[FAQ] UCC5310: Logic input characteristics of UCC53XX family

The UCC53XX devices, excluding UCC5304, have "CMOS inputs." This means the input thresholds change as VCC1 changes. The threshold levels are:

When designing your system, you need to make sure the input threshold levels of the UCC53XX are compatible with the output driving voltage of your controller. The maximum current required to drive the inputs is 240 uA, due to the pull-up/pull-down resistors on the IN+ and IN- pins. The IN+ pin has a pull-down resistor to GND1 of nominally 25 kohm (+/- 20%).  The IN- pin has a pull-up resistor to VCC1 of nominally 25 kohm (+/- 20%). These resistors are placed inside the IC to prevent logic errors as your controller is powering up, or if there is a problem on your PCB. They will keep the output low in case of floating inputs.

When powering the UCC53XX from the minimum supply voltage of 3V, your controller needs to be able to drive at least 2.1V into a 25 kohm load (+/- 20%) in order for the UCC53XX to recognize the input as a valid HIGH signal. Similarly, if your UCC53XX is powered from the maximum recommended supply voltage of 15V, your controller would need to output at least 10.5V into the 25 kohm nominal load.

Similarly, in order to recognize a valid low signal, your controller would need to pull the inputs to below 0.9V when operating with VCC1=3V, and below 4.5V when operating with VCC1=15V.

The VIH requirements of the UCC53XX family may present a problem for TTL-compatible output controllers, or controllers run off lower voltages like 1.8V. In these cases, a level-shifter is required to translate the logic signals to levels compatible with the UCC53XX family's inputs. This can be accomplished by using a level translator IC from TI (see our offerings here), or by using a discrete level translator which will most likely require you to invert your logic signal of your controller in software.

The UCC5304 is designed for TTL-compatible inputs. Here are the logic thresholds required:

One other point of confusion about logic inputs involves the MIN and MAX values in the datasheet. When designing your system, you need to make sure your controller outputs a voltage greater than the MAX value for VIH, and outputs a voltage less than the MIN value for VIL in order for the receiving device to recognize a valid high and low, respectively. In the above example for the UCC5304, there is a MIN value of 1.6V for VIH. All this means is you may find some devices that switch with a voltage as low as 1.6V, but ALL devices are not guaranteed to switch until 2V or higher. So, 2V is your design target for VIH. Likewise, for VIL, some devices may switch with a voltage as high as 1.25V, but ALL devices are not guaranteed to switch until you get to 0.8V or lower. So, 0.8V is your target for VIL. The voltage range of 1.6V down to 1.25V should be avoided as the output state is not guaranteed in this range, and could be low or high, depending on various variables. 

If you have further questions about this topic, or any other topic, please post a new/related question and our team will be happy to help you.