LT3509 LINER [Linear Technology], LT3509 Datasheet - Page 19

• Manufacturer Part Number: LT3509
• Description: Dual 36V, 700mA Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Hot Plugging Considerations
The small size, reliability and low impedance of ceramic
capacitors make them attractive for the input capacitor.
Unfortunately they can be hazardous to semiconductor
devices if combined with an inductive supply loop and a
fast power transition such as through a mechanical switch
or connector. The low-loss ceramic capacitor combined
with the just a small amount of wiring inductance forms
an underdamped resonant tank circuit and the voltage at
the V
input voltage. See Linear Technology Application Note 88
for more details.
PCB Layout and Thermal Design
The PCB layout is critical to both the electrical and thermal
performance of the LT3509. Most important is the connec-
tion to the exposed pad which provides the main ground
connection and also a thermal path for cooling the chip.
This must be soldered to a topside copper plane which
is also tied to backside and/or internal plane(s) with an
array of thermal vias.
To obtain the best electrical performance particular at-
tention should be paid to keeping the following current
paths short:
• The loop from the V
• The loops from the switch pins to the catch diodes
tor back to the ground pad and plane. This sees high
di/dt transitions as the power switches turn on an
off. Excess impedance will degrade the minimum us-
able input voltage and could cause crosstalk between
channels.
and back to the DA pins. The fast changing currents
and voltage here combined with long PCB traces will
cause ringing on the switch pin and may result in
unwelcome EMI.
IN
pin of the LT3509 can ring to twice the nominal
IN
pin through the input capaci-
• The loop from the regulated outputs through the
The area of the SW and BOOST nodes should as small as
possible. Also the feedback components should be placed
as close as possible to the FB pins so that the traces are
short and shielded from the SW and BOOST nodes by the
ground planes.
Figure 13 shows a detail view of a practical board layout
showing just the top layer. The complete board is somewhat
larger at 7.5cm x 7.5cm. The device has been evaluated
on this board in still air running at 700kHz switching fre-
quency. One channel was set to 5V and the other to 3.3V
and both channels were fully loaded to 700mA. The device
temperature reached approximately 15˚C above ambient
for input voltages below 12V. At 24V input it was slightly
higher at 17˚ above ambient.
output capacitor back to the ground plane. Excess
impedance here will result in excessive ripple at the
output.
Figure 13. Sample PCB Layout (Top Layer Only)
LT3509
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3509f