ISL6208CB-T Datasheet, PDF (9/13 Pages) Intersil Corporation – High Voltage Synchronous Rectified Buck MOSFET Drivers

ISL6208, ISL6208B
Layout Considerations
Reducing Phase Ring
The parasitic inductances of the PCB and power devices (both upper
and lower FETs) could cause increased PHASE ringing, which may
lead to voltages that exceed the absolute maximum rating of the
devices. When PHASE rings below ground, the negative voltage
could add charge to the bootstrap capacitor through the internal
bootstrap diode. Under worst-case conditions, the added charge
could overstress the BOOT and/or PHASE pins. To prevent this from
happening, the user should perform a careful layout inspection to
reduce trace inductances, and select low lead inductance MOSFETs
and drivers. D²PAK and DPAK packaged MOSFETs have high
parasitic lead inductances, as opposed to SOIC-8. If higher
inductance MOSFETs must be used, a Schottky diode is
recommended across the lower MOSFET to clamp negative PHASE
ring.
A good layout would help reduce the ringing on the phase and
gate nodes significantly:
• Avoid using vias for decoupling components where possible,
especially in the BOOT-to-PHASE path. Little or no use of vias
for VCC and GND is also recommended. Decoupling loops
should be short.
• All power traces (UGATE, PHASE, LGATE, GND, VCC) should be
short and wide, and avoid using vias. If vias must be used, two
or more vias per layer transition is recommended.
• Keep the SOURCE of the upper FET as close as thermally
possible to the DRAIN of the lower FET.
• Keep the connection in between the SOURCE of lower FET and
power ground wide and short.
• Input capacitors should be placed as close to the DRAIN of the
upper FET and the SOURCE of the lower FET as thermally
possible.
Note: Refer to Intersil Tech Brief TB447 for more information.
Thermal Management
For maximum thermal performance in high current, high
switching frequency applications, connecting the thermal pad of
the QFN and DFN parts to the power ground with multiple vias, or
placing a low noise copper plane underneath the SOIC part is
recommended. This heat spreading allows the part to achieve its
full thermal potential.
9
FN9115.6
January 31, 2012