LT3501
25
3501fd
Place additional vias near the catch diodes. Adding more
copper to the top and bottom layers and tying this copper
to the internal planes with vias can further reduce ther-
mal resistance. With these steps, the thermal resistance
from die (or junction) to ambient can be reduced to θ
JA
= 45°C/W.
The power dissipation in the other power components
such as catch diodes, boost diodes and inductors, cause
additional copper heating and can further increase what
the IC sees as ambient temperature. See the LT1767 data
sheet’s Thermal Considerations section.
Single, Low Ripple 6A Output
The LT3501 can generate a single, low ripple 6A output
if the outputs of the two switching regulators are tied
together and share a single output capacitor. By tying the
two FB pins together and the two V
C
pins together, the
two channels will share the load current. There are several
advantages to this 2-phase buck regulator. Ripple currents
at the input and output are reduced, reducing voltage ripple
and allowing the use of smaller, less expensive capacitors.
Although two inductors are required, each will be smaller
than the inductor required for a single-phase regulator. This
may be important when there are tight height restrictions
on the circuit.
There is one special consideration regarding the 2-phase
circuit. When the difference between the input voltage and
output voltage is less than 2.5V, then the boost circuits may
prevent the two channels from properly sharing current.
If, for example, channel 1 gets started first, it can supply
the load current, while channel 2 never switches enough
current to get its boost capacitor charged.
In this case, channel 1 will supply the load until it reaches
current limit, the output voltage drops, and channel 2 gets
started. Two solutions to this problem are shown in the
Typical Applications section.
The single 3.3V/6A output converter generates a boost sup-
ply from either SW that will service both switch pins.
The synchronized 3.3V/12A output converter utilizes un-
dervoltage lockout to prevent the start-up condition.
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
Note DN100 shows how to generate a dual (+ and –)
output supply using a buck regulator.
applicaTions inForMaTion
