8.3.2 Input Stage

The input pins of UCC2753x device are based on a TTL and CMOS compatible input threshold logic that is independent of the VDD supply voltage. With typical high threshold = 2 V and typical low threshold = 1 V, the logic level thresholds can be conveniently driven with PWM control signals derived from 3.3-V or 5-V logic. Wider hysteresis (typically 1 V) offers enhanced noise immunity compared to traditional TTL logic implementations, where the hysteresis is typically less than 0.5 V. This device also features tight control of the input pin threshold voltage levels which eases system design considerations and ensures stable operation across temperature. The very low input capacitance, typically 20 pF, on these pins reduces loading and increases switching speed.

The device features an important safety function wherein, whenever the input pin is in a floating condition, the output is held in the low state. This is achieved using pull-up or pull-down resistors on the input pins as shown in the block diagrams.

The input stage of the driver should preferably be driven by a signal with a short rise or fall time. Caution must be exercised whenever the driver is used with slowly varying input signals, especially in situations where the device is located in a separate daughter board or PCB layout has long input connection traces:

• High dI/dt current from the driver output coupled with board layout parasitics can cause ground bounce. Because the device features just one GND pin which may be referenced to the power ground, this may interfere with the differential voltage between Input pins and GND and trigger an unintended change of output state. Because of fast 17 ns propagation delay, this can ultimately result in high-frequency oscillations, which increases power dissipation and poses risk of damage
• 1-V Input threshold hysteresis boosts noise immunity compared to most other industry standard drivers.

If limiting the rise or fall times to the power device to reduce EMI is necessary, then an external resistance is highly recommended between the output of the driver and the power device instead of adding delays on the input signal. This external resistor has the additional benefit of reducing part of the gate charge related power dissipation in the gate driver device package and transferring it into the external resistor itself.

Finally, because of the unique input structure that allows negative voltage capability on the Input and Enable pins, caution must be used in the following applications:

• Input or Enable pins are switching to amplitude > 15 V
• Input or Enable pins are switched at dV/dt > 2 V/ns

If both of these conditions occur, it is advised to add a series 150-Ω resistor for the pin(s) being switched to limit the current through the input structure.