ST L4974A User Manual

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.

3.5AOUTPUTCURRENT

.5.1VTO40VOUTPUTVOLTAGERANGE

.

0 TO 90%DUTY CYCLERANGE

.INTERNALFEED-FORWARD LINE REG.

.INTERNALCURRENTLIMITING

.PRECISE5.1V± 2% ONCHIP REFERENCE

.RESETAND POWERFAILFUNCTIONS

.

INPUT/OUTPUTSYNCPIN

.UNDER VOLTAGE LOCK OUT WITH HYS-

TERETICTURN-ON

.PWM LATCH FOR SINGLE PULSE PER PE-

RIOD

.VERYHIGH EFFICIENCY

.SWITCHING FREQUENCYUP TO 200KHz

.THERMALSHUTDOWN

.CONTINUOUSMODE OPERATION

DES CRIPTION

TheL4974Aisa stepdownmonolithicpowerswitch­ing regulator delivering 3.5A at a voltage variable
from5.1to 40V.

Realized with BCD mixed technology, the device
uses a DMOSoutputtransistorto obtainveryhigh
efficiencyandveryfastswitchingtimes.Featuresof

L4974A

3.5ASWITCHINGREGULATOR

MULTIPO WER B CD TECHNOLOGY

POWERDIP (16+2+2)

ORDERINGNUMBER : L4974A

the L4974A include resetand power fail for micro­processors,feed forwardline regulation,soft start,
limiting current and thermal protection.The device
ismountedin aPowerdip16+2 +2plasticpackage
and requires few external components. Efficient
operation at switching frequencies up to 200KHz
allowsreductioninthesizeandcostof externalfilter
component.

BLOCK DIAGRAM

June 2000

This is advanced information on a new product now in development or undergoing evaluation. Details are subject to change without notice.

1/22

L4974A

ABSOLU TE MAXIMUM RAT INGS

Symbol Parameter Valu e Unit

V

11

V

11

V

20

I

20

V

I

V

4,V8

V

3

I

3

V

2,V7,V9,V10

I

2

I

7

I

8

P

tot

T

J,Tstg

InputVoltage 55 V
InputOperatingVoltage 50 V
OutputDC Voltage

OutputPeak Voltageat t = 0.1µs f = 200khz

-1

-5
Maximum Output Current Internally Limited
BoostrapVoltage

BoostrapOperating Voltage

65

V

+15

11

InputVoltage at Pins4, 12 12 V
Reset Output Voltage 50 V
Reset Output Sink Current 50 mA
InputVoltage at Pin 2, 7, 9, 10 7 V
Reset Delay Sink Current 30 mA
ErrorAmplifier Output Sink Current 1 A
Soft Start Sink Current 30 mA
TotalPower Dissipation at T

at T

90°C

≤

PINS

=70°C(No copper area on PCB)

amb

5

1.3

Junctionand Storage Temperature -40 to 150 °C

V
V

V
V

W
W

PI N CONNECTION (top view)

THERMAL DATA

Symbol Parame t er Value Uni t

R

th j-pins

R

thj-amb

Thermal ResistanceJunction-Pins max
Thermal ResistanceJunction-ambient max

12
60

°C/W
°C/W

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L4974A

PIN FUNCTIONS

o

N

1 BOOTSTRAP A C

2 RESET DELAY A C

3 RESET OUT Open CollectorReset/power Fail Signal Output. This output is high whenthe supply

4 RESET INPUT Input of Power Fail Circuit. The threshold is 5.1V. It may be connected via a divider

5, 6

GROUND Common GroundTerminal

15,16

7 FREQUENCY

COMPENSATION

8 SOFT START Soft StartTime Constant.A capacitoris connectedbetween thi sterminalandground

9 FEEDBACK INPUT The Feedback Terminal of the Regulation Loop.The output is connected directly to

10 SYNC INPUT Multiple L4974A’s are synchronized by connecting pin 10 inputs together or via an

11 SUPPLYVOLTAGE Unregulated InputVoltage.

12,19 N.C. Not Connected.

13 V
14 V

ref
start

17 OSCILLATOR R

18 OSCILLATOR C

20 OUTPUT Regulator Output.

Name Function

capacitor connected between this terminal and the output allows to drive

boot

properlythe internalD-MOS transistor.

capacitor connected between this terminal and ground determines the reset

d

signaldelay time.

and theoutput voltages are safe.

tothe inputforpowerfailfunction.It mustbe connectedtothepin14anexternal30K
resistorwhen power fail signal not required.

A series RC network connected between this terminal and ground determines the
regulation loop gaincharacteristics.

to define the soft start time constant.

thisterminal for5.1V operation; It is connectedvia a dividerfor higher voltages.

externalsyncr. pulse.

5.1VV

DeviceReferenceVoltage.

ref

InternalStart-up Circuit to Drive the PowerStage.

. Externalresistorconnected to grounddeterminestheconstantchargingcurrent

osc

of C

.

osc

. External capacitor connected to ground determines (with R

osc

frequency.

) the switching

osc

Ω

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L4974A

CIRCU I T OP ER ATION

The L4974Ais a 3.5Amonolithic stepdownswitch­ingregulatorworkingincontinuousmoderealizedin
the new BCD Technology. This technologyallows
theintegrationofisolatedverticalDMOSpowertran­sistorsplusmixed CMOS/Bipolartransistors.

Thedevicecandeliver3.5Aat anoutputvoltagead­justable from 5.1V to 40V and containsdiagnostic
and control functionsthat make it particularly suit­able for microprocessorbasedsystems.

BLOCKDIAGRAM
The block diagram shows the DMOSpowertran-

sistorsand the PWM controlloop. Integratedfunc­tions include a reference voltage trimmed to 5.1V
±2%,softstart,undervoltagelockout,oscillatorwith
feedforward control, pulse by pulse current limit,
thermal shutdown and finally the reset and power
fail circuit.The reset and power failcircuit provides
an outputsignalfora microprocessorindicatingthe
statusof the system.

Deviceturn on is around11V witha typical1V hys­terysis,thisthresholdporvidesa correctvoltagefor
the driving stageof the DMOSgateand thehyste­rysispreventsinstabilities.

Anexternalbootstrapcapacitorchargeto 12Vbyan
internalvoltagereferenceis neededto providecor­rect gatedriveto the powerDMOS.Thedrivingcir­cuit is able to source and sink peak currents of
around0.5A to the gate of the DMOS transistor.A
typical switching time of the current in the DMOS
transistor is 50ns. Due to the fast commutation
switchingfrequenciesup to 200kHzare possible.

The PWMcontrolloopconsistsof a sawtoothoscil­lator,erroramplifier,comparator,latch andthe out­putstage.An errorsignalisproducedby comparing
theoutputvoltagewiththeprecise5.1V±2%onchip
reference.This error signal is then compared with
the sawtooth oscillator in order to generate frixed
frequencypulsewidth modulateddrive for the out­put stage. A PWM latch is included to eliminate
multiple pulsingwithin a period even in noisy envi­ronments.

Thegainand stabilityoftheloopcanbe adjustedby

an externalRC networkconnectedto the outputof
the error amplifier. A voltage feedforward control
has beenaddedto the oscillator,this maintainssu­perior line regulation over a wide input voltage
range.Closingthe loopdirectlygivesan outputvol­tageof 5.1V,highervoltagesareobtainedbyinsert­ing a voltagedivider.

Atturnon,outputovercurrentsarepreventedbythe
soft start function(fig. 2). The error amplifier is in­itiallyclampedbyan externalcapacitor,Css,and al­lowedto riselinearlyunderthe chargeof aninternal
constantcurrentsource.

Outputoverloadprotectionis providedby a current
limitcircuit. Theload currentis sensedby a internal
metalresistorconnectedtoacomparator.Whenthe
loadcurrentexceedsa presetthreshold,the output
of the comparatorsetsa flipflop whichturns offthe
powerDMOS.Thenextclockpulse,fromaninternal
40kHzoscillator,willresettheflipflopandthepower
DMOS will again conduct. This current protection
method,ensuresaconstantcurrentoutputwhenthe
systemis overloadedorshortcircuitedandlimitsthe
switchingfrequency,inthiscondition,to 40kHz.The
Reset and Power fail circuit (fig. 4), generates an
output signal when the supply voltage exceeds a
threshold programmed by an external voltage di­vider. The reset signal, is generated with a delay
timeprogrammedbya externalcapacitoronthede­lay pin. When the supply voltage falls below the
thresholdor the outputvoltage goes below5V, the
resetoutputgoeslowimmediately.The resetoutput
is an opendrain.

Fig. 4A shows thecasewhen the supplyvoltageis
higherthan the threshold,but the output voltage is
not yet 5V.

Fig.4Bshowsthecasewhentheoutputis5.1V,but
the supply voltage is not yet higher than the fixed
threshold.

The thermal protection disables circuit operation
when the junction temperature reaches about
150°C and has a hysterysis to prevent unstable
conditions.

4/22

Figure1 : FeedforwardWaveform.

Figure2 : SoftStart Function.

L4974A

Figure3 : LimitingCurrent Function.

5/22

L4974A

Figure4 : Resetand PowerFail Functions.
A

B

6/22

L4974A

ELECTRICALCHARACTERISTICS (referto the test circuit, TJ=25°C,Vi=35V, R4= 30KΩ,

= 2.7nF,fSW= 100KHztyp, unless otherwisespecified)

C

9

DYNAMICCHARACTERISTICS

Symbol Parameter Test Conditions Min. Typ. Max. Unit Fig.

V

V

∆V

∆V

V

I

20L

InputVolt. Range (pin 11) Vo=V

i

OutputVoltage Vi=15V to 50V

o

Line Regulation VI = 15V to 50V

o

Load Regulation VO =V

o

Dropout Voltage between

d

Pin11 and 20
Max Limiting Current Vi= 15V to 50V

I

o

I

o

I

o

I

o

Io=2A
I

o

V

η

Efficiency I

o

V
Vo= 12V

SVR Supply VoltageRipple

Rejection

V
f = 100Hz; V

f SwitchingFrequency 90 100 110 KHz 5

∆f/∆Vi Voltage Stability of

V
Switching
Frequency

∆f/T

Temperature Stability of

j

Tj=0 to 125°C1%5
SwitchingFrequency

f

max

(*) Pulse testing with a low duty cycle

Maximum Operating
SwitchingFrequency

Vo=V

I

o

ref

to 40V

15 50 V 5

= 3.5A(*)

5 5.1 5.2 V 5

= 2A;Vo=V

ref

12 30 mV

= 1A; Vo=V

refIo

=2Ato3A

= 3.5A

ref

= 1A to 3.5A

825mV
410mV

0.25

0.45

0.4

0.7

V

4 4.75 5.5 A

to 40V

o=Vref

= 3.5A,f = 100KHz

o=Vref

80 85

90

= 2VRMS;Io=5A

i

o=Vref

= 15V to 45V 2 6 % 5

i

= 15KΩ

refR4

56 60 dB 5

200 KHz 5

%
%

= 3.5AC9= 2.2nF

V

SECTION(pin13)

ref

Symbol Parameter Test C o n di tion Mi n . Typ. Max. U n i t Fi g.

ReferenceVoltage 5 5.1 5.2 V 7
Line Regulation Vi= 15V to 50V 10 25 mV 7

13

Load Regulation I13 = 0 to 1mA 20 40 mV 7

13

Average Temperature

13

T

=0°C to 125°C 0.4 mV/°C7

j

Coefficient Reference

∆V
∆V

∆

V

13

V

T

∆

Voltage

V

START

I

13 short

ShortCircuit Current Limit V13= 0 70 mA 7

SECTION(pin15)

Symbol Parameter Test C o n di tion Mi n . Typ. Max. U n i t Fi g.

V

V

∆

V

∆

I

14 short

14

ReferenceVoltage 11.4 12 12.6 V 7
Line Regulation Vi= 15 to 50V 0.6 1.4 V 7

14

Load Regulation I14= 0 to 1mA 50 200 mV 7

14

ShortCircuit Current Limit V15=0V 80 mA 7

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