LT3504
17
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 8 shows the
recommended component placement with trace, ground
plane, and via locations.
Note that large, switched currents flow in the LT3504’s V
IN
,
SW and DA pins, the catch diodes (D1, D2, D3, D4) and
the input capacitors (C5, C6). The loop formed by these
components should be as small as possible and tied to
system ground in only one place. These components,
along with the inductors (L1, L2, L3, L4, L5) and output
capacitors (C1, C2, C3, C4, C7), should be placed on the
same side of the circuit board, and their connections
should be made on that layer. Place a local, unbroken
ground plane below these components, and tie this
ground plane to system ground at one location (ideally
at the ground terminal of the output capacitors). Ground
pins (Pins 10, 27) are provided near the V
IN
pins so that
the V
IN
pins can be bypassed to these ground pins. The
SW nodes should be kept as small as possible and kept
far away from the RT/SYNC and FB nodes. Keep the RT/
SYNC node and FB nodes small so that the ground pin
and ground traces will shield them from the SW nodes. If
the user plans on using a SYNC signal to set the oscillator
frequency then the RT/SYNC node should be kept away
from the FB nodes. Include vias near the exposed pad of
the LT3504 to help transfer heat from the LT3504 to the
ground plane. Keep the SW5 pad/trace as far away from
the FB pads as possible.
High Temperature Considerations
While the LT3504 is capable of delivering total output
current up to 4A, total power dissipation for an applica
-
tion circuit and the resulting temperature rise must be
considered, especially if all four channels are operating
at high duty cycle.
The
die
temperature of the LT3504 must be lower than the
maximum rating of 125°C. This is generally not a concern
unless the ambient temperature is above 85°C. For higher
temperatures, extra care should be taken in the layout of
the circuit to ensure good heat sinking of the LT3504. The
maximum load current should be derated as the ambient
temperature approaches 125°C. Programming the LT3504
to a lower switching frequency will improve efficiency and
reduce the dependence of efficiency on input voltage. The
die temperature is calculated by multiplying the LT3504
power dissipation by the thermal resistance from junc
-
tion to ambient. Power dissipation within the LT3504 can
be estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
losses. Thermal resistance depends on the layout of the
circuit board, but 43°C/W is typical for the QFN package.
Thermal shutdown will turn off the buck regulators and
the boost regulator when the die temperature exceeds
175°C, but this is not a warrant to allow operation at die
temperatures exceeding 125°C.
Outputs Greater Than 9V
For outputs greater than 9V, add a 1k resistor in series with
a 1nF capacitor across the inductor to damp the discon
-
tinuous ringing of the SW node, preventing unintended
SW current. An application with a 15V output (back page)
shows the location of this damping network.
Other Linear Technology Publications
Application Notes 19, 35, 44 contain more detailed descrip
-
tions and design information for step-down regulators and
other switching regulators. Design Note 318 shows how
to generate a bipolar output supply using a step-down
regulator
.
