ST L4915 User Manual

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ADJUSTABLEVOLTAGEREGULATOR PLUS FILTER

.

OUTPUTVOLTAGEADJUSTABLE FROM
4TO11V

.

HIGH OUTPUTCURRENT (up to 250mA)

.

HIGH RIPPLEREJECTION

.

HIGH LOAD REGULATION

.

HIGH LINE REGULATION

.

SHORTCIRCUITPROTECTION

.

THERMAL SHUT DOWN WITH HYSTERESIS

.

DUMP PROTECTION

L4915

PRELIMINARY DATA

DESCRIPTION

This circuit combines both a filter and a voltage
regulatorin order to provide a high ripple rejection
overa wideinput voltage range.

Asupervisorlow-passloopoftheelementprevents
the output transistor from saturation at low input
voltage.

The non linear behaviour of this control circuitry
allowsa fast settling of the filter.

PIN CONNECTION

POWER MINIDIP

ORDERING NUMBER : L4915

June1993

1/7

L4915

BLOCKDIAGRAM

ABSOLUTE MAXIMUMRATINGS

Symbol Parameter Value Unit

V
V

I

O

P

tot

T

stg

Peak Input Voltage(300 ms) 40 V

i

DC Input Voltage 28 V

i

Output Current Internally Limited
Power Dissipation Internally Limited
Storage and Junction Temperature – 40 to 150 °C

THERMAL DATA

Symbol Parameter Value Unit

R

th j-amb

R

th j-pins

Thermal Resistance Junction-ambient Max. 80 °C/W
Thermal Resistance Junction-pins Max. 20 °C/W

ELECTRICALCHARACTERISTICS

=25oC; Vi= 13.5 V, VO= 8.5V,circuitof Fig. 1, unless otherwise specified)

(T

amb

Symbol Parameter Test Conditions Min. Typ. Max. Unit

V

Input Voltage 20 V

i

V

∆V

∆V

∆V
∆V

V

∆I
I

Output Voltage Vi= 6 to 18V,Io= 5 to 150mA 4 11 V

o

Controlled Input-output Dropout Voltage Io= 5 to 150mA, Vi= 6 to 10V 1.6 2.1 V

I/O

Line Regulation Vi= 12 to 18V,Io= 10mA 1 20 mV

O

Load Regulation Io= 5 to 250mA,

o

Load Regulation (filter mode) Vi= 8.5V, Io= 5 to 150mA

o

Internal Voltage Reference 2.5 V

ref

I

Quiescent Current Io=5mA 1 2 mA

q

Quiescent CurrentChange Vi= 6 to 18V,Io= 5 to 150mA 0.05 mA

q

Adjust Input Current 40 nA

AD

=30µs, t

t

on

=30µs, t

t

on

= ≥ 1ms

off

=≥ 1ms

off

50 100 mV

150 250 mV

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L4915

ELECTRICALCHARACTERISTICS (continued)

=25oC; Vi= 13.5 V, VO= 8.5V,circuitof Fig. 1, unless otherwise specified)

(T

amb

Symbol Parameter Test Conditions Min. Typ. Max. Unit

∆V

O

∆T

SVR Supply Voltage Rejection V

I
T

(*) Depending of the CFTcapacitor

Output VoltageDrift I

Short Circuit Current 250 300 mA

SC

Switch On Time Io= 150mA

on

Thermal ShutdownJunction

T

j

Temperature

= 10mA 1.2 mV/°C

o

=1V

iac

Regulator
Filter Mode

Regulator
Filter Mode

, f = 100Hz, Io= 150mA

RMS

71

35(*)

300

500(*)

145 °C

dB

ms

PRINCIPLE OF OPERATION

Duringnormal operation(inputvoltage upper than
V

I MIN=VOUT NOM

+ ∆V

). The deviceworks as a

I/O

normal voltage regulator built around the OP1 of
the block diagram.

Theseriespasselementuses aPNP-NPNconnec­tion to reduce the dropout. The reference voltage
of the OP1 is derivedfrom a REF throughthe 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 characteristicof the non linear drop control
unit (see Figure 1).

Figure1 : NonlinerTransfer Characteristicof

the Drop ControlUnit

Theoutput voltage is fixed to itsnominal value :

V

OUT NOM

= V

REF



1 +







= V

CFT



R1
R2





1+




R1R2

The ripple rejection is quite high (70dB) and inde­pendentto C

FT

value.

On the usual voltage regulators, when the input
vol-tagegoes belowthe nominalvalue,theregula­tion transistors (series element) saturate bringing
the system out of regulation and making it very
sensibleto every variationof the input voltage.On

the contrary,a control loop on theL4915 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 de­creasesbelow (V

the supervisorloop, utilizing

I MIN

a non linear OTA, forces the reference voltage at
pin 2 to decrease by discharging C

. So, during

FT

the staticmode, whentheinputvoltagegoesbelow

thedropout iskept fixedto about1.6 V. Inthis

V

MIN

condition the device works as a low pass filter in
the range (2) of the OTAcharacteristic.The ripple

R1R2

FT

as













rejection is externally adjustable acting on C
follows:

SVR (jΩ) =



V

(jΩ)

i



V

(jΩ)

OUT







= 1 +







jw C

gm

−6

10



1 +



FT



Where:
gm ==2 ⋅ 10

-5Ω-1

OTA’S typical transconductancevalue on
linear region

R1R2= fixed ratio

C

= value of capacitorin µF

FT

The reactiontimeofthe supervisorloop isgivenby
the transconductanceofthe OTAandby C

. When

FT

the value of the ripple voltage is so high and its
negativepeakis fastenoughto determineanistan­taneousdecrease of the dropouttill 1.2V,the OTA
works in a higher transconductance condition
[range (3) of the characteristic] anddischargesthe
capacitorrapidously.
If theripplefrequencyis highenoughthecapacitor
won’tcharge itself completely, andthe outputvolt­age reaches a smallvalue allowing a betterripple
rejection; thedevice’sagainworkingasafilter (fast
transientrange).

WithC

=10µF; f =100Hz;Vo=8.5V a SVRof 35

FT

is obtained.

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