LT3060 查看數據表（PDF） - Linear Technology
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LT3060 Datasheet PDF : 20 Pages
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LT3060
APPLICATIONS INFORMATION
Overload Recovery
Like many IC power regulators, the LT3060 has safe
operating area protection. The safe operating 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 LT3060 provides some output current at all
values of input-to-output voltage up to the specified 45V
operational maximum.
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 LT3060. The problem occurs
with a heavy output load when the input voltage is high
and the output voltage is low. Common situations are: (1)
immediately after the removal of a short-circuit or (2) 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 creating two stable output operating
points for the regulator. With this double intersection, the
input power supply needs to be cycled down to zero and
brought up again for the output to recover.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C for
LT3060E, LT3060I, LT3060MP or 150°C for LT3060H).
Two components comprise the power dissipated by the
device:
1. Output current multiplied by the input/output voltage
differential: IOUT • (VIN– VOUT), and
2. GND pin current multiplied by the input voltage:
IGND • VIN
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 LT3060 regulator has internal thermal limiting that
protects the device during overload conditions. For
continuous normal conditions, the maximum junction
temperature of 125°C (E-grade, I-grade, MP-grade) or
150°C (H-grade) must not be exceeded. Carefully consider
all sources of thermal resistance from junction-to-ambient
including other heat sources mounted in proximity to the
LT3060.
The underside of the LT3060 DFN package has exposed
metal (1mm2) from the lead frame to the die attachment.
The package allows 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 LT3060 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.
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on a 4 layer FR-4 board with 1oz
solid internal planes and 2oz top/bottom external trace
planes with a total board thickness of 1.6mm. The four
layers were electrically isolated with no thermal vias
present. PCB layers, copper weight, board layout and
thermal vias will affect the resultant thermal resistance.
For more information on thermal resistance and high
thermal conductivity test boards, refer to JEDEC standard
JESD51, notably JESD51-12 and JESD51-7. Achieving
low thermal resistance necessitates attention to detail
and careful PCB layout.
3060f
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