L4915
ADJUSTABLE VOLTAGE REGULATOR PLUS FILTER
June1993
PRELIMINARY DATA
PIN CONNECTION
POWER MINIDIP
ORDERING NUMBER : L4915
.
OUTPUT VOLTAGE ADJUSTABLE FROM
4 TO 11V
.
HIGH OUTPUT CURRENT (up to 250mA)
.
HIGH RIPPLE REJECTION
.
HIGH LOAD REGULATION
.
HIGH LINE REGULATION
.
SHORT CIRCUIT PROTECTION
.
THERMAL SHUT DOWN WITH HYSTERESIS
.
DUMP PROTECTION
DESCRIPTION
This circuit combines both a filter and a voltage
regulator in order to provide a high ripple rejection
over a wide input voltage range.
A supervisor low-pass loop of the element prevents
the output transistor from saturation at low input
voltage.
The non linear behaviour of this control circuitry
allows a fast settling of the filter.
1/7
BLOCK DIAGRAM
ABSOLUTE MAXIMUM RATINGS
Symbol
Parameter
Value
Unit
V
i
Peak Input Voltage (300 ms)
40
V
V
i
DC Input Voltage
28
V
I
O
Output Current
Internally Limited
P
tot
Power Dissipation
Internally Limited
T
stg
Storage and Junction Temperature
40 to 150
C
THERMAL DATA
Symbol
Parameter
Value
Unit
R
th j-amb
Thermal Resistance Junction-ambient
Max.
80
C/W
R
th j-pins
Thermal Resistance Junction-pins
Max.
20
C/W
ELECTRICAL CHARACTERISTICS
(T
amb
= 25
o
C; V
i
= 13.5 V, V
O
= 8.5V, circuit of Fig. 1, unless otherwise specified)
Symbol
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
V
i
Input Voltage
20
V
V
o
Output Voltage
V
i
= 6 to 18V, I
o
= 5 to 150mA
4
11
V
V
I/O
Controlled Input-output Dropout Voltage
I
o
= 5 to 150mA, V
i
= 6 to 10V
1.6
2.1
V
V
O
Line Regulation
V
i
= 12 to 18V, I
o
= 10mA
1
20
mV
V
o
Load Regulation
I
o
= 5 to 250mA,
t
on
= 30
s, t
off
=
1ms
50
100
mV
V
o
Load Regulation (filter mode)
V
i
= 8.5V, I
o
= 5 to 150mA
t
on
= 30
s, t
off
=
1ms
150
250
mV
V
ref
Internal Voltage Reference
2.5
V
I
q
Quiescent Current
I
o
= 5 mA
1
2
mA
I
q
Quiescent Current Change
V
i
= 6 to 18V, I
o
= 5 to 150mA
0.05
mA
I
AD
Adjust Input Current
40
nA
L4915
2/7
ELECTRICAL CHARACTERISTICS (continued)
(T
amb
= 25
o
C; V
i
= 13.5 V, V
O
= 8.5V, circuit of Fig. 1, unless otherwise specified)
Symbol
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
V
O
T
Output Voltage Drift
I
o
= 10mA
1.2
mV/
C
SVR
Supply Voltage Rejection
V
iac
= 1V
RMS
, f = 100Hz, I
o
= 150mA
Regulator
Filter Mode
71
35(*)
dB
I
SC
Short Circuit Current
250
300
mA
T
on
Switch On Time
I
o
= 150mA
Regulator
Filter Mode
300
500(*)
ms
T
j
Thermal Shutdown Junction
Temperature
145
C
(*) Depending of the C
FT
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 uses 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
V
REF
.
In this condition the device works in the range (1)
of the characteristic of the non linear drop control
unit (see Figure 1).
Figure 1 :
Nonliner Transfer Characteristic of
the Drop Control Unit
The output voltage is fixed to its nominal value :
V
OUT
NOM
=
V
REF
1
+
R1
R2
=
V
CFT
1
+
R1
R2
The ripple rejection is quite high (70dB) and inde-
pendent to C
FT
value.
On the usual voltage regulators, when the input
vol-tage goes below the nominal value, the regula-
tion transistors (series element) saturate bringing
the system out of regulation and making it very
sensible to every variation of the input voltage. On
the contrary, a control loop on the L4915 consents
to avoid the saturation of the series element by
regulating the value of the reference voltage
(pin 2). In fact, whenever the input voltage de-
creases below (V
I MIN
the supervisor loop, utilizing
a non linear OTA, forces the reference voltage at
pin 2 to decrease by discharging C
FT
. So, during
the static mode, when the input voltage goes below
V
MIN
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 characteristic. The ripple
rejection is externally adjustable acting on C
FT
as
follows :
SVR
(
j
) =
V
i
(
j
)
V
OUT
(
j
)
=
1
+
10
-
6
gm
jw C
FT
1
+
R1
R2
Where:
gm =
=
2
10
-5
-1
OTA'S typical transconductance value on
linear region
R1
R2
= fixed ratio
C
FT
= value of capacitor in
F
The reaction time of the supervisor loop is given by
the transconductanceof the OTAand by C
FT
. When
the value of the ripple voltage is so high and its
negative peak is fast enough to determine an istan-
taneous decrease of the dropout till 1.2V, the OTA
works in a higher transconductance condition
[range (3) of the characteristic] and discharges 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
rejection ; the device's again working as a filter (fast
transient range).
With C
FT
= 10
F; f = 100Hz; V
o
= 8.5V a SVR of 35
is obtained.
L4915
3/7
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No
license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned
in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.
SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express
written approval of SGS-THOMSON Microelectronics.
1994 SGS-THOMSON Microelectronics - All Rights Reserved
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