L4916 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L4916' PDF : 8 Pages View PDF
L4916
Figure 2 : P.C. Board and Component Layout of Fig. 1 (1 : 1 scale).
PRINCIPLE OF OPERATION
During normal operation (input voltage upper than
VI MIN = VOUT NOM + ∆VI/O). The device works as a
normal voltage regulatorbuilt aroundthe 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 OP2and
Q3, acting as an active zener diode of value VREF.
In this conditionthe device works in the range (1) of
the characteristic of the non linear drop control unit
(see fig.3).
The output voltage is fixed to its nominal value:
VOUT NOM = VREF (1 + R1 ) =
R2
VCFT (1 + R1 )
R2
R1
= INTERNALLY FIXED RATIO = 2.4
R2
The ripple rejection is quite high (70 dB) and inde-
pendent from CFT value.
On the usualvoltage regulators, when the input vol-
tage 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. On the contrary,
a control loop on the L4916 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 voltage decreases below VI MIN the
supervisor loop, utilizing a non linear OTA, forces
the reference voltage at pin 2 to decrease by dis-
charging CFT. So, during the static mode, when the
input voltage goes below VMIN the drop out is kept
fixed to about 1.6V. In this condition the device
works as a low pass filter in the range (2) of the OTA
characteristic. The ripple rejection is externally ad-
justable acting on CFT as follows :
SVR (jw) = VI (jw) =
Vout (jw)
1+
10-6
gm
R1
(1 + )
Where:
jwCFT
R2
gm = 2 . 10-5 Ω-1 = OTA’S typical transconductance
value on linear region
R1
= fixed ratio
R2
CFT
= value of capacitor in µF
The reaction time of the supervisor loop is given by
the transconductanceof the OTA and by CFT. When
the value of the ripple voltage is so high and its ne-
gative peak is fast enough to determine an istanta-
neous decrease of the dropout till 1.2 V, the OTA
works in a higher transconductance condition
[range (3) of the characteristic] and discharge the
capacitor rapidously.
If the ripple frequency is high enough the capacitor
won’t charge itself completely, and the output volt-
age reachesa small value allowinga betterripple re-
jection ; the device’s again working as a filter (fast
transient range).
With CFT = 10 µF; f = 100Hz a SVR of 35 is obtained.
4/8
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