ETC LX8385

A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
The LX8385/85A/85B Series ICs are
positive regulators designed to provide 3A
output current. These regulators yield
higher efficiency than currently available
devices with all internal circuitry designed
to operate down to a 1V input-to-output
differential. In each of these products, the
dropout voltage is fully specified as a
function of load current. Dropout is
guaranteed at a maximum of 1.3V
(8385A/B) and 1.5V (8385) at maximum
output current, decreasing at lower load
currents.
In addition, on-chip trimming adjusts the
reference voltage tolerance to 1% maximum
at room temperature and 2% maximum over
the 0 to 125°C range for the LX8385A. The
LX8385B offers 0.8% tolerance at room
temperature and 1.0% maximum over line,
load and temperature. Fixed versions are
also available and specified in the Available
Options table below.
The LX8385/85A/85B Series devices are
pin-compatible with earlier 3-terminal
regulators, such as the 117 series products,
but they do require input and output
capacitors. A minimum 10µF capacitor is
required on the input and a 15µF or greater
on the output of these new devices for
stability. Although, these capacitors are
generally included in most regulator designs.
The LX8385/85A/85B Series quiescent
current flows into the load, thereby
increasing efficiency. This feature contrasts
with PNP regulators where up to 10% of the
output current is wasted as quiescent
current.
The LX8385-xxI is specified over the
industrial temperature range of -25°C to
125°C, while the LX8385-xxC / 85A-xxC /
85B-xxC is specified over the commercial
range of 0°C to 125°C.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
Three-Terminal Adjustable Or
Fixed Output
Guaranteed 1% Voltage
Reference Accuracy Over Line,
Load, And Temperature
(LX8385B)
Guaranteed < 1.5V Headroom At
3A (LX8385)
Guaranteed < 1.3V Headroom At
3A (LX8385A/8385B)
0.015% Line Regulation
0.15% Load Regulation
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High Efficiency Linear
Regulators
Post Regulators For Switching
Power Supplies
Battery Chargers
Constant Current Regulators
Graphics & Standard Supplies
Processor I/O Supply
Low Voltage Memory & Chipset
Supplies
IN
OUT
3.3V
at 3A
LX 8385A
5V
+
*150 0µ F
6M V 1 500G X
S anyo
121 Ω
1%
AD J
+
150 0µ F
2x 6M V 1 500G X
S anyo
200 Ω
1%
LX8385/85A/85B-00
Adjustable
LX8385/85A/85B-33
3.3V
LX8385/85A/85B-05
5.0V
Table 1 - Available Options
LX8385X
Max Ref Max Dropout
TA (°C)
Accuracy
Voltage
0 to 125
2.0%
1.5V
2.0%
1.3V
1.0%
1.3V
-25 to 125 2.0%
1.5V
Plastic TO-252
(D-Pak) 3-Pin
LX8385-xxCDT
LX8385A-xxCDT
LX8385B-xxCDT
LX8385-xxIDT
DT
Plastic TO-220
3-Pin
LX8385-xxCP
LX8385A-xxCP
LX8385B-xxCP
LX8385-xxIP
P
Plastic TO-263
3-Pin
LX8385-xxCDD
LX8385A-xxCDD
LX8385B-xxCDD
LX8385-xxIDD
DD
Note: Available in Tape & Reel.
Append the letter “T” to the part number. (i.e. LX8385-xxCPT)
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
Power Dissipation....................................................................................Internally Limited
Input Voltage .................................................................................................................10V
Input to Output Voltage Differential..............................................................................10V
Operating Junction Temperature
Plastic (DT, DD, P Packages)................................................................................ 150°C
Storage Temperature Range .......................................................................-65°C to 150 °C
Lead Temperature (Soldering, 10 Seconds)............................................................... 300°C
W W W . Microsemi . COM
TA B is V O U T
3
V IN
2
V OUT
1
AD J /
GND*
DD PACKAGE (3-PIN)
(Top View)
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 specified terminal.
TA B is V O UT
V IN
3
2
DD
Plastic TO-263 3-Pin
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
THERMAL RESISTANCE-JUNCTION TO TAB, θJT
AD J/
G ND *
1
60°C/W
2.7°C/W
DT PACKAGE (3-PIN)
(Top View)
TA B is V O U T
P
Plastic TO-220 3-Pin
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
THERMAL RESISTANCE-JUNCTION TO TAB, θJT
V O UT
60°C/W
2.7°C/W
3
V IN
2
V OUT
AD J /
G N D*
1
DT
Plastic TO-252 3-Pin
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
THERMAL RESISTANCE-JUNCTION TO TAB, θJT
P PACKAGE (3-PIN)
60°C/W
2.7°C/W
(Top View)
*Pin 1 is GND for fixed voltage versions
Junction Temperature Calculation: TJ = TA + (PD x θJT).
The θJA & θJT numbers are guidelines for the thermal performance of the device/pc-board
system. All of the above assume no ambient airflow.
V IN
Bias C ircuit
Bandgap
C ircuit
C ontrol
C ircuit
O utput
C ircuit
PACKAGE DATA
T herm al
Lim it C ircuit
V O UT
SO A
Protection
C ircuit
AD J or
GND*
C urrent
Lim it C ircuit
*Pin 1 is GND for fixed voltage versions
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
Unless otherwise specified, the following specifications apply over the operating ambient temperature for the LX8385x-xxC with
0°C ≤ TA ≤ 125°C and the LX8385-xxI with -25°C ≤ TA ≤ 125°C except where otherwise noted. Test conditions: VIN -VOUT = 3V;
IOUT = 3A. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient
temperature.
Parameter
Symbol
Min
LX8385x-xx
Typ
Max
Units
LX8385-00 / 8385A-00 / 8385B-00 (ADJUSTABLE)
Reference Voltage
(Note 4)
LX8385/85A-00
VREF
LX8385B-00
Line Regulation (Note 2)
REF
Load Regulation (Note 2)
REF
Thermal Regulation
OUT
(VIN)
(IOUT)
(Pwr)
Adjust Pin Current
1.238
1.250
1.262
V
1.225
1.250
1.270
V
1.240
1.250
1.260
V
1.238
1.250
1.262
V
1.5V < (VIN -VOUT), VIN < 7V, IOUT = 10mA
0.015
0.2
%
VOUT > VREF, VIN - VOUT = 3V,
10mA < IOUT < 3A
0.15
0.5
%
TA = 25°C, 20ms pulse
0.01
0.04
%/W
65
IADJ
Adjust Pin Current Change (Note 4)
Dropout Voltage
IOUT = 10mA, TA = 25°C
10mA < IOUT < 3A, 1.5V < (VIN -VOUT),
VIN < 10V, P < PMAX
IOUT = 10mA, TA = 25°C
10mA < IOUT < 3A, 1.5V < (VIN -VOUT),
VIN < 10V, P < PMAX
VOUT = 5V, f= 120Hz, COUT = 100µF Tantalum,
VIN = 6.5V, CADJ = 10µF, IOUT = 3A
Ripple Rejection (Note 3)
LX8385-00
LX8385A/85B-00
Minimum Load Current
Maximum Output Current
Long Term Stability (Note 3)
Test Conditions
ADJ
10mA < IOUT < IOUT(MAX), 1.5V < (VIN -VOUT), VIN<10V
REF = 1%, IOUT = 3A
REF = 1%, IOUT = 3A
IOUT(MIN)
VIN < 10V
IOUT(MAX)
(VIN - VOUT) < 7V, VIN < 10V
OUT(t)
OUT(T)
3
TA = 125°C, 1000 hours
VIN = 5V, IOUT = 0mA, TA = 25°C
LX8385/85A-33
VOUT
LX8385B-33
OUT
Load Regulation (note 2)
OUT
Thermal Regulation
OUT
(VIN)
(IOUT)
(Pwr)
Ripple Rejection (note 3)
Quiescent Current
Dropout Voltage
LX8385-33
LX8385A/85B-33
Copyright  2000
Rev. 2.0a, 2001-03-15
55
100
µA
5
µA
1.2
1.5
V
1.1
1.3
V
2
10
mA
1
%
3.5
A
0.25
%
0.003
%
3.3
3.333
V
4.75V < VIN < 10V, 0mA < IOUT < 3A, P < PMAX
3.235
3.3
3.365
V
VIN = 5V, IOUT = 0mA, TA = 25°C
3.274
3.3
3.326
V
4.75V < VIN < 10V, 0A < IOUT < 3A, P < PMAX
3.267
3.3
3.333
V
4.75V < VIN < 7V
1
6
mV
4.75V < VIN < 10V
2
10
mV
VIN = 5V, 0mA < IOUT < IOUT(MAX)
5
15
mV
0.01
0.02
%/W
TA = 25°C, 20ms pulse
COUT = 100µF (Tantalum), IOUT = 3A
IQ
0mA < IOUT < IOUT(MAX), 4.75V < V < 10V
OUT = 1%, IOUT < IOUT(MAX)
OUT = 1%, IOUT < IOUT(MAX)
60
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Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
83
dB
4
10
mA
1.2
1.5
V
1.1
1.3
V
Page 3
ELECTRICALS
Line Regulation (note 2)
3.267
dB
0.2
0.3
Temperature Stability (Note 3)
RMS Output Noise (% of VOUT)
OUT(RMS) TA = 25°C, 10Hz < f < 10kHz
(Note 3)
LX8385-33/ 8385A-33/ 8385B-33 (3.3V FIXED)
Output Voltage
(Note 4)
83
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A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
Unless otherwise specified, the following specifications apply over the operating ambient temperature for the LX8385x-xxC with
0°C ≤ TA ≤ 125°C and the LX8385-xxI with -25°C ≤ TA ≤ 125°C except where otherwise noted. Test conditions: VIN -VOUT = 3V;
IOUT = 3A. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient
temperature.
Parameter
Test Conditions
Min
LX8385x-xx
Typ
Max
Units
LX8385-33 / 8385A-33 / 8385B- (3.3V FIXED)(CONTINUED)
Maximum Output Current
Temperature Stability (Note 3)
Long Term Stability (Note 3)
Symbol
IOUT(MAX)
VIN < 7V
OUT(T)
OUT (t)
TA=125°C, 1000 hours
RMS Output Noise (% of VOUT)
VOUT (RMS)
(Note 3)
LX8385-05 / 8385A-05 / 8385B-05 (5.0V FIXED)
Output Voltage
(Note 4)
LX8385/85A-05
VOUT
LX8385B-05
Line Regulation (Note 2)
Load Regulation (Note 2)
OUT(VIN)
OUT
(IOUT)
OUT
Thermal Regulation
(Pwr)
Ripple Rejection (Note 3)
Quiescent Current
Dropout Voltage
LX8385-33
LX8385A/85B-33
Maximum Output Current
Temperature Stability (Note 3)
Long Term Stability (Note 3)
RMS Output Noise (% of VOUT)
(Note 3)
Note 2
A
0.3
TA=25°C, 10Hz < f < 10kHz
%
1
0.003
%
%
VIN=7V, IOUT=0mA, TA=25°C
4.950
5.00
5.050
V
7V < VIN < 10V, 0mA < IOUT < 3A, P < PMAX
4.900
5.00
5.100
V
VIN=7V, IOUT=0mA, TA=25°C
4.960
5.00
5.040
V
7V < VIN < 10V, 0mA < IOUT < 3A, P < PMAX
4.950
5.00
5.050
V
7V < VIN < 10V
2
10
mV
VIN=7V, 0mA < IOUT < IOUT(MAX)
5
25
mV
0.01
0.02
%/W
4
10
mA
1.2
1.5
V
1.1
1.3
V
TA=25°C, 20ms pulse
COUT=100µF (Tantalum), IOUT=3A
0mA < IOUT < IOUT(MAX), 4.75V < VIN < 10V
OUT=1%, IOUT < IOUT(MAX)
OUT=1%, IOUT < IOUT(MAX)
VIN < 10V
OUT (T)
OUT (t)
TA=125°C, 1000 hours
VOUT (RMS)
TA=25°C, 10Hz < f < 10kHz
60
3
83
dB
3.5
A
0.25
%
0.3
1
0.003
%
%
Regulation is measured at constant junction temperature, using pulse testing with a low duty cycle. Changes in output
voltage due to heating effects are covered under the specification for thermal regulation.
These parameters, although guaranteed are not tested in production.
See Maximum Output Current Section
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
ELECTRICALS
Note 3
Note 4
3.5
0.25
IQ
IOUT(MAX)
3
W W W . Microsemi . COM
Page 4
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
The LX8385/85A/85B Series ICs are easy to use LowDropout (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
LX8385/85A/85B 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 (<400mΩ ESR) 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 operation.
!"#$ %&'
10µF
10µF
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
(
None
15µF
Copyright  2000
Rev. 2.0a, 2001-03-15
IN
LX 8385 x
OUT
Min im u m Lo ad
(L arg er re sistor)
Fu ll L oa d
(S m alle r
re sisto r)
AD J
R D S O N << R L
S ta r G rou nd
10m s
1 sec
F IG U R E 1 - D Y N A M IC IN P U T A N D O U T P U T T E S T
OVERLOAD RECOVERY
Like almost all IC power regulators, the LX8385/85A/85B
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-tooutput 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 LX8385/85A/85B 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-to-output
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
Microsemi
Linfinity Microelectronics Division
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Page 5
APPLICATIONS
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 under-shoot, can be compared quickly in order to
develop an optimum solution.
P o wer
S u pp ly
W W W . Microsemi . COM
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
OVERLOAD RECOVERY (continued)
voltage across the regulator at the time the short circuit is
removed from the output. 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.
where:
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
At a Ripple frequency of 120Hz, with R1= 100 C=
1
= 13.3
(6.28 ×120Hz ×100Ω )
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 =
where:
M
VOUT
VREF
a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
VREF = 1.25V
For example, if VOUT = 2.5V the output ripple will be:
V R EF
V OUT
R1
I AD J
50µ A
R2

R2 

R1 
VOUT = V REF  1 +
 + I ADJ R 2
F IG U R E 2 - B A S IC A D JU S TA B LE R E G U LA TO R
LOAD REGULATION
Because the LX8385/85A/85B 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:
 R 2 + R1 
RPeff = RP × 

 R1 
RP Actual parasitic line resistance.
where:
When the circuit is connected as shown in Figure 3, the
parasitic resistance appears as its actual value, rather than the
higher RPeff.
IN
V IN
OUT
R P P arasitic Line
R esistance
LX 8385x
R1
C on nect R 1 to
C ase o f R eg ula tor
RL
R2
C on nect R 2 to
Lo ad
F IG U R E 3 - C O NN E C TIO N S FO R B E S T LO A D R E G U L A TIO N
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
APPLICATIONS
AD J
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.
OUTPUT VOLTAGE
The LX8385/85A/85B 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.
OUT
LX 8385x
AD J
2.5V
=2
M=
1.25V
Copyright  2000
Rev. 2.0a, 2001-03-15
IN
V IN
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:
C=
W W W . Microsemi . COM
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
LOAD REGULATION (continued)
Even when the circuit is configured optimally, parasitic
resistance can be a significant source of error. A 20 mil. wide PC
trace built from 1 oz. copper-clad circuit board material has a
parasitic resistance of about 25 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
1.5 amps of current, a 75 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 2.43 volts, which is a
3% 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 regulator 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.
V
where: VIN(MIN)
Find:
VIN
VOUT
IOUT
TA
R
=
=
=
=
=
5V
2.5V
1.5A
50°C
2.7°C/W for TO-220
Proper Heat Sink to keep IC’s junction temperature
below 125°C.**
Solution: The junction temperature is:
TJ = PD (R
where: PD
R
R
R
TS
the lowest allowable instantaneous
TJ
+R
+R
) + TA
TC
TS
R θ CS
TA
R θ SA
First, find the maximum allowable thermal resistance of the
heat sink:
the designed output voltage for the
R
SA
=
TJ − TA
− (R
PD
+R
CS
)
PD = (VIN(MAX) − VOUT ) I OUT = (5.0V − 2.5V ) × 1.5A
PD = 3.75W
R =
125°C − 50°C
− (2.7°C/W + 1.0°C/W)
(5.0V − 2.5V) * 1.5A
R = 16.3°C/W
Next, select a suitable heat sink. The selected heat sink must
have R < 3.1°C/W. Thermalloy heatsink 6230B has R =
12°C/W.
Finally, verify that junction temperature remains within specification using the selected heat sink:
TJ = 3.75W(2.7°C/W + 1.0°C/W + 12.0°C/W) + 50°C
TJ = 109°C
** Although the device can operate up to 150°C junction, it is recommended for long
term reliability to keep the junction temperature below 125°C whenever possible.
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
APPLICATIONS
THERMAL CONSIDERATIONS
The LX8385/85A/85B 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.
Junction-to-case thermal resistance is specified from the IC
junction to the back surface of the case directly opposite the die.
This is the lowest resistance path for heat flow. Proper mounting
is required to ensure the best possible thermal flow from this area
of the package to the heat sink. Thermal compound at the case to
heat sink interface is strongly recommended. If the case of the
device must be electrically isolated, a thermally conductive
spacer can be used, as long as its added contribution to thermal
resistance is considered. Note that the case of all devices in this
series is electrically connected to the output.
Dissipated power.
Thermal resistance from the junction to
the mounting tab of the package.
Thermal resistance through the
interface between the IC and the
surface on which it is mounted.
(1.0°C/W at 6 in-lbs mounting screw
torque).
Thermal resistance from the mounting
surface to ambient (thermal resistance
of the heat sink).
Heat Sink Temperature.
R θ JT
power supply system.
VDROPOUT(MAX) the specified dropout voltage for the
installed regulator.
Copyright  2000
Rev. 2.0a, 2001-03-15
Example
Given:
= VOUT + VDROPOUT(MAX)
voltage at the input pin.
VOUT
W W W . Microsemi . COM
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
IN
(N ote A)
OUT
LX 8385x
V IN
IN
V IN
(N ote A)
OUT
LX 8385x
C 1*
10µ F
V OUT
R1
121 Ω 1%
AD J
+
R1
121 Ω 1%
10µ F
AD J
V O U T **
+
+
C2
100µ F
150µ F
+
R2
365 Ω 1%
R2
1k
C1
10µ F *
* C 1 im proves ripple rejection.
X C should be ≈ R 1 at ripple
frequency.
W W W . Microsemi . COM
* N eeded if device is far from filter capacitors.

F IG U R E 4 - IM P R O V IN G R IP P LE R E JE C TIO N
* *VO U T = 1.25V  1 +



R1 
R2
F IG U R E 5 - 1.2V - 8V A D JU S TA B LE R E G U LA TO R
IN
V IN
LX 8385x
OUT
5V
(n o te A )
+
10µ F
121 Ω
1%
ADJ
+
1k
100µ F
2N3904
TTL
O u tp u t
365 Ω
1%
1k
F IG U R E 6 - 5V R EG UL ATO R W ITH S HU TD O W N
10µ F Tantalum or
100µ F A lum inum
LX 8385x
APPLICATIONS
IN
V IN
OUT
ADJ
3.3 V
M in. 15µ F T antalum or
100µ F A lum inum capacitor. M ay
be increased without lim it. E S R
m ust be less than <400m Ω .
F IG U R E 7 - FIXE D 3.3V O U TP U T R E G U LA TO R
Note A:
VIN(MIN) = (Intended VOUT ) + VDROPOUT(MAX)
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 8
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
P
W W W . Microsemi . COM
3-Pin Plastic TO-220
B
S
Dim
F
T
Q
A
B
C
D
F
G
H
J
K
L
N
Q
R
S
T
U
U
A
1
2
C
3
R
H
K
D
L
J
G
MILLIMETERS
MIN
MAX
14.22
15.88
9.65
10.67
3.56
4.83
0.51
1.14
3.53
4.09
2.54 BSC
6.35
0.30
1.14
12.70
14.73
1.14
1.27
5.08 TYP
2.54
3.05
2.03
2.92
1.14
1.40
5.84
6.86
0.508
1.14
INCHES
MIN
MAX
0.560
0.625
0.380
0.420
0.140
0.190
0.020
0.045
0.139
0.161
0.100 BSC
0.250
0.012
0.045
0.500
0.580
0.045
0.050
0.200 TYP
0.100
0.120
0.080
0.115
0.045
0.055
0.230
0.270
0.020
0.045
MILLIMETERS
MIN
MAX
10.03
10.67
8.51
9.17
4.19
4.59
1.14
1.40
0.330
0.51
1.19
1.34
2.41
2.66
2.29
2.79
–
1.65
0
0.25
14.60
15.87
7°
3°
INCHES
MIN
MAX
0.395
0.420
0.335
0.361
0.165
0.181
0.045
0.055
0.013
0.020
0.047
0.053
0.095
0.104
0.090
0.110
–
0.065
0
0.010
0.575
0.625
7°
3°
N
DD
3-Pin Plastic TO-263
I
A
Dim
D
C
B
K
M
N
F
0° -8°
E
G
MECHANICALS
H
A
B
C
D
E
F
G
H
I
J
K
M
N
J
S ea tin g P la n e
Note: Dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006”) on any side. Lead dimension shall
not include solder coverage.
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 9
A
M I C R O S E M I
LX8385x-xx
3A Low Dropout Positive Regulators
C O M P A N Y
P RODUCTION D ATA S HEET
DT
W W W . Microsemi . COM
3-Pin Plastic TO-252
Dim
P
U
J
G
N
W
L
A
F
Q
O
V
M
H
C
R
D
E
B
X
I
K
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
U
V
W
X
MILLIMETERS
MIN
MAX
6.47
6.73
5.97
6.23
2.16
2.42
0.68
0.94
0.38
0.64
0.63
0.89
2.16
2.42
0.84
1.10
0.89
1.15
2.44
2.70
9.55
9.81
5.20
5.46
7.0°
0.51
0.77
0.51
0.77
4.19
4.45
0.76
1.02
0.48
0.74
0.51
0.77
45°
1.44
1.70
0
0.10
INCHES
MIN
MAX
0.255
0.265
0.235
0.245
0.085
0.095
0.027
0.037
0.015
0.025
0.025
0.035
0.085
0.095
0.033
0.043
0.035
0.045
0.096
0.106
0.376
0.386
0.205
0.215
7.0°
0.020
0.030
0.020
0.030
0.165
0.175
0.030
0.040
0.019
0.029
0.020
0.030
45°
0.057
0.067
0
0.004
MECHANICALS
Copyright  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
A
M I C R O S E M I
C O M P A N Y
LX8385x-xx
3A Low Dropout Positive Regulators
P RODUCTION D ATA S HEET
W W W . Microsemi . COM
NOTES
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  2000
Rev. 2.0a, 2001-03-15
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11