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LT1681I

Dual Transistor Synchronous Forward Controller

Linear Technology 

LT1681

APPLICATIO S I FOR ATIO

Layout Considerations—Grounding

The LT1681 is typically used in high current converter

designs that involve substantial switching transients. The

switch drivers on the IC are designed to drive large

capacitances and, as such, generate significant transient

currents. Careful consideration must be made regarding

input and local power-supply bypassing to avoid corrupt-

ing the ground references used by the error amplifier and

current sense circuitry.

Effective grounding of the two-transistor synchronous

forward topology where the LT1681 is used is inherently

difficult. The situation is complicated further by the num-

ber of bypass elements that must be considered.

Typically, high current paths and transients from the input

supply and any local drive supplies must be kept isolated

from SGND, to which sensitive circuits such as the error

amp reference and the current sense circuits, as well as the

local 5VREF supply, are referred. By virtue of the topologies

used in LT1681 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 connection of the sense resistor as the refer-

ence point for both SGND and PGND. In nonisolated

applications where SGND is the output reference, we

now have a condition where every bypass capacitor in

the converter is referenced to the same point.

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

bypass elements must be oriented such that transient

currents in the return paths of VIN and VCC do not mix. Care

must be taken to keep these transients away from the

SGND reference. An effective approach is to use a 2-layer

ground plane, reserving an entire layer for SGND. The

5VREF and non-isolated converter output bypasses can

then be directly connected to the SGND plane.

LT1681

VBST

VBST

VIN

5VREF

SGND

BSTREF

VCC

VCC

PGND

1681 F06

Figure 6. High Current Transient Return Paths

1681f

14

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