Microsemi LX8117-28CDD 0.8, 1 & 1.2a low dropout positive regulator Datasheet

LIN D O C #: 8117
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
T
H E
I
N F I N I T E
P
O W E R
O F
I
P
N N O VA T I O N
R O D U C T I O N
DESCRIPTION
The LX8117/8117A/8117B series are
positive Low Dropout (LDO) regulators.
At the designed maximum load current,
the LX8117 series dropout voltage is guaranteed to be 1.2V or lower at 0.8A
(LX8117A 1.3V @ 1A). The dropout
voltage decreases with load current.
An adjustable output voltage version
of the LX8117/17A/17B is available, as
well as versions with fixed outputs of
2.5V, 2.85V, 3.3V and 5V. The 2.85V
version is specifically designed for use
as a component of active termination
networks for the SCSI bus. On-chip
trimming of the internal voltage reference
allows specification of the initial output
voltage to within ±1% of its nominal
value. The output current-limit point is
also trimmed, which helps to minimize
stress on both the regulator and the
system power source when they are
operated under short-circuit conditions.
The regulator's internal circuitry will
D
A T A
S
H E E T
K E Y F E AT U R E S
operate at input-to-output differential
voltages down to 1V.
Most regulator circuit designs include
output capacitors with values in the range
of tens to hundreds of microfarads or
more. The LX8117/17A/17B typically
requires at least 10µF of output capacitance for stable operation.
PNP-type regulators can waste current
equal to as much as 10 percent of their
output as a quiescent current which flows
directly to ground, bypassing the load.
Quiescent current from the LX8117/17A/
17B flows through the load, increasing
power-use efficiency and allowing cooler
operation.
The LX8117 is available in low-profile
plastic SOT-223 and D-Pak packages for
applications where space is at a premium.
The LX8117 is also available in a plastic
TO-263 package for instances when the
thermal resistance from the circuit die to
the environment must be minimized.
NOTE: For current data & package dimensions, visit our web site: http://www.linfinity.com.
PRODUCT HIGHLIGHT
ACTIVE TERMINATOR FOR SCSI-2 BUS
110Ω
■ 0.2% Line Regulation Maximum
■ 0.4% Load Regulation Maximum
■ Output Current Of 800mA
■ Regulates Down To 1.2V Dropout
(LX8117) And 1.3V Dropout (LX8117A)
■ Operates Down To 1V Dropout
■ Space Saving SOT-223 Surface
Mount Package
■ Guaranteed Dropout Voltage At Multiple
Current Levels
■ Three-Terminal Adjustable Or Fixed 2.5V,
2.85V, 3.3V & 5V
A P P L I C AT I O N S
■
■
■
■
■
Battery Chargers
Active SCSI Terminators
5V To 3.3V Linear Regulators
High-Efficiency Linear Regulators
Post Regulators For Switching Supplies
A VA I L A B L E O P T I O N S
PER
Part #
P A RT #
Output
Voltage
LX8117/8117A/8117B-00
LX8117/8117A/8117B-25
Adjustable
2.5V
LX8117/8117A/8117B-28
LX8117/8117A/8117B-33
LX8117/8117A/8117B-05
2.85V
3.3V
5V
110Ω
LX8117-28
IN
110Ω
OUT
18 to 27
Lines
GND
4.75V to
5.25V
22µF
10µF
110Ω
PA C K A G E O R D E R I N F O R M AT I O N
TA (°C)
0 to 125
O/P
Current
SOT-223
ST Plastic
3-pin
T0-263
DD Plastic
3-pin
T0-252
DT Plastic
(D-Pak) 3-pin
0.8A
LX8117-xxCST
LX8117-xxCDD
LX8117-xxCDT
1.0A
LX8117A-xxCST
LX8117A-xxCDD
LX8117A-xxCDT
1.2A
LX8117B-xxCST
LX8117B-xxCDD
LX8117B-xxCDT
Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number
(i.e. LX8117-28CSTT). "xx" refers to output voltage, please see table above.
Copyright © 1999
Rev. 1.4 3/99
LINFINITY MICROELECTRONICS INC.
11861 W ESTERN A VENUE, G ARDEN G ROVE, CA. 92841, 714-898-8121, F AX: 714-893-2570
1
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
A B S O LUT E M AXIM UM R ATINGS
D
A T A
S
H E E T
PACKAGE PIN OUTS
(Note 1)
Power Dissipation .................................................................................. Internally Limited
Input Voltage
LX8117-00/8117A-00/8117B-00 (Adj.) ..................................................................... 15V
LX8117-33/8117A-33/8117B-33 (3.3V), LX8117-05/8117A-05/8117B-05 (5.0V) .... 15V
LX8117-25/8117A-25/8117B-25 (2.5V), LX8117-28/8117A-28/8117B-28 (2.85V) .. 12V
Surge Voltage ............................................................................................................... 15V
Operating Junction Temperature
Plastic (ST, DD & DT Packages) .......................................................................... 150°C
Storage Temperature Range ...................................................................... -65°C to 150°C
Lead Temperature (Soldering, 10 seconds) ............................................................. 300°C
Short-Circuit Protection ....................................................................................... Indefinite
TAB IS V OUT
3. IN
2. OUT
1. ADJ / GND
ST PACKAGE
(Top View)
TAB IS VOUT
Note 1. Exceeding these ratings could cause damage to the device. All voltages are with
respect to Ground. Currents are positive into, negative out of the specified terminal.
T H E R MAL DATA
ST PACKAGE:
THERMAL RESISTANCE-JUNCTION TO TAB, θJT
15°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
3
IN
2
OUT
1
ADJ / GND
DD PACKAGE (D2 Pak)
(Top View)
*150°C/W
DD PACKAGE:
THERMAL RESISTANCE-JUNCTION TO TAB, θJT
10°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
TAB IS V OUT
*60°C/W
3. IN
DT PACKAGE:
THERMAL RESISTANCE-JUNCTION TO TAB, θJC
2. OUT
9°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
*80°C/W
1. ADJ / GND
Junction Temperature Calculation: TJ = TA + (PD x θJA). The θJA numbers are guidelines for the
thermal performance of the device/pc-board system. All of the above assume no ambient airflow.
* θ JAcan be improved with package soldered to 0.5IN2 copper area over backside ground
plane or internal power plane. θJAcan vary from 20ºC/W to > 40ºC/W depending on
mounting technique. (See Application Notes Section: Thermal Considerations)
DT PACKAGE (D-Pak)
(Top View)
BLOCK D IA GR A M
VIN
Bias
Circuit
Thermal
Limit Circuit
Bandgap
Circuit
Control
Circuit
Output
Circuit
VOUT
ADJ
2
Current
Limit Circuit
Copyright © 1999
Rev. 1.4 3/99
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
R E C O M M E N D E D O P E R AT I N G C O N D I T I O N S
Parameter
Symbol
(Note 2)
Recommended Operating Conditions
Min.
Typ.
Max.
Input Voltage
Operating Voltage
LX8117(A/B)-00 / 8117(A/B)-05
LX8117(A/B)-25 / -28 / -33
Input-Output Differential
LX8117(A/B)-00
Operating Ambient Temperature Range
Units
15
12
10
125
0
V
V
V
°C
Note 2. Range over which the device is functional.
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified: 0°C ≤ TJ ≤ 125°C, IMAX = 0.8A for the LX8117-xx, I MAX = 1.0A for the LX8117A-xx, and IMAX = 1.2A for the LX8117B-xx.)
LX8117-00 / 8117A-00 / 8117B-00 (Adjustable)
Parameter
Reference Voltage
Line Regulation (Note 3)
Load Regulation (Note 3)
Dropout Voltage
(Note 4)
LX8117-00
LX8117A/B-00
Current Limit
LX8117-00
LX8117A-00
LX8117B-00
Minimum Load Current (Note 5)
Thermal Regulation
Ripple Rejection
Adjust Pin Current
Adjust Pin Current Change
Temperature Stability
Long Term Stability
RMS Output Noise (% of VOUT)
Symbol
Test Conditions
VREF
IOUT = 10mA, (VIN - VOUT) = 2V, TJ = 25°C
10mA ≤ IOUT ≤ IOUT (MAX), 1.4V ≤ (VIN - VOUT ) ≤ 10V
∆VREF(VIN) IOUT = 10mA, 1.5V ≤ (VIN - VOUT ) ≤ 7V
∆VREF (IOUT) (VIN - VOUT) = 3V, 10mA ≤ IOUT ≤ IOUT (MAX)
∆V
IOUT = 100mA
IOUT = 500mA
IOUT = IOUT (MAX)
I OUT = IOUT (MAX)
IOUT (MAX) (VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
I OUT (MIN) VIN ≤ 10V
∆VOUT (Pwr) TA = 25°C, 30ms pulse
fRIPPLE =120Hz, (VIN - V OUT) = 3V, VRIPPLE = 1Vp-p
I ADJ
∆IADJ
10mA ≤ IOUT ≤ IOUT (MAX) , 1.4V ≤ (VIN - VOUT) ≤ 10V
∆VOUT (T)
∆VOUT (t) TA = 125°C, 1000 hours
VOUT (RMS) 10Hz ≤ f ≤ 10kHz
LX8117 / 17A / 17B-00
Min. Typ.
Max.
1.238
1.225
800
1000
1200
60
1.250
1.250
0.05
0.15
0.97
1.00
1.05
1.15
950
1200
1500
0.5
0.08
75
45
0.2
0.5
0.3
0.003
1.262
1.270
0.2
0.4
1.10
1.15
1.20
1.30
5
0.2
100
5
Units
V
V
%
%
V
V
V
V
mA
mA
mA
mA
%/W
dB
µA
µA
%
%
%
Notes: 3. See thermal regulation specification for changes in output voltage due to heating effects. Load regulation and line regulation are measured at a constant junction
temperature by low duty cycle pulse testing.
4. Dropout voltage is specified over the full output current range of the device. Dropout voltage is defined as the minimum input/output differential measured
at the specified output current. Test points and limits are also shown on the Dropout Voltage Curve.
5. Minimum load current is defined as the minimum output current required to maintain regulation.
(Other Voltage Options on following pages.)
Copyright © 1999
Rev. 1.4 3/99
3
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
ELECTRICAL CHARACTERISTICS
(continued)
LX8117-25 / 8117A-25 / 8117B-25 (2.5V Fixed)
Parameter
Output Voltage
Test Conditions
VOUT
Line Regulation (Note 3)
Load Regulation (Note 3)
Dropout Voltage
(Note 4)
Current Limit
Symbol
LX8117-25
LX8117A/B-25
LX8117-25
LX8117A-25
LX8117B-25
Quiescent Current
Thermal Regulation
Ripple Rejection
Temperature Stability
Long Term Stability
RMS Output Noise (% of VOUT)
IOUT = 10mA, VIN = 5V, TA = 25°C
0mA ≤ IOUT ≤ IOUT (MAX) , 4.75V ≤ VIN ≤ 10V
∆V OUT(VIN) IOUT = 0mA, 4.25V ≤ VIN ≤ 10V
∆VOUT (IOUT) VIN = 4.25V, 0mA ≤ IOUT ≤ IOUT (MAX)
∆V
IOUT = 100mA
IOUT = 500mA
IOUT = IOUT (MAX)
I OUT = IOUT (MAX)
IOUT (MAX) (VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
IQ
VIN ≤ 10V
∆VOUT (Pwr) TA = 25°C, 30ms pulse
fRIPPLE =120Hz, (VIN - V OUT) = 3V, VRIPPLE = 1Vp-p
∆VOUT (T)
∆VOUT (t) TA = 125°C, 1000 hours
VOUT (RMS) 10Hz ≤ f ≤ 10kHz
LX8117 / 17A / 17B-25
Min. Typ.
Max.
2.475
2.450
800
1000
1200
60
2.500
2.500
1
2
0.97
1.00
1.05
1.15
950
1200
1500
4.5
0.08
75
0.5
0.3
0.003
2.525
2.550
6
10
1.10
1.15
1.20
1.30
10
0.2
Units
V
V
mV
mV
V
V
V
V
mA
mA
mA
mA
%/W
dB
%
%
%
LX8117-28 / 8117A-28 / 8117B-28 (2.8V Fixed)
Parameter
Output Voltage
VOUT
Line Regulation (Note 3)
Load Regulation (Note 3)
Dropout Voltage
(Note 4)
Current Limit
Symbol
LX8117-28
LX8117A/B-28
LX8117-28
LX8117A-28
LX8117B-28
Quiescent Current
Thermal Regulation
Ripple Rejection
Temperature Stability
Long Term Stability
RMS Output Noise (% of VOUT)
Test Conditions
IOUT = 10mA, VIN = 4.85V, TA = 25°C
0mA ≤ IOUT ≤ IOUT (MAX) , 4.25V ≤ VIN ≤ 10V
0mA ≤ IOUT ≤ 500mA, VIN = 3.95V
∆V OUT(VIN) IOUT = 0mA, 4.25V ≤ VIN ≤ 10V
∆VOUT (IOUT) VIN = 4.25V, 0mA ≤ IOUT ≤ IOUT (MAX)
∆V
IOUT = 100mA
IOUT = 500mA
IOUT = IOUT (MAX)
I OUT = IOUT (MAX)
IOUT (MAX) (VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
(VIN - VOUT) = 5V, TJ = 25°C
IQ
VIN ≤ 10V
∆VOUT (Pwr) TA = 25°C, 30ms pulse
fRIPPLE =120Hz, (VIN - V OUT) = 3V, VRIPPLE = 1Vp-p
∆VOUT (T)
∆VOUT (t) TA = 125°C, 1000 hours
VOUT (RMS) 10Hz ≤ f ≤ 10kHz
LX8117 / 17A / 17B-28
Min. Typ.
Max.
2.820
2.790
2.790
800
1000
1200
60
2.850
2.850
2.850
1
2
0.97
1.00
1.05
1.15
950
1200
1500
4.5
0.08
75
0.5
0.3
0.003
2.880
2.910
2.910
6
10
1.10
1.15
1.20
1.30
10
0.2
Units
V
V
V
mV
mV
V
V
V
V
mA
mA
mA
mA
%/W
dB
%
%
%
Notes: 3. See thermal regulation specification for changes in output voltage due to heating effects. Load regulation and line regulation are measured at a constant junction
temperature by low duty cycle pulse testing.
4. Dropout voltage is specified over the full output current range of the device. Dropout voltage is defined as the minimum input/output differential measured
at the specified output current. Test points and limits are also shown on the Dropout Voltage Curve.
5. Minimum load current is defined as the minimum output current required to maintain regulation.
4
Copyright © 1999
Rev. 1.4 3/99
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
ELECTRICAL CHARACTERISTICS
(continued)
LX8117-33 / 8117A-33 / 8117B-33 (3.3V Fixed)
Parameter
Output Voltage
Test Conditions
V OUT
Line Regulation (Note 3)
Load Regulation (Note 3)
Dropout Voltage
(Note 4)
Current Limit
Symbol
LX8117-33
LX8117A/B-33
LX8117-33
LX8117A-33
LX8117B-33
Quiescent Current
Thermal Regulation
Ripple Rejection
Temperature Stability
Long Term Stability
RMS Output Noise (% of VOUT)
IOUT = 10mA, VIN = 5V, TA = 25°C
0mA ≤ IOUT ≤ IOUT (MAX) , 4.75V ≤ VIN ≤ 10V
∆V OUT(VIN) IOUT = 0mA, 4.25V ≤ VIN ≤ 10V
∆V OUT (IOUT) VIN = 4.25V, 0mA ≤ IOUT ≤ IOUT (MAX)
∆V
IOUT = 100mA
IOUT = 500mA
IOUT = IOUT (MAX)
IOUT = IOUT (MAX)
I OUT (MAX) (VIN - VOUT ) = 5V, TJ = 25°C
(VIN - VOUT ) = 5V, TJ = 25°C
(VIN - VOUT ) = 5V, TJ = 25°C
IQ
VIN ≤ 10V
∆V OUT (Pwr) TA = 25°C, 30ms pulse
fRIPPLE =120Hz, (VIN - VOUT ) = 3V, VRIPPLE = 1Vp-p
∆VOUT (T)
∆VOUT (t) TA = 125°C, 1000 hours
VOUT (RMS) 10Hz ≤ f ≤ 10kHz
LX8117 / 17A / 17B-33
Min.
Typ.
Max.
3.267
3.235
800
1000
1200
60
3.300
3.300
1
2
0.97
1.00
1.05
1.15
950
1200
1500
4.5
0.08
75
0.5
0.3
0.003
3.333
3.365
6
10
1.10
1.15
1.20
1.30
10
0.2
Units
V
V
mV
mV
V
V
V
V
mA
mA
mA
mA
%/W
dB
%
%
%
LX8117-05 / 8117A-05 / 8117B-05 (5.0V Fixed)
Parameter
Output Voltage
V OUT
Line Regulation (Note 3)
Load Regulation (Note 3)
Dropout Voltage
(Note 4)
Current Limit
Symbol
LX8117-05
LX8117A/B-05
LX8117-05
LX8117A-05
LX8117B-05
Quiescent Current
Thermal Regulation
Ripple Rejection
Temperature Stability
Long Term Stability
RMS Output Noise (% of VOUT)
Test Conditions
IOUT = 10mA, VIN = 7V, TA = 25°C
0mA ≤ IOUT ≤ IOUT (MAX) , 6.50V ≤ VIN ≤ 10V
∆V OUT(VIN) IOUT = 0mA, 6.5V ≤ VIN ≤ 10V
∆V OUT (IOUT) VIN = 6.5V, 0mA ≤ IOUT ≤ IOUT (MAX)
∆V
IOUT = 100mA
IOUT = 500mA
IOUT = IOUT (MAX)
IOUT = IOUT (MAX)
I OUT (MAX) (VIN - VOUT ) = 5V, TJ = 25°C
(VIN - VOUT ) = 5V, TJ = 25°C
(VIN - VOUT ) = 5V, TJ = 25°C
IQ
VIN ≤ 10V
∆V OUT (Pwr) TA = 25°C, 30ms pulse
fRIPPLE =120Hz, (VIN - VOUT ) = 3V, VRIPPLE = 1Vp-p
∆VOUT (T)
∆VOUT (t) TA = 125°C, 1000 hours
VOUT (RMS) 10Hz ≤ f ≤ 10kHz
LX8117 / 17A / 17B-05
Min.
Typ.
Max.
4.950
4.900
800
1000
1200
60
5.000
5.000
1
2.5
0.97
1.00
1.05
1.15
950
1200
1500
4.5
0.08
75
0.5
0.3
0.003
5.050
5.100
10
15
1.10
1.15
1.20
1.30
10
0.2
Units
V
V
mV
mV
V
V
V
V
mA
mA
mA
mA
%/W
dB
%
%
%
Notes: 3. See thermal regulation specification for changes in output voltage due to heating effects. Load regulation and line regulation are measured at a constant junction
temperature by low duty cycle pulse testing.
4. Dropout voltage is specified over the full output current range of the device. Dropout voltage is defined as the minimum input/output differential measured
at the specified output current. Test points and limits are also shown on the Dropout Voltage Curve.
5. Minimum load current is defined as the minimum output current required to maintain regulation.
Copyright © 1999
Rev. 1.4 3/99
5
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
A P P L I C AT I O N N O T E S
The LX8117 series ICs are easy to use Low-Dropout (LDO) voltage
regulators. They have all of the standard self-protection features
expected of a voltage regulator: short circuit protection, safe
operating area protection and automatic thermal shutdown if the
device temperature rises above approximately 165°C.
Use of an output capacitor is REQUIRED with the LX8117 series.
Please see the table below for recommended minimum capacitor
values.
These regulators offer a more tightly controlled reference voltage
tolerance and superior reference stability when measured against
the older pin-compatible regulator types that they replace.
STABILITY
The output capacitor is part of the regulator’s frequency compensation system. Many types of capacitors are available, with different
capacitance value tolerances, capacitance temperature coefficients,
and equivalent series impedances. For all operating conditions,
connection of a 220µF aluminum electrolytic capacitor or a 47µF
solid tantalum capacitor between the output terminal and ground
will guarantee stable operation.
If a bypass capacitor is connected between the output voltage
adjust (ADJ) pin and ground, ripple rejection will be improved
(please see the section entitled “RIPPLE REJECTION”). When ADJ
pin bypassing is used, the required output capacitor value increases.
Output capacitor values of 220µF (aluminum) or 47µF (tantalum)
provide for all cases of bypassing the ADJ pin. If an ADJ pin bypass
capacitor is not used, smaller output capacitor values are adequate.
The table below shows recommended minimum capacitance values
for stable operation.
RECOMMENDED CAPACITOR VALUES
INPUT
OUTPUT
ADJ
10µF
10µF
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
None
15µF
In order to ensure good transient response from the power supply
system under rapidly changing current load conditions, designers
generally use several output capacitors connected in parallel. Such
an arrangement serves to minimize the effects of the parasitic
resistance (ESR) and inductance (ESL) that are present in all
capacitors. Cost-effective solutions that sufficiently limit ESR and
ESL effects generally result in total capacitance values in the range
of hundreds to thousands of microfarads, which is more than
adequate to meet regulator output capacitor specifications. Output
capacitance values may be increased without limit.
The circuit shown in Figure 1 can be used to observe the transient
response characteristics of the regulator in a power system under
changing loads. The effects of different capacitor types and values
on transient response parameters, such as overshoot and undershoot, can be quickly compared in order to develop an optimum
solution.
6
Power Supply
IN
LX8117-xx
OUT
Full Load
(Smaller resistor)
ADJ
C1
Minumum Load
(Larger resistor)
C2
Star Ground
RDSON << RL
10ms
1 sec
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
OVERLOAD RECOVERY
Like almost all IC power regulators, the LX8117 regulators are
equipped with Safe Operating Area (SOA) protection. The SOA
circuit limits the regulator's maximum output current to progressively lower values as the input-to-output voltage difference
increases. By limiting the maximum output current, the SOA circuit
keeps the amount of power that is dissipated in the regulator itself
within safe limits for all values of input-to-output voltage within the
operating range of the regulator. The LX8117 SOA protection
system is designed to be able to supply some output current for all
values of input-to-output voltage, up to the device breakdown
voltage.
Under some conditions, a correctly operating SOA circuit may
prevent a power supply system from returning to regulated
operation after removal of an intermittent short circuit at the output
of the regulator. This is a normal mode of operation which can be
seen in most similar products, including older devices such as 7800
series regulators. It is most likely to occur when the power system
input voltage is relatively high and the load impedance is relatively
low.
When the power system is started “cold”, both the input and
output voltages are very close to zero. The output voltage closely
follows the rising input voltage, and the input-to-output voltage
difference is small. The SOA circuit therefore permits the regulator
to supply large amounts of current as needed to develop the
designed voltage level at the regulator output. Now consider the
case where the regulator is supplying regulated voltage to a resistive
load under steady state conditions. A moderate input-to-output
voltage appears across the regulator but the voltage difference is
small enough that the SOA circuitry allows sufficient current to flow
through the regulator to develop the designed output voltage across
the load resistance. If the output resistor is short-circuited to ground,
the input-to-output voltage difference across the regulator suddenly
becomes larger by the amount of voltage that had appeared across
the load resistor. The SOA circuit reads the increased input-tooutput voltage, and cuts back the amount of current that it will
permit the regulator to supply to its output terminal. When the short
circuit across the output resistor is removed, all the regulator output
current will again flow through the output resistor. The maximum
current that the regulator can supply to the resistor will be limited
by the SOA circuit, based on the large input-to-output voltage across
the regulator at the time the short circuit is removed from the output.
Copyright © 1999
Rev. 1.4 3/99
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
A P P L I C AT I O N N O T E S
OVERLOAD RECOVERY (continued)
If this limited current is not sufficient to develop the designed
voltage across the output resistor, the voltage will stabilize at some
lower value, and will never reach the designed value. Under these
circumstances, it may be necessary to cycle the input voltage down
to zero in order to make the regulator output voltage return to
regulation.
C = 1 / (6.28 * FR * R1)
≡ the value of the capacitor in Farads;
select an equal or larger standard value.
FR ≡ the ripple frequency in Hz
R1 ≡ the value of resistor R1 in ohms
where: C
At a ripple frequency of 120Hz, with R1 = 100Ω:
C = 1 / (6.28 * 120Hz * 100Ω) = 13.3µF
The closest equal or larger standard value should be used, in this
case, 15µF.
When an ADJ pin bypass capacitor is used, output ripple
amplitude will be essentially independent of the output voltage. If
an ADJ pin bypass capacitor is not used, output ripple will be
proportional to the ratio of the output voltage to the reference
voltage:
M = VOUT/VREF
where: M
VREF
≡ a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
= 1.25V.
For example, if VOUT = 2.5V the output ripple will be:
M = 2.5V/1.25V= 2
LX8117-xx
OUT
ADJ
R1
R2
FIGURE 2 — BASIC ADJUSTABLE REGULATOR
LOAD REGULATION
Because the LX8117 regulators are three-terminal devices, it is not
possible to provide true remote load sensing. Load regulation will
be limited by the resistance of the wire connecting the regulator to
the load. The data sheet specification for load regulation is
measured at the bottom of the package. Negative side sensing is a
true Kelvin connection, with the bottom of the output divider
returned to the negative side of the load. Although it may not be
immediately obvious, best load regulation is obtained when the top
of the resistor divider, (R1), is connected directly to the case of the
regulator, not to the load. This is illustrated in Figure 3. If R1 were
connected to the load, the effective resistance between the regulator
and the load would be:
RPeff = RP *
 R2+R1
 R1 
where: RP ≡ Actual parasitic line resistance.
When the circuit is connected as shown in Figure 3, the parasitic
resistance appears as its actual value, rather than the higher RPeff.
VIN
RP
Parasitic
LX8117-xx
Line Resistance
OUT
IN
ADJ
Connect
R1
OUTPUT VOLTAGE
R2
Copyright © 1999
Rev. 1.4 3/99
VREF
VOUT = VREF 1 + R2 + IADJ R2
R1
Output ripple will be twice as bad as it would be if the ADJ pin
were to be bypassed to ground with a properly selected capacitor.
The LX8117 ICs develop a 1.25V reference voltage between the output
and the adjust terminal (See Figure 2). By placing a resistor, R1,
between these two terminals, a constant current is caused to flow
through R1 and down through R2 to set the overall output voltage.
Normally this current is the specified minimum load current of 10mA.
Because IADJ is very small and constant when compared with the current
through R1, it represents a small error and can usually be ignored.
VOUT
IADJ
50µA
RIPPLE REJECTION
Ripple rejection can be improved by connecting a capacitor
between the ADJ pin and ground. The value of the capacitor should
be chosen so that the impedance of the capacitor is equal in
magnitude to the resistance of R1 at the ripple frequency. The
capacitor value can be determined by using this equation:
IN
VIN
R1 to Case
of Regulator
RL
Connect
R2
to Load
FIGURE 3 — CONNECTIONS FOR BEST LOAD REGULATION
7
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
A T A
S
H E E T
A P P L I C AT I O N N O T E S
LOAD REGULATION (continued)
THERMAL CONSIDERATIONS (continued)
Even when the circuit is optimally configured, parasitic resistance
can be a significant source of error. A 100 mil (2.54 mm) wide PC
trace built from 1 oz. copper-clad circuit board material has a
parasitic resistance of about 5 milliohms per inch of its length at
room temperature. If a 3-terminal regulator used to supply 2.50 volts
is connected by 2 inches of this trace to a load which draws 5 amps
of current, a 50 millivolt drop will appear between the regulator and
the load. Even when the regulator output voltage is precisely
2.50 volts, the load will only see 2.45 volts, which is a 2% error. It
is important to keep the connection between the regulator output
pin and the load as short as possible, and to use wide traces or
heavy-gauge wire.
The minimum specified output capacitance for the regulator
should be located near the reglator package. If several capacitors
are used in parallel to construct the power system output capacitance, any capacitors beyond the minimum needed to meet the
specified requirements of the regulator should be located near the
sections of the load that require rapidly-changing amounts of
current. Placing capacitors near the sources of load transients will
help ensure that power system transient response is not impaired
by the effects of trace impedance.
To maintain good load regulation, wide traces should be used on
the input side of the regulator, especially between the input
capacitors and the regulator. Input capacitor ESR must be small
enough that the voltage at the input pin does not drop below VIN (MIN)
during transients.
Example
Given: VIN = 5.0V ±5%, VOUT = 2.5V ±3%
IOUT = 0.5A, TA = 55°C, TJ = 125°C
RθJT = 15°C/W, RθTS = 5°C/W
VIN (MIN) = VOUT + VDROPOUT (MAX)
≡ the lowest allowable instantaneous
voltage at the input pin.
≡ the designed output voltage for the
VOUT
power supply system.
VDROPOUT (MAX) ≡ the specified dropout voltage
for the installed regulator.
where: VIN (MIN)
THERMAL CONSIDERATIONS
The LX8117 regulators have internal power and thermal limiting
circuitry designed to protect each device under overload conditions.
For continuous normal load conditions, however, maximum junction temperature ratings must not be exceeded. It is important to
give careful consideration to all sources of thermal resistance from
junction to ambient. This includes junction to case, case to heat sink
interface, and heat sink thermal resistance itself.
Find: The size of a square area of 1oz. copper circuitboard trace-foil that will serve as a heatsink,
adequate to maintain the junction temperature of the
LX8117 in the ST (SOT-223) package within
specified limits.
Solution: The junction temperature is:
TJ = PD (RθJT + RθCS + R θSA) + TA
where: PD ≡ Dissipated power.
RθJT ≡ Thermal resistance from the junction to the
mounting tab of the package.
RθTS ≡ Thermal resistance through the interface
between the IC and the surface on which
it is mounted.
RθSA ≡ Thermal resistance from the mounting surface
of the heatsink to ambient.
TS ≡ Heat sink temperature.
TJ
TC
Rq JT
TS
Rq CS
TA
Rq SA
First, find the maximum allowable thermal resistance of the
heat sink:
PD
= [[VIN * (1 + TolVIN )] - [VOUT * (1 - TolVOUT)]] * IOUT
PD
= 1.4W
RθSA =
TJ - TA
- (RθJT + R θTS) ,
PD
RθSA = 29.6°C/W
A test was conducted to determine the thermal characteristics of
1 oz. copper circuit-board trace material. The following equation
describes the observed relationship between the area of a square
copper pad, and the thermal resistance from the tab of a SOT-223
package soldered at the center of the pad to ambient.
3.1°C/W
in 2
AreaSINK =
R θSA - 22.3°C/W
Substituting the value for RθSA calculated above, we find that a
square pad with area:
AreaSINK = 0.43 in2 (0.66" x 0.66"), 280mm 2 (17 x 17 mm)
will be required to maintain the LX8117 junction temperature
within specified limits.
8
Copyright © 1999
Rev. 1.4 3/99
PRODUCT DATABOOK 1996/1997
LX8117-xx/8117A-xx/8117B-xx
0.8, 1 & 1.2A L O W D R O P O U T P O S I T I V E R E G U L AT O R S
P
R O D U C T I O N
D
S
A T A
H E E T
T Y P I C A L A P P L I C AT I O N S
(Note A)
VIN
10µF
LX8117-xx
OUT
IN
ADJ
* C1 improves ripple rejection.
XC should be ≈ R1 at ripple
frequency.
5V
R1
121Ω
1%
R2
365Ω
1%
VOUT
VIN
IN
(Note A)
LX8117-xx
OUT
ADJ
C1*
10µF
150µF
VOUT**
R1
121Ω
R2
1k
C1
10µF*
C2
100µF
* Needed if device is far from filter capacitors.
** VOUT = 1.25V 1 + R2
R1
FIGURE 4 — IMPROVING RIPPLE REJECTION
FIGURE 5 — 1.2V - 8V ADJUSTABLE REGULATOR
LX8117-33
VIN
OUT
IN
10µF Tantalum
or 100µF Aluminum
GND
3.3V
Min. 15µF Tantalum or
100µF Aluminum capacitor.
May be increased without
limit. ESR must be less
than 50mΩ.
FIGURE 6 — FIXED 3.3V OUTPUT REGULATOR
Note A: VIN (MIN) = (Intended VOUT) + (VDROPOUT (MAX))
PRODUCTION DATA - Information contained in this document is proprietary to Linfinity, and is current as of publication date. This document
may not be modified in any way without the express written consent of Linfinity. Product processing does not necessarily include testing of
all parameters. Linfinity reserves the right to change the configuration and performance of the product and to discontinue product at any time.
Copyright © 1999
Rev. 1.4 3/99
9