LT1084_15 Datasheet, PDF (13/22 Pages) Linear Technology – 3A, 5A, 7.5A Low Dropout Positive Fixed Regulators

LT1083/LT1084/LT1085
Applications Information
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.
Thermal Considerations
The LT1083 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 Section
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.
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly below 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 electri-
cally 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.
For example, using an LT1083CK (TO-3, Commercial)
and assuming:
V_IN (Max Continuous) = 9V, V_OUT = 5V, I_OUT = 6A,
T_A = 75°C, θ_{HEAT SINK} = 1°C/W,
θ_{CASE-TO-HEAT SINK} = 0.2°C/W for K package with
thermal compound.
Power dissipation under these conditions is equal to:
P_D = (V_IN – V_OUT )(I_OUT) = 24W
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 + 24W (1°C/W + 0.2°C/W + 0.6°C/W) = 118°C
118°C < 125°C = T_JMAX (Control Section
Commercial Range)
For the Power Transistor:
T_J = 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W) = 142°C
142°C < 150°C = T_JMAX (Power Transistor
Commercial Range)
In both cases the junction temperature is below the maxi-
mum rating for the respective sections, ensuring reliable
operation.
For more information www.linear.com/LT1083
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