11.1 Layout Guidelines
Proper PCB layout is extremely important in a high-current fast-switching circuit to provide appropriate device operation and design robustness. The 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
- In noisy environments, tying inputs of an unused channel of the UCC27524A device to VDD (in case of INx+) or GND (in case of INX–) 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.
- 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 (13-ns/13-ns rise and fall propagation delay versus 25-ns/35-ns rise and fall propagation delay).
11.3 Thermal Protection
The useful range of a driver is greatly affected by the drive power requirements of the load and the thermal characteristics of the device package. In order for a gate driver device to be useful over a particular temperature range the package must allow for the efficient removal of the heat produced while keeping the junction temperature within rated limits. For detailed information regarding the thermal information table, please refer to Application Note from Texas Instruments entitled, IC Package Thermal Metrics (SPRA953).
Among the different package options available for the UCC27524A device, power dissipation capability of the DGN package is of particular mention. The MSOP PowerPAD-8 (DGN) package offers a means of removing the heat from the semiconductor junction through the bottom of the package. This package offers an exposed thermal pad at the base of the package. This pad is soldered to the copper on the printed circuit board directly underneath the device package, reducing the thermal resistance to a very low value. This allows a significant improvement in heat-sinking over that available in the D package. The printed circuit board must be designed with thermal lands and thermal vias to complete the heat removal subsystem. Note that the exposed pads in the MSOP-8 (PowerPAD) package are not directly connected to any leads of the package, however, the PowerPAD is electrically and thermally connected to the substrate of the device which is the ground of the device. TI recommends to externally connect the exposed pads to GND in PCB layout for better EMI immunity.