L4954 查看數據表（PDF） - STMicroelectronics
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L4954 Datasheet PDF : 13 Pages
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Functional description
3
Functional description
L4954
The L4954 includes a monitoring circuit to warn the microprocessor if a low voltage or no
voltage condition is occurring.
When the voltage on the STCAP pin drops below 7.5 V (typ), the LVW output goes low. This
tells the microprocessor to stop executing code and save vital information. The reset output
goes low when REG4 (5 V - standby) drops below (VREG4 -250 mV) or 4.75 V is minimum
value. The RESET output doesn’t go above 0.4V until REG4 has gone back above 4.75 V
(min).
Any spike tells the microprocessor to start operating. Once the STCAP line passes 7.5 V
(typ), the LVWarn output also returns to high state.
The STCAP pin acts like a delay circuit. Due to the large capacitor (470 μF), the STCAP pin
allows the battery voltage to decay slowly giving the microprocessor time to store data.
Also, during short low voltage or negative voltage conditions, the STCAP pin protects the
5 V standby output from dropping below the RESET and LVW trip points. The four outputs
are expected to follow the battery voltage down to 11 V for REG1, 9 V for REG2, 6 V for
REG3 and REG4.
The L4954 has a standby mode to keep the microprocessor and memories alive during
ignition off conditions. The EN1 input pin is controlled by the microprocessor. A high on the
EN1 input turns on REG1, REG2, REG3, and HSD1. A Low on EN1 places the part in
stand-by mode with REG4 on. The high side driver outputs HSD2 and HSD3 are controlled
by EN2 and EN3 respectively: a low on the control input turns the corresponding high side
driver off.
Figure 4. Timing diagram
STCAP
(V)
0
5
REG4
(V)
0
5
LVWarn
(V)
5
RESET
(V)
0.4
7.5
7.5
4.75 4.75
TIME
TIME
TIME
D95AU349A
TIME
10/13
Doc ID 17719 Rev 1
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