LTC3705IGN datasheet(14/20 Pages) LINER

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LTC3705

3705fb

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.

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 ITH 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 VCC.

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 VIN. In

either case, the LTC3705 oscillator determines the fre-

quency as found from:

OSC

FS P

21 10

4200

•

()

Note that polyphase operation is not possible in the stand-

alone configuration.

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