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LT3437

3437fc

LAYOUT CONSIDERATIONS

As with all high frequency switchers, when considering

layout, care must be taken in order to achieve optimal

electrical, thermal and noise performance. For maximum

efficiency, switch rise and fall times are typically in the

nanosecond range. To prevent noise both radiated and

conducted, the high speed switching current path,

shown in Figure 8, must be kept as short as possible. This

is implemented in the suggested layouts of Figure 9.

Shortening this path will also reduce the parasitic trace

inductance of approximately 25nH/inch. At switch off, this

parasitic inductance produces a flyback spike across the

LT3437 switch. When operating at higher currents and

input voltages, with poor layout, this spike can generate

voltages across the LT3437 that may exceed its absolute

maximum rating. A ground plane should always be used

under the switcher circuitry to prevent interplane coupling

and overall noise.

The VC and FB components should be kept as far away as

possible from the switch and boost nodes. The LT3437

pinout has been designed to aid in this. The ground for

these components should be separated from the switch

current path. Failure to do so will result in poor stability or

subharmonic oscillation.

Board layout also has a significant effect on thermal

resistance. Pin 4/Pin 8 and the exposed die pad, Pin 11/Pin

17, are connected by a continuous copper plate that runs

under the LT3437 die. This is the best thermal path for heat

out of the package. Reducing the thermal resistance from

Pin 4 and the exposed pad onto the board will reduce die

temperature and increase the power capability of the

LT3437. This is achieved by providing as much copper

area as possible around the exposed pad. Adding multiple

solder filled feedthroughs, under and around this pad, to

an internal ground plane will also help. Similar treatment

to the catch diode and coil terminations will reduce any

additional heating effects.

Figure 8. High Speed Switching Path

3437 F08

VIN

LT3437

VOUT

VIN

HIGH

FREQUENCY

CIRCULATION

PATH

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