Proper PCB layout is extremely important in a
high-current, fast-switching circuit to provide appropriate device operation and design
robustness. The UCC5710x-Q1 gate driver incorporates short propagation
delays and powerful output stages capable of delivering large current peaks with very fast
rise and fall times at the gate of a power switch to facilitate voltage transitions very
quickly. Very high di/dt can cause unacceptable ringing if the trace lengths and impedances
are not well controlled. The following circuit layout guidelines are recommended when
designing with these high-speed drivers.
- Place the driver device as close as
possible to power device to minimize the length of high-current traces between the
driver output pins and the gate of the power switch device.
- Place the bypass capacitors between the
VDD pin and the GND pin and as close to the driver pins as possible to minimize the
trace length for improved noise filtering. TI recommends having two capacitors; a 100-nF
ceramic surface-mount capacitor placed less than 1mm from the VDD pin of the device and
another ceramic surface-mount capacitor of a few microfarads added in parallel. These
capacitors support high peak current being drawn from VDD during turnon of the power
switch. The use of low inductance surface-mount components such as chip capacitors is
highly recommended.
- The turnon and turn-off current loop
paths (driver device, power switch, and VDD bypass capacitor) should be minimized as
much as possible in order to keep stray inductance to a minimum. High di/dt is
established in these loops at two instances – during turnon and turn-off transients,
which induces significant voltage transients on the output pins of the driver device and
gate of the power switch.
- Wherever possible, parallel the source
and return traces of a current loop, taking advantage of flux cancellation
- Separate power traces and signal
traces, such as output and input signals.
- To minimize switch node transients and
ringing, adding some gate resistance and/or snubbers on the power devices may be
necessary. These measures may also reduce EMI.
- Star-point grounding is a good way to
minimize noise coupling from one current loop to another. The GND of the driver should
be connected to the other circuit nodes such as the source of the power switch, the
ground of the PWM controller, and so forth, at a single point. The connected paths
should be as short as possible to reduce inductance and be as wide as possible to reduce
resistance.
- Use a ground plane to provide noise
shielding. Fast rise and fall times at the OUT pin may corrupt the input signals during
transitions. The ground plane must not be a conduction path for any current loop.
Instead the ground plane should be connected to the star-point with one trace to
establish the ground potential. In addition to noise shielding, the ground plane can
help in power dissipation as well.