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LTM4646 数据表(PDF) 23 Page - Linear Technology

部件名 LTM4646
功能描述  Dual 10A or Single 20A 關Module Regulator
Download  34 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTM4646 数据表(HTML) 23 Page - Linear Technology

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LTM4646
23
4646f
For more information www.linear.com/LTM4646
for JCtop and JCbottom, respectively. In practice, power
loss is thermally dissipated in both directions away from
the package—granted, in the absence of a heat sink and
airflow, a majority of the heat flow is into the board.
Within the LTM4646, be aware there are multiple power
devices and components dissipating power, with a con-
sequence that the thermal resistances relative to differ-
ent junctions of components or die are not exactly linear
with respect to total package power loss. To reconcile this
complication without sacrificing modeling simplicity—
but also, not ignoring practical realities—an approach
has been taken using FEA software modeling along with
laboratory testing in a controlled-environment chamber
to reasonably define and correlate the thermal resistance
values supplied in this data sheet: (1) Initially, FEA soft-
ware is used to accurately build the mechanical geometry
of the LTM4646 and the specified PCB with all of the cor-
rect material coefficients along with accurate power loss
source definitions; (2) this model simulates a software-
defined JEDEC environment consistent with JESD51-12
to predict power loss heat flow and temperature readings
at different interfaces that enable the calculation of the
JEDEC-defined thermal resistance values; (3) the model
and FEA software is used to evaluate the LTM4646 with
heat sink and airflow; (4) having solved for and analyzed
these thermal resistance values and simulated various
operating conditions in the software model, a thorough
laboratory evaluation replicates the simulated conditions
with thermocouples within a controlled-environment
chamber while operating the device at the same power
loss as that which was simulated. The outcome of this
process and due diligence yields a set of derating curves
provided in other sections of this data sheet. After these
laboratory tests have been performed, then the JB and
BA are summed together to correlate quite well with the
LTM4646 model with no airflow or heat sinking in a define
chamber. This JB + BA value should accurately equal
the JA value because approximately 100% of power loss
flows from the junction through the board into ambient
with no airflow or top mounted heat sink. Each system has
its own thermal characteristics, therefore thermal analysis
must be performed by the user in a particular system.
The LTM4646 has been designed to effectively remove
heat from both the top and bottom of the package. The
APPLICATIONS INFORMATION
bottom substrate material has very low thermal resistance
to the printed circuit board. An external heat sink can be
applied to the top of the device for excellent heat sinking
with airflow.
Safety Considerations
The LTM4646 modules do not provide isolation from VIN
to VOUT. There is no internal fuse. If required, a slow blow
fuse with a rating twice the maximum input current needs
to be provided to protect each unit from catastrophic
failure. The fuse or circuit breaker should be selected
to limit the current to the regulator during overvoltage
in case of an internal top MOSFET fault. If the internal
top MOSFET fails, then turning it off will not resolve the
overvoltage, thus the internal bottom MOSFET will turn
on indefinitely trying to protect the load. Under this fault
condition, the input voltage will source very large cur-
rents to ground through the failed internal top MOSFET
and enabled internal bottom MOSFET. This can cause
excessive heat and board damage depending on how
much power the input voltage can deliver to this system.
A fuse or circuit breaker can be used as a secondary fault
protector in this situation.
The device does support over current protection.
Temperature diodes are provided for monitoring inter-
nal temperature, and can be used to detect the need for
thermal shutdown that can be done by controlling the
RUN pin.
Power Derating
The 5V, 8V and 12V power loss curves in Figures 9
through 11 can be used in coordination with the load
current derating curves in Figures 12 to 16 for calculating
an approximate JA thermal resistance for the LTM4646
with airflow conditions. The power loss curves are taken
at room temperature, and are increased with a 1.35 to 1.4
multiplicative factor at 125°C. These factors come from
the fact that the power loss of the regulator increases
about 45% from 25°C to 150°C, thus a 50% spread


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