L7250
4
POWER MONITOR, VOLTAGE REGULATORS AND SHOCK SENSOR
4.1 NPOR - Power ON Reset
The Power On Reset circuit monitors 12V and 5V power supplies as well as 3.3V and 2.5V regulators. If any
monitored voltage falls below its under voltage threshold, NPOR is latched low after an internal glitch filter delay.
When the positive regulators are in position, a delay time is added, the POR delay, before NPOR goes high and
the reset condition is cleared. During this delay time, any power fault will reset the POR delay and start the
process over again.
T
= 0.520 * C
ext
Delay
Where :
= External capacitor on pin CPOR measured in uF.
C
ext
4.2 Linear Voltage Regulators:1.8V & 3.3V
The 3.3V linear use an external NPN transistor connected to the 5V power supply line, instead the 1.8V linear
regulator use an external NPN transistor that could be connected to the 3.3V line or to the 5V power supply line.
To fix the 1.8V regulator voltage output an external resistor divider as to be used.
The regulated voltage could be varied around the 1.8V value (from 1.3V to 2V) choosing the external divider
appropriately.
The stability of the two regulators is guarantee by the external filter capacitor .
The internal Vbg reference is trimmed at the wafer level.
Figure 16. Linear positive regulators
V C C 5
V bg
3 3 _ B a se
15
R 2
3 3 _ F e e d
3.3V output
16
R 1
C ext
V C C 5
V b g
25_B ase
1 3
1 4
1.8V output
R 1 ex t
R 2 ex t
25_Feed
C ex t
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STMICROELECTRONICS [ ST ]
