Proper PCB layout is extremely
important in a high-current fast-switching circuit to provide appropriate device
operation and design robustness. The UCC27524A 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 power MOSFET to facilitate voltage transitions very quickly. At
higher VDD voltages, the peak current capability is even higher (5-A peak
current is at VDD = 12 V). Very high di/dt causes unacceptable ringing if
the trace lengths and impedances are not well controlled. The following circuit
layout guidelines are strongly recommended when designing with these high-speed
drivers.
- Locate the driver device as close
as possible to power device in order to minimize the length of high-current
traces between the output pins and the gate of the power device.
- Locate the VDD bypass
capacitors between VDD and GND as close as possible to the driver
with minimal trace length to improve the noise filtering. These capacitors
support high peak current being drawn from VDD during turnon of power
MOSFET. The use of low inductance surface-mounted-device (SMD) components such
as chip resistors and chip capacitors is highly recommended.
- The turnon and turnoff current
loop paths (driver device, power MOSFET and VDD bypass capacitor)
must be minimized as much as possible in order to keep the stray inductance to a
minimum. High di/dt is established in these loops at two instances during turnon
and turnoff transients which induces significant voltage transients on the
output pin of the driver device and Gate of the power MOSFET.
- Wherever possible, parallel the
source and return traces to take advantage of flux cancellation
- Separate power traces and signal
traces, such as output and input signals.
- Star-point grounding is a good
way to minimize noise coupling from one current loop to another. The GND of the
driver is connected to the other circuit nodes such as source of power MOSFET
and ground of PWM controller at one, single point. The connected paths must 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 OUT may corrupt the input signals
during transition. The ground plane must not be a conduction path for any
current loop. Instead the ground plane must be connected to the star-point with
one single trace to establish the ground potential. In addition to noise
shielding, the ground plane can help in power dissipation as well
- Exercise caution when replacing
the UCC2732x/UCC2742x devices with the UCC27524A device:
- The UCC27524A device is a much stronger
gate driver (5-A peak current versus 4-A peak current).
- The UCC27524A device is a much faster gate
driver (17-ns/17-ns rise and fall propagation delay versus 25-ns/35-ns
rise and fall propagation delay).