Datasheet

LT3055
20
3055f
For more information www.linear.com/LT3055
Spreading the devices on the PC board also spreads the
heat. Series input resistors can further spread the heat if
the input-to-output differential is high.
Overload Recovery
Like many IC power regulators, the LT3055 has safe oper-
ating area protection. The safe area protection decreases
current limit as input-to-output voltage increases, and
keeps the power transistor inside a safe operating region
for all values of input-to-output voltage. The LT3055 pro-
vides some output current at all values of input-to-output
voltage up to the device breakdown.
When power is first applied, the input voltage rises and the
output follows the input; allowing the regulator to start-up
into very heavy loads. During start-up, as the input voltage
is rising, the input-to-output voltage differential is small,
allowing the regulator to supply large output currents.
With a high input voltage, a problem can occur wherein
the removal of an output short will not allow the output
to recover. Other regulators, such as the LT1083/LT1084/
LT1085 family and LT1764A also exhibit this phenomenon,
so it is not unique to the LT3055. The problem occurs with
a heavy output load when the input voltage is high and the
output voltage is low. Common situations are immediately
after the removal of a short circuit or if the shutdown pin is
pulled high after the input voltage is already turned on. The
load line intersects the output current curve at two points. If
this happens, there are two stable output operating points
for the regulator. With this double intersection, the input
power supply needs to be cycled down to zero and back
up again to recover the output.
Thermal Considerations
The LT3055’s maximum rated junction temperature of
125°C (E-, I-grades) or 150°C (MP-, H-grades) limits its
power handling capability. Two components comprise the
power dissipated by the device:
1. Output current multiplied by the input/output voltage
differential:
I
OUT
• (V
IN
– V
OUT
), and
2. GND pin current multiplied by the input voltage:
I
GND
• V
IN
GND pin current is determined using the GND Pin Current
curves in the Typical Performance Characteristics section.
Power dissipation equals the sum of the two components
listed above.
The LT3055 regulator has internal thermal limiting that
protects the device during overload conditions. For con-
tinuous normal conditions, do not exceed the maximum
junction temperature of 125°C (E-, I-grades) or 150°C
(MP-, H-grades). Carefully consider all sources of thermal
resistance from junction-to-ambient including other heat
sources mounted in proximity to the LT3055.
The undersides of the LT3055 DFN and MSE packages have
exposed metal from the lead frame to the die attachment.
These packages allow heat to directly transfer from the
die junction to the printed circuit board metal to control
maximum operating junction temperature. The dual-in-
line pin arrangement allows metal to extend beyond the
ends of the package on the topside (component side) of a
PCB. Connect this metal to GND on the PCB. The multiple
IN and OUT pins of the LT3055 also assist in spreading
heat to the PCB.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes also can spread the heat generated by
power devices.
Tables 3 and 4 list thermal resistance as a function of copper
area in a fixed board size. All measurements were taken
in still air on a 4-layer FR-4 board with 1oz solid internal
planes, and 2oz external trace planes with a total board
thickness of 1.6mm. For further information on thermal
resistance and using thermal information, refer to JEDEC
standard JESD51, notably JESD51-12.
APPLICATIONS INFORMATION
Table 3. MSOP Measured Thermal Resistance
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE BACKSIDE
2500 sq mm 2500 sq mm 2500 sq mm 35°C/W
1000 sq mm 2500 sq mm 2500 sq mm 36°C/W
225 sq mm 2500 sq mm 2500 sq mm 37°C/W
100 sq mm 2500 sq mm 2500 sq mm 39°C/W