Datasheet

10
LT1086 Series
sn1086 1086ffs
APPLICATIONS INFORMATION
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Load Regulation
Because the LT1086 is a 3-terminal device, it is not
possible to provide true remote load sensing. Load regu-
lation will be limited by the resistance of the wire connect-
ing 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 connec-
tion, with the bottom of the output divider returned to the
negative side of the load. Although it may not be immedi-
ately obvious, best load regulation is obtained when the
top of the resistor divider R1 is connected directly to the
case not to the load , as illustrated in Figure 2. If R1 were
connected to the load, the effective resistance between the
regulator and the load would be:
R
P
, R
P
= Parasitic Line Resistance
R2 + R1
R1
()
Thermal Considerations
The LT1086 series of regulators have internal power and
thermal limiting circuitry designed to protect the device
under overload conditions. For continuous normal load
conditions however, maximum junction temperature rat-
ings 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 resistance itself. New
thermal resistance specifications have been developed to
more accurately reflect device temperature and ensure
safe operating temperatures. The data section for these
new regulators provides a separate thermal resistance and
maximum junction temperature for both the Control Sec-
tion and the Power Transistor . Previous regulators, with a
single junction-to-case thermal resistance specification,
used an average of the two values provided here and
therefore could allow excessive junction temperatures
under certain conditions of ambient temperature and heat
sink resistance. To avoid this possibility, calculations
should be made for both sections to ensure that both
thermal limits are met.
For example, using a LT1086CK (TO-3, Commercial) and
assuming:
V
IN
(max continuous) = 9V, V
OUT
= 5V, I
OUT
= 1A,
T
A
= 75°C, θ
HEAT SINK
= 3°C/W,
θ
CASE-TO-HEAT SINK
= 0.2°C/W for T package with
thermal compound.
Power dissipation under these conditions is equal to:
P
D
= (V
IN
– V
OUT
)(I
OUT
) = 4W
Junction temperature will be equal to:
T
J
= T
A
+ P
D
(θ
HEAT SINK
+ θ
CASE-TO-HEAT SINK
+ θ
JC
)
For the Control Section:
T
J
= 75°C + 4W(3°C/W + 0.2°C/W + 1.5°C/W) = 94.6°C
95°C < 125°C = T
JMAX
(Control Section
Commercial Range)
For the Power Transistor:
T
J
= 75°C + 4W(3°C/W + 0.2°C/W + 4°C/W) = 103.8°C
103.8°C < 150°C = T
JMAX
(Power Transistor
Commercial Range)
LT1086 OUTINV
IN
ADJ
R
P
PARASITIC
LINE RESISTANCE
R1*
*CONNECT R1 TO CASE
CONNECT R2 TO LOAD
1086 • F02
R
L
R2*
Figure 2. Connections for Best Load Regulation
Connected as shown, R
P
is not multiplied by the divider
ratio. R
P
is about 0.004Ω per foot using 16-gauge wire.
This translates to 4mV/ft at 1A load current, so it is
important to keep the positive lead between regulator and
load as short as possible and use large wire or PC board
traces.
Note that the resistance of the package leads for the H
package ≈0.06Ω/inch. While it is usually not possible to
connect the load directly to the package, it is possible to
connect larger wire or PC traces close to the case to avoid
voltage drops that will degrade load regulation.
For fixed voltage devices the top of R1 is internally Kelvin
connected and the ground pin can be used for negative
side sensing.