www.fairchildsemi.com
REV. 1.0.5 10/31/01
Features
Fast transient response
Low dropout voltage at up to 4.5A
Load regulation: 0.5% typical
On-chip thermal limiting
Standard TO-220, TO-263 center cut, and TO-252
packages
Applications
Desktop PCs, RISC and embedded processors' supply
GTL, SSTL logic Reference bus supply
Low voltage V
CC
logic supply
Battery-powered circuitry
Post regulator for switching supply
Cable and ADSL modems' DSP core supply
Set Top Boxes and Web Boxes modules' supply
Description
The FAN1084, FAN1084-1.5, and FAN1084-3.3 are low
dropout three-terminal regulators with 4.5A output current
capability. These devices have been optimized for low voltage
applications including V
TT
bus termination, where transient
response and minimum input voltage are critical. The
FAN1084 is ideal for low voltage microprocessor applica-
tions requiring a regulated output from 1.5V to 3.6A with an
input supply of 5V or less. The FAN1084-1.5 offers fixed
1.5V with 4.5A current capabilities for GTL+ bus V
TT
termination. The FAN1084-3.3 offers a fixed 3.3V output
at 4.5A.
On-chip thermal limiting provides protection against any
combination of overload and ambient temperature that
would create excessive junction temperatures.
The FAN1084 series regulators are available in the industry-
standard TO-220, TO-263 center cut, and TO-252 (DPAK)
power packages.
Application Diagrams
10
F
22
F
124
124
V
IN = 5V
2.5V at 4.5A
+
+
V
IN
ADJ
V
OUT
FAN1084
10
F
22
F
V
IN = 3.3V
1.5V at 4.5A
+
+
V
IN
GND
V
OUT
FAN1084-1.5
FAN1084
4.5A Adjustable/Fixed Low Dropout Linear Regulator
FAN1084
PRODUCT SPECIFICATION
2
REV. 1.0.5 10/31/01
Pin Assignments
Absolute Maximum Ratings
Parameter
Min.
Max.
Unit
V
IN
7
V
Operating Junction Temperature Range
0
125
C
Storage Temperature Range
65
150
C
Lead Temperature (Soldering, 10 seconds)
300
C
GND OUT
FRONT VIEW
3-Lead Plastic TO-220
JC
=3
C/W*
3-Lead Plastic TO-263
JC
=3
C/W*
*With package soldered to 0.5 square inch copper area over backside ground plane or internal power plane,
JA
can vary from
30
C/W to more than 40
C/W. Other mounting techniques may provide better power dissipation than 30
C/W.
FAN1084M-3.3 also available with uncut center lead.
FAN1084M-3.3
FAN1084M-1.5
IN
1
2
3
ADJ OUT
FRONT VIEW
IN
1
2
3
GND
FRONT VIEW
IN
1
2
3
FAN1084M
FAN1084T
FAN1084T-3.3
FAN1084T-1.5
ADJ
FRONT VIEW
IN
1
2
3
3-Lead Plastic TO-252
JC=3
C/W*
FAN1084D-3.3
FAN1084D-1.5
GND
FRONT VIEW
IN
1 2
3
FAN1084D
ADJ
FRONT VIEW
IN
1 2
3
Tab is out.
Tab is out.
PRODUCT SPECIFICATION
FAN1084
REV. 1.0.5 10/31/01
3
Electrical Characteristics
Operating Conditions: 4.75
V
IN
< 5.25V, T
j
= 25C unless otherwise specified.
Notes:
1. See thermal regulation specifications for changes in output voltage due to heating effects. Load and line regulation are
measured at a constant junction temperature by low duty cycle pulse testing.
2. Line and load regulation are guaranteed up to the maximum power dissipation. Power dissipation is determined by input/
output differential and the output currrent. Guaranteed maximum output power will not be available over the full input/output
voltage range.
3. FAN1084 only.
4. Guaranteed by design.
5. FAN1084-1.5 only.
6. FAN1084-3.3 only.
Parameter
Conditions
Min.
Typ.
Max
Units
Reference Voltage
3
Adj connected to ground, I
OUT
= 10mA
1.23
1.250
1.27
V
Output Voltage
5
I
OUT
= 10mA
1.475
1.5
1.525
V
Output Voltage
6
I
OUT
= 10mA
3.234
3.3
3.366
V
Line Regulation
1, 2
I
OUT
= 10mA
0.5
2
%
Load Regulation
1, 2
10mA
I
OUT
4.5A
0.5
2.5
%
Dropout Voltage
V
REF%
= 2%, I
OUT
= 4.5A
1.5
V
Current Limit
(V
IN
V
OUT
) = 2V
5.5
A
Adjust Pin Current
3
35
100
Mimimum Load Current
4
1.5V
(V
IN
V
OUT
)
5.75V
10
mA
Quiescent Current
4
V
IN
= 5V
4
mA
Thermal Resistance,
Junction to Case
TO-220
3
C/W
TO-263 Center Cut, TO-252
3
C/W
Thermal Shutdown
4
150
C
Typical Performance Characteristics
Figure 1. Maximum Power Dissipation
20
15
10
5
0
25 45 65 85 105 125
POWER (W)
CASE TEMPERATURE
FAN1084
PRODUCT SPECIFICATION
4
REV. 1.0.5 10/31/01
Applications Information
General
The FAN1084, FAN1084-1.5, and FAN1084-3.3 are three-
terminal regulators optimized for GTL+ V
TT
termination
and logic applications. These devices are short-circuit pro-
tected, and offer thermal shutdown to turn off the regulator
when the junction temperature exceeds about 150C. The
FAN1084 series provides low dropout voltage and fast
transient response. Frequency compensation uses capacitors
with low ESR while still maintaining stability. This is critical
in addressing the needs of low voltage high speed micro-
processor buses like GTL+.
Stability
The FAN1084 series requires an output capacitor as a part of
the frequency compensation. It is recommended to use a 22F
solid tantalum or a 100F aluminum electrolytic on the output
to ensure stability. The frequency compensation of these devices
optimizes the frequency response with low ESR capacitors.
In general, it is suggested to use capacitors with an ESR of
<0.2
. It is also recommended to use bypass capacitors such
as a 22F tantalum or a 100F aluminum on the adjust pin of
the FAN1084 for low ripple and fast transient response.
When these bypassing capacitors are not used at the adjust pin,
smaller values of output capacitors provide equally good
results.
Protection Diodes
In normal operation, the FAN1084 series does not require
any protection diodes. For the FAN1084, internal resistors
limit internal current paths on the adjust pin. Therefore, even
with bypass capacitors on the adjust pin, no protection diode
is needed to ensure device safety under short-circuit condi-
tions.
A protection diode between the input and output pins is
usually not needed. An internal diode between the input and
the output pins on the FAN1084 series can handle micro-
second surge currents of 50A to 100A. Even with large value
output capacitors it is difficult to obtain those values of surge
currents in normal operation. Only with large values of
output capacitance, such as 1000
F to 5000
F, and with the
input pin instantaneously shorted to ground can damage
occur. A crowbar circuit at the input can generate those
levels of current; a diode from output to input is then recom-
mended, as shown in Figure 2. Usually, normal power supply
cycling or system "hot plugging and unplugging" will not
generate current large enough to do any damage.
The adjust pin can be driven on a transient basis 7V with
respect to the ouput, without any device degradation. As with
any IC regulator, exceeding the maximum input-to-output
voltage differential causes the internal transistors to break
down and none of the protection circuitry is then functional.
Figure 2. Optional Protection
Ripple Rejection
In applications that require improved ripple rejection, a bypass
capacitor from the adjust pin of the FAN1084 to ground
reduces the output ripple by the ratio of V
OUT
/1.25V. The
impedance of the adjust pin capacitor at the ripple frequency
should be less than the value of R1 (typically in the range of
100
to 120
) in the feedback divider network in Figure 2.
Therefore, the value of the required adjust pin capacitor is a
function of the input ripple frequency. For example, if R1
equals 100
and the ripple frequency equals 120Hz, the
adjust pin capacitor should be 22F. At 10kHz, only 0.22F is
needed.
Output Voltage
The FAN1084 regulator develops a 1.25V reference voltage
between the ouput pin and the adjust pin (see Figure 3). Placing
a resistor R1 between these two terminals causes a constant
current 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.
FAN1084
ADJ
C2
22
F
V
OUT
+
C1
10
F
+
+
IN
OUT
D1
1N4002
(OPTIONAL)
V
IN
FAN1084-1.5
GND
C2
22
F
V
OUT
+
C1
10
F
+
IN
OUT
D1
1N4002
(OPTIONAL)
V
IN
R1
R2
C
ADJ
PRODUCT SPECIFICATION
FAN1084
REV. 1.0.5 10/31/01
5
The current out of the adjust pin adds to the current from R1.
Its output voltage contribution is small and only needs consid-
eration when a very precise output voltage setting is required.
Figure 3. Connection for Best Load Regulation
Load Regulation
It is not possible to provide true remote load sensing because
the FAN1084 series are three-terminal devices. Load regula-
tion is limited by the resistance of the wire connecting the reg-
ulator to the load. Load regulation per the data sheet
specification is measured at the bottom of the package.
For fixed voltage devices, negative side sensing is a true
Kelvin connection with the ground pin of the device returned
to the negative side of the load. This is illustrated in Figure 4.
Figure 4. Connection for Best Load Regulation
For adjustable voltage devices, negative side sensing is a true
Kelvin connection with the bottom of the output divider
returned to the negative side of the load. The best load regula-
tion is obtained when the top of the resistor divider R1 connects
directly to the regulator output and not to the load. Figure 5
illustrates this point.
If R1 connects to the load, then the effective resistance
between the regulator and the load would be:
R
P
X (1 + R2/R1), R
P
= Parasitic line Resistance
The connection shown in Figure 5 does not multiply R
P
by the
divider ration. As an example, R
P
is about four milliohms per
foot with 16-gauge wire. This translates to 4mV per foot at 1A
load current. At higher load currents, this drop represents a
significant percentage of the overall regulation. It is important
to keep the positive lead between the regulator and the load as
short as possible and to use large wire or PC board traces.
Figure 5. Connection for Best Load Regulation
Thermal Conditions
The FAN1084 series protect themselves under overload con-
ditions with internal power and thermal limiting circuitry.
However, for normal continuous load conditions, do not
exceed maximum junction temperature ratings. It is impor-
tant to consider all sources of thermal resistance from junc-
tion-to-ambient. These sources include the junction-to-case
resistance, the case-to-heatsink interface resistance, and the
heat sink resistance. Thermal resistance specifications have
been developed to more accurately reflect device tempera-
ture and ensure safe operating temperatures. The electrical
characteristics section provides a separate thermal resistance
and maximum junction temperature for both the control cir-
cuitry and the power transistor. Calculate the maximum junc-
tion temperature for both sections to ensure that both thermal
limits are met.
For example, look at using a FAN1084T to generate 4.5A @
1.5V from a 3.3V source (3.2V to 3.6V).
Assumptions
V
IN
= 3.4V worst case
V
OUT
= 1.475V worst case
I
OUT
= 4.5A continuous
T
A
= 60
C
Case-to-Ambient
= 5
C/W (assuming both a heatsink and
a thermally conductive material)
The power dissipation in this application is:
P
D
= (V
IN
V
OUT
) * (I
OUT
) = (3.6 1.475) * (4.5) = 9.6W
From the specification table:
T
J
= T
A
+ (P
D
) * (
Case-to-Ambient
+
JC
)
= 60 + (9.6) * (5 + 3) = 137
C
The junction temperature is below the maximum thermal limit.
FAN1084
ADJ
C2
22
F
V
OUT
+
C1
10
F
I
ADJ
35
A
+
IN
OUT
V
IN
R1
R2
V
REF
FAN1084-1.5
GND
R
L
R
P
Parasitic
Line Resistance
IN
OUT
V
IN
FAN1084
ADJ
R
L
R1*
R2*
*Connect R1 to case
Connect R2 to load
R
P
Parasitic
Line Resistance
IN
OUT
V
IN
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