ST L4918 User Manual

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VOLTAGE REGULATORSPLUS FILTER

.FIXEDOUTPUT VOLTAGE8.5V

.250mAOUTPUT CURRENT

.HIGH RIPPLEREJECTION

.HIGH LOADREGULATION

.HIGH LINE REGULATION

.SHORTCIRCUIT PROTECTION

.THERMALSHUTDOWN WITHHYSTERESIS

.DUMPPROTECTION

DESCRIP TIO N

TheL4918combinesbothafilterandavoltageregu­latorinordertoprovidea high ripplerejectionovera
widerinput voltagerange.

A supervisorlow-passloop of theelement prevents
the outputtransistorfrom saturationat low inputvol
tages.

The non linear behaviourof this control circuitry al­lowsa fastsettingofthe filter.

L4918

PRELI MINA RY DATA

PENTAWATT

ORDERING NUMBER : L4918

BLOCK DIAGRAM

October1988

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L4918

PIN CONNECTIO N (top view)

Figure 1 : Applicationand TestCircuit.

ABSOLUTE MAXIMUM RATINGS

Symbol Parameter Value Unit

Peak Input Voltage (300 ms) 40 V

V

i

DC Input Voltage 28 V

V

i

Output Current Internally Limited

I

O

Power Dissipation Internally Limited

P

tot

T

stg,Tj

Storage and Junction Temperature – 40 to 150 °C

THERMAL DATA

Symbol Parameter Value Unit

Thermal Resistance Junction-case Max 4 °C/W

2/6

R

th j-case

L4918

ELECTRICAL CHARACTERISTICS (T

=25°C;Vi= 13.5 V, unlessotherwisespecified)

amb

Symbol Parameter Test Conditions Min. Typ. Max. Unit

Input Voltage 20 V

V

i

V

O

∆V

I/O

∆V

O

∆V

O

∆V

O

I

q

∆I

q

∆V

O

Output Voltage Vi=12to18V

= 5 to 150 mA

I

O

Controlled Input-output Dropout
Voltage

Vi= 5 to 10 V

= 5 to 150 mA

I

O

Line Regulation Vi=12to18V

=10mA

I

O

Load Regulation IO= 5 to 250 mA

=30µs

t

on

= ≥ 1ms

t

off

Load Regulation
(filter mode)

Vi= 8.5 V
I

= 5 to 150 mA

O

=30µs

t

on

t

= ≥ 1ms

off

Quiescent Current IO= 5 mA 1 2 mA
Quiescent Current Change Vi= 6 to 18 V

= 5 to 150 mA

I

O

Output Voltage Drift I

= 10 mA 1.2 mV/°C

O

8.1 8.5 8.9 V

1.6 2.1 V

120mV

100 mV

150 250 mV

0.05 mA

∆T

SVR Supply Voltage Rejection V

iac

=1V

rms

f = 100 Hz

= 150 mA

I

O

V

=12to18V

IDC

= 6 to 11 V

V

IDC

Short Circuit Current 250 300 mA

I

SC

Switch On Time IO= 150 mA

t

on

T

Thermal Shutdown Junction

JSD

= 5 to 11 V

V

i

=11to18V

V

i

71

35(*)

500(*)

300
150 °C

Temperature

dB
dB

ms
ms

(*) Depending of the CFTcapacitor

PRINCI P LE OF OPERATION

Duringnormal operation (input voltage upper than
V

I MIN=VOUT NOM

+ ∆V

). The device works as a

I/O

normalvoltageregulatorbuiltaroundtheOP1ofthe
blockdiagram.

The series passelement usea PNP-NPNconnec­tiontoreducethedropout.Thereferencevoltageof
theOP1isderivedfromaREFthroughtheOP2and
Q3, acting as an activezener diodeof value V

REF

In this conditionthe deviceworksin the range(1)of
the characteristicof thenon lineardropcontrolunit
(seefig.2).

The output voltageis fixed to itsnominalvalue:

R1 R1

V

OUT NOM=VREF

(1 + ) = V

R2 R2

(1 + )

CFT

R1

= INTERNALLYFIXED RATIO= 2.4

R2

The ripple rejectionis quite high (71 dB)and inde­pendentfromC

FT

value.

Ontheusualvoltageregulators,whentheinputvolt­age goes below the nominal value, the regulation
transistors (series element) saturate bringing the
systemout of regulationmaking it very sensibleto
everyvariationofthe inputvoltage.Onthecontrary,

.

a controlloop on the L4918 consents to avoidthe
saturationof the series element by regulating the
valueofthereferencevoltage(pin2). In fact,when­evertheinputvoltagedecreasesbelowV
pervisorloop,utilizing a nonlinear OTA,forces the
referencevoltageat pin2 to decreaseby discharg­ing C
voltagegoes belowV

. So,duringthe staticmode, whenthe input

FT

the dropout is keptfixed

MIN

to about 1.6 V. Inthisconditionthe deviceworksas
a lowpass filterin the range(2)of theOTA charac-

IMIN

thesu-

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L4918

teristic. The ripplerejection is externallyadjustable
actingon C

asfollows:

FT

SVR(jw) = =

1+

gm R1
jwC

FT

V

(jw)

I

(jw)

V

out

–6

10

(1 + )

R

2

Where:
gm = 2 .10

-5Ω-1

=OTA’S typicaltransconductance
valueon linearregion

R1

=fixed ratio

R2
Figure 2 : Nonliner TransferCharacteristicof the

DropControlUnit.

=valueof capacitorin µF

C

FT

Thereactiontimeof the supervisorloopis given by
thetransconductanceoftheOTAandbyC

.When

FT

thevalueoftheripplevoltageissohighanditsnega­tive peak is fast/enoughto determinean istantane­ousdecreaseofthedropouttill1.2V,theOTAworks
in a highertransconductancecondition[range(3) of
the characteristic]and dischargethe capacitorrapi­dously.
If theripplefrequencyis highenoughthe capacitor
won’t charge itself completely,and the output vol­tage reaches a small value allowing a better ripple
rejection;the device’s againworking as afilter(fast
transientrange).

WithC

=10µF;f=100HzaSVRof35isobtained.

FT

Figure 3 : SupplyVoltageRejectionvs.Fre-

1) Normal oper at ing r ange (high rippl e rejection)

2) Dr op c ontrol led r ange (medi um ripple reject i on)

3) F ast discharge of C

FT

Figure 4 : Supply voltageRejectionvs.Input Volt-

age.

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Figure 5 : Output Voltagevs. InputVoltage.

PENTAWATT PACKAGE MECHANICAL DATA

L4918

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
F 0.8 1.05 0.031 0.041

F1 1 1.4 0.039 0.055

G 3.4 0.126 0.134 0.142
G1 6.8 0.260 0.268 0.276
H2 10.4 0.409
H3 10.05 10.4 0.396 0.409

L 17.85 0.703
L1 15.75 0.620
L2 21.4 0.843
L3 22.5 0.886
L5 2.6 3 0.102 0.118
L6 15.1 15.8 0.594 0.622
L7 6 6.6 0.236 0.260

M 4.5 0.177

M1 4 0.157

Dia 3.65 3.85 0.144 0.152

A

H3

L

L1

C

D1

L5

Dia.

L7

L6

D

L2
L3

F1

H2

E

MM1

G1

G

F

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L4918

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 infringementof 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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