Datasheet LX8385B-33CP, LX8385B-05CDD, LX8385B-00CP, LX8385B-00CDD, LX8385A-33CP Datasheet (Microsemi Corporation)
...Page 1
DESCRIPTION KEY FEATURES
■■
■■
■ Three-Terminal Adjustable Or Fixed
Output Voltage
■■
■■
■ 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)
■■
■■
■ Output Current Of 3A Minimum
p 0.015% Line Regulation
p 0.1% Load Regulation
The LX8385/8385A/8385B series ICs are
positive regulators designed to provide
3A output current. Each regulator yields
higher efficiency than currently available
devices with all internal circuitry designed
to operate down to a 1V input-to-outputdifferential. In these products, the dropout voltage is fully specified as a function
of load current. The LX8385's dropout is
guaranteed at a maximum of 1.5V at maximum output current, decreasing at lower
load currents. The LX8385A/85B have a
maximum dropout voltage of 1.3V.
The LX8385B offers a tighter voltage
reference tolerance: ±1.0% over line, load
and temperature, with 0.8% initial accuracy. The LX8385/85A have 1.0% initial
accuracy, with 2% over line, load and temperature.
The LX8385/85A/85B series devices are
pin-compatible with earlier 3-terminal
regulators, such as 117 series products.
While a 10µF output capacitor is required
on both input and output of these new
devices, this is generally included in most
regulator designs.
The quiescent current of the LX8385/
85A/85B series products flows into the
load, increasing efficiency. This feature
contrasts with PNP regulator designs,
where up to 10% of the output current is
wasted as quiescent current.
The LX8385-xxI is rated for -25°C to
+125°C applications, and the LX8385-xxC
/8385B-xxC is rated for 0°C to +125°C
applications.
PRODUCT HIGHLIGHT
3.3V, 3 A REGULATOR
APPLICATIONS
■ 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
3A LOW D
ROPOUT POSITIVE REGULATORS
P RODUCTION DATA SHEET
T HE I NFINITE P OWER OF I NNOVATION
LX8385-xx/8385A-xx/8385B-xx
LIN DOC #:
8385
LX8385
200
Ω
1%
1%
3.3V at 3A
5V
OUT
IN
ADJ
1500µF, 6.3V
6MV1500GX
2x 1500µF, 6.3V
6MV1500GX
from Sanyo
121Ω
Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number
(i.e. LX8385-00CDDT). "xx" refers to output voltage, please see table above.
* Consult factory for availability of TO-3 Metal Can.
LX8385/85A/85B-00 Adjustable
LX8385/85A/85B-33 3.3V
LX8385/85A/85B-05 5.0V
Part #
Output
Voltage
AVAILABLE OPTIONS PER PART #
Other voltage options may be available —
Please contact factory for details.
PACKAGE ORDER INFORMATION*
T
A
(°C)
P
1.5VLX8385-xxCPLX8385-xxCD
0 to 1251.3VLX8385A-xxCPLX8385A-xxCD
1.3VLX8385B-xxCPLX8385B-xxCD
-25 to 1251.5VLX8385-xxIPLX8385-xxID
Dropout
Voltage
Plastic TO-220
3-pin
Plastic TO-263
3-pin
DD
Copyright © 1999
Rev. 2.0a 10/00
1
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841, 714-898-8121, FAX: 714-893-2570
L INF INITY MICROELECTRONICS INC.
NOTE: For current data & package dimensions, visit our web site: http://www.linfinity.com.
Page 2
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 2.0a 10/00
2
P
RODUCTION DATA SHEET
ABSOLUTE MAXIMUM RATINGS (Note 1)
Power Dissipation .................................................................................. Internally Limited
Input Voltage................................................................................................................ 10V
Input to Output Voltage Differential ........................................................................... 10V
Operating Junction Temperature
Plastic (P, DD & DL Packages)............................................................................. 150°C
Storage Temperature Range ...................................................................... -65°C to 150°C
Lead Temperature (Soldering, 10 seconds) ............................................................. 300°C
PACKAGE PIN OUTS
Thermal
Limit Circuit
Control
Circuit
Bandgap
Circuit
Output
Circuit
Bias
Circuit
V
OUT
V
IN
SOA Protection
Circuit
Current
Limit Circuit
ADJ or
GND*
BLOCK DIAGRAM
P PACKAGE:
THERMAL RESISTANCE-JUNCTION TO TAB,
θθ
θθ
θ
JT
3.0°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θθ
θ
JA
60°C/W
DD PACKAGE:
THERMAL RESISTANCE-JUNCTION TO TAB,
θθ
θθ
θ
JT
3.0°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θθ
θθ
θ
JA
*60°C/W
Junction Temperature Calculation: TJ = TA + (P
D
x θ
JA
).
The θ
JA
numbers are guidelines for the thermal performance of the device/pc-board system.
All of the above assume no ambient airflow.
* θ
JA
can be improved with package soldered to 0.5IN2 copper area over backside ground
plane or internal power plane. θ
JA
can vary from 20ºC/W to > 40ºC/W depending on
mounting technique.
THERMAL DATA
3
2
1
P PACKAGE
(Top View)
* Pin 1 is GND for fixed voltage versions.
3
2
1
DD PACKAGE
(Top View)
* Pin 1 is GND for fixed voltage versions.
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.
TAB IS V
OUT
TAB IS V
OUT
* This pin GND for fixed voltage versions.
V
IN
V
OUT
ADJ / GND*
V
IN
V
OUT
ADJ / GND*
Page 3
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
3
Copyright © 1999
Rev. 2.0a 10/00
P RODUCTION DATA SHEET
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for the LX8385-xxC/85A/85B-xxC with 0°C ≤ TA ≤
125°C and the LX8385-xxI with -25°C ≤ TA ≤ 125°C; V
IN
- V
OUT
= 3V; I
OUT
= 3A. Low duty cycle pulse testing techniques are used which maintains
junction and case temperatures equal to the ambient temperature.)
Parameter
Symbol
Test Conditions Units
LX8385/85A/85B-00
Min. Typ. Max.
Reference Voltage LX8385-00 V
REFIOUT
= 10mA, TA = 25°C
(Note 4) 10mA ≤ I
OUT
≤ I
OUT (MAX)
, 1.5V ≤ (V
IN
- V
OUT
), V
IN
≤ 10V, P ≤ P
MAX
LX8385A/85B-00
I
OUT
= 10mA, TA = 25°C
10mA ≤ I
OUT
≤ I
OUT (MAX)
, 1.5V ≤ (V
IN
- V
OUT
), V
IN
≤ 10V, P ≤ P
MAX
Line Regulation (Note 2)
∆V
REF
(VIN)
1.5V ≤ (V
IN
- V
OUT
) ≤ 7V, I
OUT
= 10mA
Load Regulation (Note 2)
∆V
REF
(I
OUT
)
VIN - V
OUT
= 3V, 10mA ≤ I
OUT
≤ 3A
Thermal Regulation
∆V
OUT
(Pwr)
TA = 25°C, 20ms pulse
Ripple Rejection (Note 3) V
OUT
= 5V, f =120Hz, C
OUT
= 100µf Tantalum, VIN = 6.5V
C
ADJ
= 10µF, I
OUT
= 3A
Adjust Pin Current I
ADJ
Adjust Pin Current Change (Note 4) ∆ I
ADJ
10mA ≤ I
OUT
≤ I
OUT (MAX)
, 1.5V ≤ (V
IN
- V
OUT
), V
IN
≤ 10V
Dropout Voltage LX8385-00 ∆ V ∆V
REF
= 1%, I
OUT
= 3A
LX8385A/85B-00
∆V
REF
= 1%, I
OUT
= 3A
Minimum Load Current I
OUT (MIN)VIN
≤ 10V
Maximum Output Current I
OUT (MAX)(VIN
- V
OUT
) ≤ 7V, VIN ≤ 10V
Temperature Stability (Note 3) ∆V
OUT
(T)
Long Term Stability (Note 3) ∆V
OUT
(t) TA = 125°C, 1000 hours
RMS Output Noise (% of V
OUT
) (Note 3) V
OUT (RMS)TA
= 25°C, 10Hz ≤ f ≤ 10kHz
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
0.015 0.2 %
0.2 0.5 %
0.01 0.04 %/W
65 83 dB
55 100 µA
0.2 5 µA
1.2 1.5 V
1.1 1.3 V
210mA
3 3.5 A
0.25 %
0.3 1 %
0.003 %
Parameter
Symbol
Test Conditions Units
LX8385/85A/85B-33
Min. Typ. Max.
Output Voltage LX8385-33 V
OUTVIN
= 5V, I
OUT
= 0mA, TA = 25°C
(Note 4) 4.75V ≤ VIN ≤ 10V, 0mA ≤ I
OUT
≤ 3A, P ≤ P
MAX
LX8385A/85B-33
VIN = 5V, I
OUT
= 0mA, TA = 25°C
4.75V ≤ VIN ≤ 10V, 0mA ≤ I
OUT
≤ 3A, P ≤ P
MAX
Line Regulation (Note 2) ∆V
OUT
4.75V ≤ VIN ≤ 7V
(VIN)
4.75V ≤ VIN ≤ 10V
Load Regulation (Note 2)
∆V
OUT(IOUT
)
V
IN
= 5V, 0mA ≤ I
OUT
≤ I
OUT (MAX)
Thermal Regulation
∆V
OUT
(Pwr)
TA = 25°C, 20ms pulse
Ripple Rejection (Note 3) C
OUT
= 100µF (Tantalum), I
OUT
= 3A
Quiescent Current I
Q
0mA ≤ I
OUT
≤ I
OUT (MAX)
, 4.75V ≤ VIN ≤ 10V
Dropout Voltage LX8385-33 ∆V ∆V
OUT
= 1%, I
OUT
≤ I
OUT (MAX)
LX8385A/85B-33
∆V
OUT
= 1%, I
OUT
≤ I
OUT (MAX)
Maximum Output Current I
OUT (MAX)VIN
≤ 7V
Temperature Stability (Note 3) ∆V
OUT
(T)
Long Term Stability (Note 3) ∆V
OUT
(t) TA = 125°C, 1000 hours
RMS Output Noise (% of V
OUT
) (Note 3) V
OUT (RMS)TA
= 25°C, 10Hz ≤ f ≤ 10kHz
3.267 3.3 3.333 V
3.235 3.3 3.365 V
3.274 3.3 3.326 V
3.267 3.3 3.333 V
16mV
210mV
515mV
0.01 0.02 % / W
60 83 dB
410mA
1.2 1.5 V
1.1 1.3 V
3 3.5 A
0.25 %
0.3 1 %
0.003 %
Note 2. 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.
Note 3. These parameters, although guaranteed, are not tested in production.
Note 4. See Maximum Output Current Section above.
LX8385-00/85A/85B-00 (Adjustable)
LX8385-33/85A-33/85B-33 (3.3V Fixed)
Page 4
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 2.0a 10/00
4
P
RODUCTION DATA SHEET
ELECTRICAL CHARACTERISTICS
Parameter
Symbol
Test Conditions Units
LX8385/85A/85B-05
Min. Typ. Max.
Output Voltage LX8385-05 V
OUTVIN
= 7V, I
OUT
= 0mA, TA = 25°C
(Note 4) 7V ≤ VIN ≤ 10V, 0mA ≤ I
OUT
≤ 3A, P ≤ P
MAX
LX8385A/85B-05
VIN = 7V, I
OUT
= 0mA, TA = 25°C
7V ≤ VIN ≤ 10V, 0mA ≤ I
OUT
≤ 3A, P ≤ P
MAX
Line Regulation (Note 2)
∆V
OUT (VIN
)
4.75V ≤ VIN ≤ 10V
Load Regulation (Note 2)
∆V
OUT(IOUT
)
V
IN
= 7V, 0mA ≤ I
OUT
≤ I
OUT (MAX)
Thermal Regulation
∆V
OUT
(Pwr)
TA = 25°C, 20ms pulse
Ripple Rejection (Note 3) C
OUT
= 100µF (Tantalum), I
OUT
= 3A
Quiescent Current I
Q
0mA ≤ I
OUT
≤ I
OUT (MAX)
, 7V ≤ VIN ≤ 10V
Dropout Voltage LX8385-
05
∆V ∆V
OUT
= 1%, I
OUT
≤ I
OUT (MAX)
LX8385A/85B-05
∆V
OUT
= 1%, I
OUT
≤ I
OUT (MAX)
Maximum Output Current I
OUT (MAX)VIN
≤ 10V
Temperature Stability (Note 3) ∆V
OUT
(T)
Long Term Stability (Note 3) ∆V
OUT
(t) TA = 125°C, 1000 hours
RMS Output Noise (% of V
OUT
) (Note 3) V
OUT (RMS)TA
= 25°C, 10Hz ≤ f ≤ 10kHz
4.950 5.00 5.050 V
4.900 5.00 5.100 V
4.960 5.00 5.040 V
4.950 5.00 5.050 V
210mV
525mV
0.01 0.02 % / W
60 83 dB
410mA
1.2 1.5 V
1.1 1.3 V
3 3.5 A
0.25 %
0.3 1 %
0.003 %
Note 2. 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.
Note 3. These parameters, although guaranteed, are not tested in production.
Note 4. See Maximum Output Current Section above.
LX8385-05/85A-05/85B-05 (5.0V Fixed)
Page 5
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
5
Copyright © 1999
Rev. 2.0a 10/00
P RODUCTION DATA SHEET
APPLICATION NOTES
The LX8385/85A/85B series ICs are easy to use Low-Dropout (LDO)
voltage regulators. They have all of the standard self-protection
features expected of voltage regulators: 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
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.
In order to ensure good transient response from the power supply
INPUT OUTPUT ADJ
10µF 15µF Tantalum, 100µF Aluminum None
10µF 47µF Tantalum, 220µF Aluminum 15µF
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.
RECOMMENDED CAPACITOR VALUES
FIGURE 1 — Dynamic Input And Output Test
LX8385
/85A/85B
Power Supply
OUT
IN
ADJ
Star Ground
1 sec
10ms
R
DSON
<< R
L
Full Load
(Smaller resistor)
Minumum Load
(Larger resistor)
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-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 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-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.
Page 6
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 2.0a 10/00
6
P
RODUCTION DATA SHEET
APPLICATION NOTES
FIGURE 2 — Basic Adjustable Regulator
R2+R1
R1
LX8385/85A/85B
OUT
IN
ADJ
V
OUT
V
IN
R1
R2
V
REF
I
ADJ
50µA
V
OUT
= V
REF
1 + + I
ADJ
R2
R2
R1
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.
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 = 1 / (6.28 * F
R
* R1)
where: C ≡ the value of the capacitor in Farads;
select an equal or larger standard value.
F
R
≡ the ripple frequency in Hz
R1 ≡ the value of resistor R1 in ohms
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 = V
OUT/VREF
where: M ≡ a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
V
REF
= 1.25V.
For example, if V
OUT
= 2.5V the output ripple will be:
M = 2.5V/1.25V= 2
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 I
ADJ
is very small and constant when compared with the current
through R1, it represents a small error and can usually be ignored.
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
Peff
= RP
*
where: R
P
≡ 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 R
Peff
.
LX8385/85A/85B
OUT
IN
ADJ
V
IN
R1
R2
R
L
R
P
Parasitic
Line Resistance
Connect
R1 to Case
of Regulator
Connect
R2
to Load
FIGURE 3 — Connections For Best Load Regulation
Page 7
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
7
Copyright © 1999
Rev. 2.0a 10/00
P RODUCTION DATA SHEET
APPLICATION NOTES
Example
Given: V
IN
= 5V
V
OUT
= 2.5V, I
OUT
= 1.5A
Ambient Temp., T
A
= 50°C
R
θJT
= 2.7°C/W for TO-220
Find: Proper Heat Sink to keep IC's junction
temperature below 125°C.**
Solution: The junction temperature is:
T
J
= PD (R
θJT
+ R
θCS
+ R
θSA
) + T
A
where: P
D
≡ Dissipated power.
R
θJT
≡ Thermal resistance from the junction to the
mounting tab of the package.
R
θCS
≡ 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.)
R
θSA
≡
Thermal resistance from the mounting surface
to ambient (thermal resistance of the heat sink).
T
S
≡ Heat sink temperature.
First, find the maximum allowable thermal resistance of the
heat sink:
R
θSA
= - (R
θJT
+ R
θCS
)
P
D
=(V
IN(MAX)
- V
OUT
) I
OUT
= (5.0V-2.5V) * 1.5A
= 3.75W
R
θSA
= - (2.7°C/W + 1.0°C/W)
= 16.3°C/W
Next, select a suitable heat sink. The selected heat sink must have
R
θSA
≤ 16.3°C/W. Thermalloy heatsink 6230B has R
θSA
= 12.0°C/W.
Finally, verify that junction temperature remains within speci-
fication using the selected heat sink:
T
J
= 3.75W (2.7°C/W + 1.0°C/W + 12.0°C/W) + 50°C = 109°C
T
J
T
C
T
S
T
A
R
θ
JT
R
θ
CS
R
θ
SA
TJ - T
A
P
D
125°C - 50°C
3.75W
**
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.
LOAD REGULATION (continued)
Even when the circuit is optimally configured, 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 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 V
IN (MIN)
during transients.
V
IN (MIN)
= V
OUT
+ V
DROPOUT (MAX)
where: V
IN (MIN)
≡ the lowest allowable instantaneous
voltage at the input pin.
V
OUT
≡ the designed output voltage for the
power supply system.
V
DROPOUT (MAX)
≡ the specified dropout voltage
for the installed regulator.
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-toheat-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.
Page 8
3A LOW D
ROPOUT POSITIVE REGULATORS
LX8385-xx/8385A-xx/8385B-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 2.0a 10/00
8
P
RODUCTION DATA SHEET
TYPICAL APPLICATIONS
LX8385/85A/85B
OUT
IN
ADJ
150µF
V
IN
R2
365Ω
1%
10µF
R1
121
Ω
1%
C1
10µF*
* C1 improves ripple rejection.
X
C
should be ≈ R1 at ripple
frequency.
V
OUT
5V
(Note A)
FIGURE 5 — 1.2V - 8V Adjustable RegulatorFIGURE 4 — Improving Ripple Rejection
LX8385/85A/85B
OUT
IN
ADJ
V
OUT
**
V
IN
R1
121Ω
R2
1k
C1*
10µF
* Needed if device is far from filter capacitors.
** V
OUT
= 1.25V 1 +
C2
100µF
R2
R1
(Note A)
FIGURE 6 — 5V Regulator With Shutdown
(Note A)
LX8385/85A/85B
OUT
IN
ADJ
5V
V
IN
121Ω
1%
10µF
100µF
365
Ω
1%
1k
1k
2N3904
TTL
O
utput
Note A: V
IN (MIN)
= (Intended V
OUT
) + (V
DROPOUT (MAX)
)
FIGURE 7 — Fixed 3.3V Output Regulator
LX8385/85A/85B-33
OUT
IN
GND
3.3V
V
IN
10µF Tantalum
or 100µF Aluminum
Min. 15µF Tantalum or
100µF Aluminum capacitor.
May be increased without
limit. ESR must be less
than 50mΩ.
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.
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