LTM4634_15 Datasheet, PDF (17/32 Pages) Linear Technology – Triple Output 5A/5A/4A Step-Down DC/DC Module Regulator

LTM4634
Applications Information
ceramic capacitor to ground at EXTV_CC and ensure the
voltage at CNTL_PWR is always greater than the voltage
at EXTV_CC at all times during start-up and shutdown.
Connecting V_OUT3 to EXTV_CC may present a convenient
way to meet the sequencing requirement. Otherwise float
EXTV_CC if not used.
Thermal Considerations and Output Current Derating
The thermal resistances reported in the Pin Configuration
section of the data sheet are consistent with those param-
eters defined by JESD51-12 and are intended for use with
finite element analysis (FEA) software modeling tools that
leverage the outcome of thermal modeling, simulation,
and correlation to hardware evaluation performed on a
µModule package mounted to a hardware test board.
The motivation for providing these thermal coefficients is
found in JESD51-12 (“Guidelines for Reporting and Using
Electronic Package Thermal Information”).
Many designers may opt to use laboratory equipment
and a test vehicle such as the demo board to predict the
µModule regulator’s thermal performance in their appli-
cation at various electrical and environmental operating
conditions to compliment any FEA activities. Without FEA
software, the thermal resistances reported in the Pin Con-
figuration section are, in and of themselves, not relevant
to providing guidance of thermal performance; instead,
the derating curves provided in the data sheet can be used
in a manner that yields insight and guidance pertaining to
one’s application usage, and can be adapted to correlate
thermal performance to one’s own application.
The Pin Configuration section gives four thermal coeffi-
cients explicitly defined in JESD51-12. These coefficients
are quoted or paraphrased as follows:
1. θ_JA: The thermal resistance from junction to ambient, is
the natural convection junction-to-ambient air thermal
resistance measured in a one cubic foot sealed enclo-
sure. This environment is sometimes referred to as
“still air” although natural convection causes the air to
move. This value is determined with the part mounted
to a 95mm × 76mm PCB with four layers.
2. θ_JCbottom: The thermal resistance from the junction to
the bottom of the product case, is determined with all
of the internal power dissipation flowing through the
bottom of the package. In a typical µModule regulator,
the bulk of the heat flows out the bottom of the pack-
age, but there is always heat flow out into the ambient
environment. As a result, this thermal resistance value
may be useful for comparing packages but the test
conditions don’t generally match the user’s application.
3. θ_JCtop: The thermal resistance from junction to top of
the product case, is determined with nearly all of the
component power dissipation flowing through the top of
the package. As the electrical connections of the typical
µModule regulator are on the bottom of the package, it
is rare for an application to operate such that most of
the heat flows from the junction to the top of the part.
As in the case of θ_JCbottom, this value may be useful
for comparing packages but the test conditions don’t
generally match the user’s application.
4. θ_JB: The thermal resistance from junction to the printed
circuit board, is the junction-to-board thermal resistance
where almost all of the heat flows through the bottom
of the µModule package and into the board, and is really
the sum of the θ_JCbottom and the thermal resistance of
the bottom of the part through the solder joints and
through a portion of the board. The board temperature
is measured at a specified distance from the package.
A graphical representation of the aforementioned ther-
mal resistances is given in Figure 4; blue resistances are
contained within the μModule regulator, whereas green
resistances are external to the µModule package.
As a practical matter, it should be clear to the reader that
no individual or sub-group of the four thermal resistance
parameters defined by JESD51-12 or provided in the Pin
Configuration section replicates or conveys normal oper-
ating conditions of a μModule regulator. For example, in
normal board-mounted applications, never does 100%
of the device’s total power loss (heat) thermally con-
duct exclusively through the top or exclusively through
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