth_LDO1,2. To calculate the sawtooth period “Twop”, taking care that the microcontroller triggers the

positive edge during the discharge phase of Ctwx (Td), the following equation can be used:

Vwℎtℎ_LDO1,2−Vwltℎ_LDO1,2 ×Ctw=ICWd_LDO1,2×Td

(2)

Vwℎtℎ_LDO1,2−Vwltℎ_LDO1,2 ×Ctw=ICWc_LDO1,2×Twol

(3)

Twop=Td+Twol

(4)

L99VR02XP

Application information

DS14686 - Rev 1

page 26/50

L99VR02XP 데이터시트(PDF) 26 Page - STMicroelectronics