Datasheet

LTM4637
16
4637fc
For more information www.linear.com/LTM4637
applicaTions inForMaTion
INTV
CC
Regulator
The LTM4637 has an internal low dropout regulator from
V
IN
called INTV
CC
. This regulator output has a 2.2µF
ceramic capacitor
internal. An additional 2.2µF ceramic
capacitor is needed on this pin to ground. This regulator
powers the internal controller and MOSFET drivers. The
gate driver current is ~20mA for 750kHz operation. The
regulator loss can be calculated as:
(V
IN
– 5V) • 20mA = P
LOSS
EXTV
CC
external voltage source ≥ 4.7V can be applied to
this pin to eliminate the internal INTV
CC
LDO power loss and
increase regulator efficiency. A 5V supply can be applied
to run the internal circuitry and power MOSFET driver. If
unused, leave pin floating. EXTV
CC
must be less than V
IN
at all times during power-on and power-off sequences.
Stability Compensation
The LTM4637 has already been internally compensated
for all output voltages. Table 5 is provided for most ap
-
plication requirements.
LTpowerCAD is available for other
control loop optimization.
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
in 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 this 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 below:
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 junction to the
bottom of the product case, is determined with all of
the component power dissipation flowing through the
bottom of the package. In the 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 a
portion of the board. The board temperature is measured
a specified distance from the package.