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[FAQ] LMH1219: Internal Voltage Thresholds for 4-Level Input Configuration Pins

Part Number: LMH1219

Tool/software:

Hi All,

It is common to wonder what is going on inside of ICs. Being an applications engineer at TI gives me the unique opportunity to report on this so that people can to understand our devices a little bit more.

In this E2E FAQ, I will be discussing a common pin configuration implementation that is used in our redrivers and in most if not all of our SDI retimers. 4-level input configuration pins have internal voltage thresholds to configure settings on the device. Some common settings that can be configured are ENSMB, OUT_CTRL, VOD_DE and many others. 

Using this excerpt from the LMH1219EVM user's guide, we can delve a little deeper into this topic. The four pin options are H,F,R and L, and they correspond to 4 different resistor configurations. If the pin was pulled low 1k ohms that would be L, if the pin was pulled 1k to VIN, that would be H, if the pin was pulled 20k ohms to GND, that would be R, and if the pin was left floating, that would be F. 

What a lot of customers do not realise is that typically this resistor ladder configuration includes an internal pullup resistor and pulldown resistor. These resistors effect Vout by ohm's law and the internal threshold values that eventually decide the device's operation are determined by the result of this resistor scheme. For example, for the LMH1219 the value for the pullup resistor is 30k ohms and the value of the pulldown resistor is 60k ohms.

Say we have a 2.5V power supply. This means that the threshold between 0 and R would be 0.5 V and the threshold between R and F would be 1.25 V. As long as we fall between these two voltages, then the pin level will be R. When adding a 20 kΩ to ground, we can determine the voltage at the output based on adding a resistor in parallel with the 60 kΩ pull-down resistor, and this would give 0.83 V at Vout. By the same calculation with a 22 kΩ resistor, Vout would be approximately 0.87 V.

Best Regards,

Nick Peabody

HSSC Applications