L4988D STMicroelectronics, L4988D Datasheet - Page 17

• Manufacturer Part Number: L4988D
• Description: IC REG LDO 5V 200MA 8SOIC
• Manufacturer: STMicroelectronics

Specifications of L4988D

• Regulator Topology: Positive Fixed
• Voltage - Output: 5V
• Voltage - Input: 5.6 ~ 31 V
• Voltage - Dropout (typical): 0.27V @ 200mA
• Number Of Regulators: 1
• Current - Output: 200mA
• Current - Limit (min): 200mA
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Number Of Outputs: 1
• Polarity: Negative, Positive
• Output Voltage: - 0.3 V to + 6 V
• Output Type: Adjustable
• Dropout Voltage (max): 0.5 V at 200 mA
• Output Current: 200 mA
• Line Regulation: 25 mV
• Load Regulation: 25 mV
• Voltage Regulation Accuracy: 2 %
• Maximum Operating Temperature: + 150 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Reference Voltage: 5.1 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

L4988
3.3
Watchdog
A connected microcontroller is monitored by the watchdog input Wi. If pulses are missing,
the Reset output pin is set to low. The pulse sequence time 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 the microcontroller must generate a positive edge during the
discharge of the capacitor before the voltage has reached the threshold Vwlth. In order to
calculate the minimum time t, during which the micro-controller must output the positive
edge, the following equation can be used:
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.
Figure 27. Watchdog timing diagram
Vcw
Res
Wi
(Vwhth-Vwlth) x Ctw = Icwd x t
twol
twop
Application information
Vwhth
Vwlth
Vwlth
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