L4918 Datasheet, PDF (3/6 Pages) STMicroelectronics – VOLTAGE REGULATORS PLUS FILTER

L4918
ELECTRICAL CHARACTERISTICS (T_amb = 25 °C; V_i = 13.5 V, unless otherwise specified)
Symbol
Parameter
Test Conditions
V_i Input Voltage
V_O Output Voltage
V_i = 12 to 18 V
IO = 5 to 150 mA
∆V_I/O Controlled Input-output Dropout V_i = 5 to 10 V
Voltage
IO = 5 to 150 mA
∆VO Line Regulation
Vi = 12 to 18 V
IO = 10 mA
∆V_O Load Regulation
I_O = 5 to 250 mA
ton = 30 µs
t_off = ≥ 1 ms
∆VO Load Regulation
(filter mode)
Vi = 8.5 V
I_O = 5 to 150 mA
ton = 30 µs
t_off = ≥ 1 ms
I_q Quiescent Current
I_O = 5 mA
∆I_q Quiescent Current Change
V_i = 6 to 18 V
I_O = 5 to 150 mA
∆V_O Output Voltage Drift
∆T
IO = 10 mA
SVR Supply Voltage Rejection
Viac = 1 Vrms
f = 100 Hz
IO = 150 mA
V_IDC = 12 to 18 V
VIDC = 6 to 11 V
ISC Short Circuit Current
t_on Switch On Time
I_O = 150 mA
V_i = 5 to 11 V
V_i = 11 to 18 V
TJSD Thermal Shutdown Junction
Temperature
Min. Typ. Max. Unit
20
V
8.1
8.5
8.9
V
1.6
2.1
V
1
20
mV
100 mV
150 250 mV
1
2
mA
0.05
mA
1.2
mV/°C
71
dB
35(*)
dB
250 300
mA
500(*)
ms
300
ms
150
°C
(*) Depending of the CFT capacitor
PRINCIPLE OF OPERATION
During normal operation (input voltage upper than
V_{I MIN} = V_{OUT NOM} + ∆V_I/O). The device works as a
normal voltage regulator built around the OP1 of the
block diagram.
The series pass element use a PNP-NPN connec-
tion to reduce the dropout. The reference voltage of
the OP1 is derived from a REF throughthe OP2 and
Q3, acting as an active zener diode of value VREF.
In this condition the device works in the range (1) of
the characteristic of the non linear drop control unit
(see fig.2).
The output voltage is fixed to its nominal value:
R1
R1
VOUT NOM = VREF (1 + _R2 ) = VCFT (1 + _R2 )
R1
= INTERNALLY FIXED RATIO = 2.4
R2
The ripple rejection is quite high (71 dB) and inde-
pendent from C_FT value.
On the usual voltage regulators, when the input volt-
age goes below the nominal value, the regulation
transistors (series element) saturate bringing the
system out of regulation making it very sensible to
every variation of the input voltage. Onthe contrary,
a control loop on the L4918 consents to avoid the
saturation of the series element by regulating the
value of the reference voltage (pin 2). In fact, when-
ever the input voltagedecreasesbelow V_{I MIN} the su-
pervisor loop, utilizing a non linear OTA, forces the
reference voltage at pin 2 to decrease by discharg-
ing C_FT. So, during the static mode, when the input
voltage goes below VMIN the drop out is kept fixed
to about 1.6 V. In this condition the device works as
a low pass filter in the range (2) of the OTA charac-
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