12.1 Layout Guidelines
Proper PCB layout is extremely important in a high-current fast-switching circuit to provide appropriate device operation and design robustness. The UCC27511 and UCC27512 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 switch to facilitate voltage transitions very quickly. At higher VDD voltages, the peak-current capability is even higher (4-A/8-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 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) should 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 will induce significant voltage transients on the output pin of the driver device and gate of the power switch.
- Wherever possible, parallel the source and return traces, taking 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 should be connected to the other circuit nodes such as source of power switch, ground of PWM controller, and so forth, at one, 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 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.
- In noisy environments, tying the unused Input pin of UCC27511 and UCC27512 to VDD (in case of IN+) or GND (in case of IN-) using short traces in order to ensure that the output is enabled and to prevent noise from causing malfunction in the output may be necessary.
- The UCC27512 device offers two ground pins, pin 2 and pin 5. Shorting the two pins together using the PCB trace is extremely important. The shortest trace should be located as close as possible to the device.