LTC3705_15 Datasheet, PDF (14/20 Pages) Linear Technology – 2-Switch Forward Controller and Gate Driver

LTC3705
APPLICATIO S I FOR ATIO
For the LTC3706, the master performs soft-start and
voltage-loop regulation by driving all slaves to the same
current as the master using the I_TH pins. Faults and
shutdowns are communicated via the interconnection of
the RUN/SS pins. The LTC3706 is put into slave mode by
tying the FB pin to V_CC.
Standalone Primary-Side Operation
The LTC3705 can be used to implement a standalone
forward converter using optoisolator feedback and a
secondary-side voltage reference. Alternately the LTC3705
can be used to implement an open-loop forward converter
using the VSLMT pin to regulate against changes in V_IN. In
either case, the LTC3705 oscillator determines the fre-
quency as found from:
f
OSC
=
21• 10⁹
R_FS(P) + 4200
Note that polyphase operation is not possible in the stand-
alone configuration.
Grounding Considerations
The LT3705 is typically used in high current converter
designs that involve substantial switching transients. Fig-
ure 5 illustrates these currents. The switch drivers on the
IC are designed to drive large capacitances and, as such,
generate significant transient currents. Careful consider-
ation must be made regarding input and local power
supply bypassing to avoid corrupting the ground refer-
ences used by the UVLO and frequency set circuitry.
Typically, high current paths and transients from the input
supply and any local drive supplies must be kept isolated
from GND. By virtue of the topologies used in LT3705
applications, the large currents from the primary switches,
as well as the switch drive transients, pass through the
sense resistor to ground. This defines the ground connec-
tion of the sense resistor as the reference point for both
GND and PGND.
Effective grounding can be achieved by considering the
return current paths from the sense resistor to each
respective bypass capacitor. Don’t be tempted to run
small traces to separate the grounds. A power ground
plane is important as always in high power converters, but
care must be taken to keep high current paths away from
the GND reference. An effective approach is to use a 2-
layer ground plane, reserving an entire layer for GND and
an entire layer for PGND. The UVLO and frequency set
resistors can then be directly connected to the GND plane.
3705fb
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