FAN1084 Fairchild Semiconductor, FAN1084 Datasheet - Page 5

• Manufacturer Part Number: FAN1084
• Description: 4.5A Adjustable/Fixed Low Dropout Linear Regulator
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
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.
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.
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
The connection shown in Figure 5 does not multiply R
divider ration. As an example, R
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.
REV. 1.0.6 11/18/02
V
IN
P
X (1 + R2/R1), R
+
Figure 3. Connection for Best Load Regulation
Figure 4. Connection for Best Load Regulation
V
IN
C1
10 F
I
35 A
IN
ADJ
IN
FAN1084-1.5
FAN1084
GND
P
ADJ
= Parasitic line Resistance
OUT
OUT
P
Line Resistance
is about four milliohms per
Parasitic
V
REF
R
P
R1
R2
+
R
L
P
C2
22 F
by the
V
OUT
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
• V
• I
• T
•
The power dissipation in this application is:
P
From the specification table:
T
The junction temperature is below the maximum thermal limit.
D
J
a thermally conductive material)
= T
= 60 + (9.6) * (5 + 3) = 137 C
OUT
= (V
A
Case-to-Ambient
IN
OUT
Figure 5. Connection for Best Load Regulation
= 60 C
A
= 3.4V worst case
V
IN
+ (P
= 4.5A continuous
IN
= 1.475V worst case
– V
*Connect R1 to case
D
Connect R2 to load
) * (
OUT
IN
FAN1084
) * (I
Case-to-Ambient
= 5 C/W (assuming both a heatsink and
ADJ
OUT
OUT
) = (3.6 – 1.475) * (4.5) = 9.6W
Line Resistance
Parasitic
R1*
R2*
+
R
P
JC
)
R
L
FAN1084
5