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LTM4646

4646f

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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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