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L4916
VOLTAGE REGULATOR PLUS FILTER
L4916June 2000
VOLTAGE REGULATOR PLUS FILTER FIXED OUTPUT VOLTAGE 8.5 V. 250 mA OUTPUT CURRENT. HIGH RIPPLE REJECTION. HIGH LOAD REGULATION. HIGH LINE REGULATION. SHORT CIRCUIT PROTECTION. THERMAL SHUT DOWN WITH HYSTERESIS. DUMP PROTECTION
This circuit combines both a filter and a voltage regu-
lator in order to provide a high ripple rejection over a
wider input voltage range.
A supervisor low-pass loop of the element prevents
the output transistor from saturation at low input volt-
ages.
The non linear behaviour of this control circuitry
allows a fast settling of the filter.
BLOCK DIAGRAM
DESCRIPTION1/8
ABSOLUTE MAXIMUM RATINGS
PIN CONNECTION (top view)
THERMAL DATA
L49162/8
ELECTRICAL CHARACTERISTICS (Tamb = 25 °C; Vi = 13.5 V, test circuit of fig. 1, unless otherwise specified)(*) Depending of the CFT capacitor.
Figure 1 : Test and Application Circuit.
L49163/8
Figure 2 : P.C. Board and Component Layout of Fig. 1 (1 : 1 scale).
PRINCIPLE OF OPERATIONDuring normal operation (input voltage upper than
VI MIN = VOUT NOM + ΔVI/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 through the 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.3).
The output voltage is fixed to its nominal value: VOUT NOM = VREF (1 + ) =VCFT (1 + ) = INTERNALLY FIXED RATIO = 2.4R2
The ripple rejection is quite high (70 dB) and inde-
pendent from CFT value.
On the usual voltage 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 :
VI (jw) SVR (jw) = =
Vout (jw)-6
1 + R1(1 + )
jwCFT R2Where:
gm = 2 . 10-5 Ω-1 = OTA’S typical transconductance
value on linear region = fixed ratio
CFT = value of capacitor in μF
The reaction time of the supervisor loop is given by
the transconductance of 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 reaches a small value allowing a better ripple re-
jection ; the device’s again working as a filter (fast
transient range).
With CFT = 10 μF; f = 100 Hz a SVR of 35 is obtained.
L49164/8
Figure 3 : Nonliner Transfer Characteristic of the Drop Control Unit.
Figure 4 : Supply Voltage Rejection vs. Input Voltage.
Figure 5 : Supply voltage Rejection vs. Frequency.
Figure 6 : Vo vs. Supply Voltage. Figure 7 : Quiescent Current vs. Input Voltage.
L49165/8
Figure 8 : Dropout vs. Load Current.
Figure 9 : Inhibit Function Realized on CFT Pin.
L49166/8
