SGS Thomson Microelectronics L4915 Datasheet

ADJUSTABLEVOLTAGEREGULATOR PLUS FILTER
.
.
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
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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
VV
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
(j)
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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