LT3782A
14
3782afc
For more information www.linear.com/LT3782A
3. The I
2
R losses in the sense resistor can be calculated
almost by inspection:
P
R(SENSE)
=
I
O(MAX)
2
1– D
MAX
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
2
•R •D
MAX
4. The losses in the inductor are simply the DC input cur-
rent squared times the winding resistance. Expressing
this loss as a function of the output current yields:
P
R(WINDING)
=
I
O(MAX)
2
1– D
MAX
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
2
•R
W
5. Losses in the boost diode. The power dissipation in the
boost diode is:
P
DIODE
=
O(MAX)
• V
D
The boost diode can be a major source of power loss
in a boost converter. For 13.2V input, 42V output at 3A,
a Schottky diode with a 0.4V forward voltage would
dissipate 600mW, which represents about 1% of the
input power. Diode losses can become significant at
low output voltages where the forward voltage is a
significant percentage of the output voltage.
6. Other losses, including C
IN
and C
O
ESR dissipation and
inductor core losses, generally account for less than
2% of the total losses.
PCB Layout Considerations
To achieve best performance from an LT3782A circuit,
the PC board layout must be carefully done. For lower
power applications, a two-layer PC board is sufficient.
However, at higher power levels, a multiplayer PC board
is recommended. Using a solid ground plane under the
circuit is the easiest way to ensure that switching noise
does not affect the operation.
In order to help dissipate the power from MOSFETs and
diodes, keep the ground plane on the layers closest to the
layers where power components are mounted. Use power
planes for MOSFETs and diodes in order to improve the
spreading of the heat from these components into the PCB.
For best
electrical performance, the LT3782A circuit should
be laid out as follows:
Place all power components in a tight area. This will
minimize the size of high current loops. Orient the input
and output capacitors and current sense resistors in a way
that minimizes the distance between the pads connected
to ground plane.
Place the LT3782A and associated components tightly
together and next to the section with power components.
Use a local via to ground plane for all pads that connect to
ground. Use multiple vias for power components.
Connect the current sense inputs of LT3782A directly
to the current sense resistor pads. Connect the current
sense traces on the opposite sides of pads from the traces
carrying the MOSFETs source currents to ground. This
technique is referred to as Kelvin sensing.
applicaTions inForMaTion
