The input pins of the UCC27511A device is based on a TTL and CMOS compatible input-threshold logic that is independent of the V_DD supply voltage. With a typically high threshold of 2.2 V and a typically low threshold of 1.2 V, the logic-level thresholds can be conveniently driven with PWM control signals derived from 3.3-V and 5-V digital-power controllers. Wider hysteresis (1 V typical) offers enhanced noise immunity compared to traditional TTL-logic implementations, where the hysteresis is typically less than 0.5 V. This device also feature 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 on these pins reduces loading and increases switching speed.

Whenever any of the input pins are in a floating condition, the output of the respective channel is held in the low state. This function is achieved using V_DD-pullup resistors on all the inverting inputs (IN– pin) or GND-pulldown resistors on all the non-inverting input pins (IN+ pin), (see Section 6.2).

The device also features a dual-input configuration with two input pins available to control the state of the output. The user has the flexibility to drive the device using either a non-inverting input pin (IN+) or an inverting input pin (IN–). The state of the output pin is dependent on the bias on both the IN+ and IN– pins. For additional clarification, refer to the I/O-logic truth table (Table 6-3) and the typical application diagrams, (Figure 6-4 and Figure 6-5).

When an input pin is selected for PWM drive, the other input pin (the unused input pin) must be properly biased in order to enable the output. As previously stated, the unused input pin cannot remain in a floating condition because whenever any input pin is left in a floating condition the output is disabled. Alternatively, the unused input pin can effectively be used to implement an enable or disable function. The following explains this function:

• In order to drive the device in a non-inverting configuration, apply the PWM-control input signal to IN+ pin. In this case, the unused input pin, IN–, must be biased low (such as tied to GND) in order to enable the output.
  • Alternately, the IN– pin is used to implement the enable or disable function using an external logic signal. OUT is disabled when IN– is biased high and OUT is enabled when IN– is biased low.
• In order to drive the device in an inverting configuration, apply the PWM-control input signal to IN– pin. In this case, the unused input pin, IN+, must be biased high (such as tied to V_DD) in order to enable the output.
  • Alternately, the IN+ pin is used to implement the enable or disable function using an external logic signal. OUT is disabled when IN+ is biased low and OUT is enabled when IN+ is biased high.
• Finally, note that the output pin can be driven into a high state only when the IN+ pin is biased high and the IN– input is biased low.

The input stage of the driver is preferably driven by a signal with a short rise or fall time. Use caution whenever the driver is used with slowly varying input signals, especially in situations where the device is located in a mechanical socket or PCB layout that is not optimal:

• High dI/dt current from the driver output coupled with board layout parasitics can cause ground bounce. The differential voltage between input pins and GND is modified and triggers an unintended change of output state because of fast 13-ns propagation delay which can ultimately result in high-frequency oscillations that increase power dissipation and pose a risk of damage to the device.
• A 1-V input-threshold hysteresis boosts noise immunity compared to most other industry standard drivers.
• In the worst case, when a slow input signal is used and PCB layout is not optimal, adding a small capacitor (1 nF) between the input pin and GND pin very close to the driver device may be necessary which helps to convert the differential mode noise with respect to the input-logic circuitry into common-mode noise and avoid unintended change of output state.

If limiting the rise or fall times to the power device is the primary goal, 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 the power dissipation into the external resistor.