LTM4630IV View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTM4630IV Datasheet PDF : 34 Pages
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LTM4630
APPLICATIONS INFORMATION
The junction temperatures are monitored while ambient
temperature is increased with and without airflow. The
power loss increase with ambient temperature change
is factored into the derating curves. The junctions are
maintained at ~120°C maximum while lowering output
current or power while increasing ambient temperature.
The decreased output current will decrease the internal
module loss as ambient temperature is increased.
The monitored junction temperature of 120°C minus
the ambient operating temperature specifies how much
module temperature rise can be allowed. As an example in
Figure 15, the load current is derated to ~25A at ~86°C with
no air or heat sink and the power loss for the 12V to 1.0V
at 25A output is a ~5.5W loss. The 5.5W loss is calculated
with the ~4.1W room temperature loss from the 12V to
1.0V power loss curve at 25A, and the 1.35 multiplying
factor at 125°C ambient. If the 86°C ambient temperature
is subtracted from the 120°C junction temperature, then
the difference of 34°C divided 5.5W equals a 6.2°C/W ΘJA
thermal resistance. Table 2 specifies a 7°C/W value which
is pretty close. The airflow graphs are more accurate due
to the fact that the ambient temperature environment is
controlled better with airflow. As an example in Figure 19,
the load current is derated to ~30A at ~72°C with 200LFM
of airflow and the power loss for the 12V to 1.5V at 30A
output is a ~7.9W loss. The 7.9W loss is calculated with
the ~5.9W room temperature loss from the 12V to 1.5V
power loss curve at 22A, and the 1.35 multiplying factor
at 125°C ambient. If the 72°C ambient temperature is
subtracted from the 120°C junction temperature, then
the difference of 48°C divided 7.9W equals a 6.0°C/W
θJA thermal resistance. Table 2 specifies a 6.0°C/W value
which is pretty close. Tables 2 and 3 provide equivalent
thermal resistances for 1.0V and 1.5V outputs with and
without airflow and heat sinking.
The derived thermal resistances in Tables 2 and 3 for the
various conditions can be multiplied by the calculated
power loss as a function of ambient temperature to derive
temperature rise above ambient, thus maximum junction
temperature. Room temperature power loss can be derived
from the efficiency curves and adjusted with the above
ambient temperature multiplicative factors. The printed
circuit board is a 1.6mm thick four layer board with two
ounce copper for the two outer layers and one ounce
copper for the two inner layers. The PCB dimensions are
101mm × 114mm. The BGA heat sinks are listed in Table 3.
Layout Checklist/Example
The high integration of LTM4630 makes the PCB board
layout very simple and easy. However, to optimize its electri-
cal and thermal performance, some layout considerations
are still necessary.
• Use large PCB copper areas for high current paths,
including VIN, GND, VOUT1 and VOUT2. It helps to mini-
mize the PCB conduction loss and thermal stress.
• Place high frequency ceramic input and output capaci-
tors next to the VIN, PGND and VOUT pins to minimize
high frequency noise.
• Place a dedicated power ground layer underneath the
unit.
• To minimize the via conduction loss and reduce module
thermal stress, use multiple vias for interconnection
between top layer and other power layers.
• Do not put via directly on the pad, unless they are
capped or plated over.
• Use a separated SGND ground copper area for com-
ponents connected to signal pins. Connect the SGND
to GND underneath the unit.
• For parallel modules, tie the VOUT, VFB, and COMP pins
together. Use an internal layer to closely connect these
pins together. The TRACK pin can be tied a common
capacitor for regulator soft-start.
• Bring out test points on the signal pins for monitoring.
Figure 12 gives a good example of the recommended
layout. LGA and BGA PCB layouts are identical with the
exception of circle pads for BGA (see Package Description).
4630fa
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For more information www.linear.com/LTM4630
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