L4949E
With this structure very low dropout voltage at
currents up to 100mA is obtained. The dropout
operation of the standby regulator is maintained
down to 3V input supply voltage. The output volt-
age is regulated up to the transient input supply
voltage of 40V. With this feature no functional in-
terruption due to overvoltage pulses is generated.
The typical curve showing the standby output
voltage as a function of the input supply voltage is
shown in Fig. 2. The current consumption of the
device (quiescent current) is less than 300µA.
To reduce the quiescent current peak in the un-
dervoltage region and to improve the transient re-
sponse in this region, the dropout voltage is con-
trolled, the quiescent current as a function of the
supply input voltage is shown in Fig. 3.
Figure 2:
Output Voltage vs. Input Voltage
Preregulator
To improve the transient immunity a preregulator
stabilized the internal supply voltage to 5V. This
internal voltage is present at Pin 3 (V
Z
). This volt-
age should not be used as an output because the
output capability is very small (
≤
10
µ
A).
This output may be used as an option when a
better transient behaviour for supply voltages less
than 8V is required (see also application note).
In this case a capacitor (100nF - 1µF) must be
connected between Pin 3 and GND. If this feature
is not used Pin 3 must be left open.
Reset Circuit
The block circuit diagram of the reset circuit is
shown in Fig. 4. The reset circuit supervises the
output voltage.
The reset thereshold of 4.5V is defined with the
internal reference voltage and standby output
drivider.
The reset pulse delay time t
RD
, is defined with the
charge time of an external capacitor C
T
:
t
RD
=
C
T
•
2V
2
µ
A
40V
Figure 3:
Quiescent Current vs. Supply Voltage
The reaction time of the reset circuit originates
from the discharge time limitation of the reset ca-
pacitor C
T
and is proportional to the value of C
T
.
The reaction time of the reset circuit increases the
noise immunity. Standby output voltage drops be-
low the reset threshold only a bit longer than the
reaction time results in a shorter reset delay time.
The nominal reset delay time will be generated for
standby output voltage drops longer than approxi-
mately 50
µ
s.
The typical reset output waveforms are shown in
Fig. 5.
Sense Comparator
The sense comparator compares an input signal
with an internal voltage reference of typical 1.23V.
The use of an external voltage divider makes this
comparator very flexible in the application.
It can be used to supervise the input voltage
either before or after the protection diode and to
give additional informations to the microprocessor
like low voltage warnings.
5/10
STMICROELECTRONICS [ STMICROELECTRONICS ]
