L4918
July 2003
VOLTAGE REGULATORS PLUS FILTER
.
FIXED OUTPUT VOLTAGE 8.5V
.
250mA OUTPUT CURRENT
.
HIGH RIPPLE REJECTION
.
HIGH LOAD REGULATION
.
HIGH LINE REGULATION
.
SHORT CIRCUIT PROTECTION
.
THERMAL SHUT DOWN WITH HYSTERESIS
.
DUMP PROTECTION
DESCRIPTION
The L4918 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 vol
tages.
The non linear behaviour of this control circuitry
allows a fast setting of the filter.
PENTAWATT
ORDERING NUMBER : L4918
BLOCK DIAGRAM
1/6
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
, T
j
Storage and Junction Temperature
40 to 150
C
PIN CONNECTION (top view)
Figure 1 : Application and Test Circuit.
THERMAL DATA
Symbol
Parameter
Value
Unit
R
th j-case
Thermal Resistance Junction-case
Max
4
C/W
L4918
2/6
ELECTRICAL CHARACTERISTICS (T
amb
= 25
C; V
i
= 13.5 V, unless otherwise specified)
Symbol
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
V
i
Input Voltage
20
V
V
O
Output Voltage
V
i
= 12 to 18 V
I
O
= 5 to 150 mA
8.1
8.5
8.9
V
V
I/O
Controlled Input-output Dropout
Voltage
V
i
= 5 to 10 V
I
O
= 5 to 150 mA
1.6
2.1
V
V
O
Line Regulation
V
i
= 12 to 18 V
I
O
= 10 mA
1
20
mV
V
O
Load Regulation
I
O
= 5 to 250 mA
t
on
= 30
s
t
off
=
1 ms
100
mV
V
O
Load Regulation
(filter mode)
V
i
= 8.5 V
I
O
= 5 to 150 mA
t
on
= 30
s
t
off
=
1 ms
150
250
mV
I
q
Quiescent Current
I
O
= 5 mA
1
2
mA
I
q
Quiescent Current Change
V
i
= 6 to 18 V
I
O
= 5 to 150 mA
0.05
mA
V
O
T
Output Voltage Drift
I
O
= 10 mA
1.2
mV/
C
SVR
Supply Voltage Rejection
V
iac
= 1 V
rms
f = 100 Hz
I
O
= 150 mA
V
IDC
= 12 to 18 V
V
IDC
= 6 to 11 V
71
35(*)
dB
dB
I
SC
Short Circuit Current
250
300
mA
t
on
Switch On Time
I
O
= 150 mA
V
i
= 5 to 11 V
V
i
= 11 to 18 V
500(*)
300
ms
ms
T
JSD
Thermal Shutdown Junction
Temperature
150
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 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 V
REF
.
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
V
OUT NOM
= V
REF
(1 + ) = V
CFT
(1 + )
R2
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 voltage decreases below V
I MIN
the
supervisor loop, utilizing a non linear OTA, forces
the reference voltage at pin 2 to decrease by dis-
charging 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
L4918
3/6
characteristic. The ripple rejection is externally ad-
justable acting on C
FT
as follows:
V
I
(jw)
SVR (jw) = =
V
out
(jw)
10
6
1 +
gm
R1
(1 + )
jwC
FT
R
2
Where:
gm = 2 . 10
-5
-1
= OTA'S typical transconductance
value on linear region
R1
= fixed ratio
R2
C
FT
= value of capacitor in
F
The reaction time of the supervisor loop is given by
the transconductance of the OTA and by C
FT
. 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 1.2V, 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 vol-
tage 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 = 100 Hz a SVR of 35 is obtained.
Figure 2 : Nonliner Transfer Characteristic of the
Drop Control Unit.
Figure 3 : Supply Voltage Rejection vs. Fre-
1) Normal operating range (high ripple rejection)
2) Drop controlled range (medium ripple rejection)
3) Fast discharge of C
FT
Figure 4 : Supply voltage Rejection vs. Input Volt-
age.
Figure 5 : Output Voltage vs. Input Voltage.
L4918
4/6
Weight: 2.00gr
Pentawatt V
DIM.
mm
inch
MIN.
TYP.
MAX.
MIN.
TYP.
MAX.
A
4.8
0.189
C
1.37
0.054
D
2.4
2.8
0.094
0.110
D1
1.2
1.35
0.047
0.053
E
0.35
0.55
0.014
0.022
E1
0.76
1.19
0.030
0.047
F
0.8
1.05
0.031
0.041
F1
1.0
1.4
0.039
0.055
G
3.2
3.4
3.6
0.126
0.134
0.142
G1
6.6
6.8
7.0
0.260
0.268
0.276
H2
10.4
0.409
H3
10.05
10.4
0.396
0.409
L
17.55
17.85
18.15
0.691
0.703
0.715
L1
15.55
15.75
15.95
0.612
0.620
0.628
L2
21.2
21.4
21.6
0.831
0.843
0.850
L3
22.3
22.5
22.7
0.878
0.886
0.894
L4
1.29
0.051
L5
2.6
3.0
0.102
0.118
L6
15.1
15.8
0.594
0.622
L7
6.0
6.6
0.236
0.260
L9
2.1
2.7
0.008
0.106
L10
4.3
4.8
0.17
0.189
M
4.23
4.5
4.75
0.167
0.178
0.187
M1
3.75
4.0
4.25
0.148
0.157
0.167
V4
40 (typ.)
V5
90 (typ.)
Dia
3.65
3.85
0.144
0.152
L
L1
A
C
L5
D1
L2
L3
E
M1
M
D
H3
Dia.
L7
L9
L10
L6
F1
H2
F
G G1
E1
F
E
V4
RESIN BETWEEN
LEADS
H2
V5
V4
PENTVME
L4
0015981
OUTLINE AND
MECHANICAL DATA
L4918
5/6
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quences 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 STMicroelectronics. Specification mentioned in this
publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMi-
croelectronics products are not authorized for use as critical components in life support devices or systems without express written
approval of STMicroelectronics.
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