CM2020-01TR Datasheet, PDF (4/12 Pages) ON Semiconductor – HDMI Transmitter Port Protection and Interface Device

CM2020-01TR
Backdrive Protection
Below, two scenarios are discussed to illustrate what can happen when a powered device is connected to an
unpowered device via an HDMI interface, substantiating the need for backdrive protection on this type of
interface.
In the first example a DVD player is connected to a TV via an HDMI interface. If the DVD player is switched off
and the TV is left on, there is a possibility of reverse current flow back into the main power supply rail of the
DVD player. Typically, the DVD’s power supply has some form of bulk supply capacitance associated with it.
Because all CMOS logic exhibits a very high impedance on the power rail node when “off,” if there may be very
little parasitic shunt resistance, and even with as little as a few milliamps of “backdrive” current flowing into the
power rail, it is possible over time to charge that bulk supply capacitance to some intermediate level. If this level
rises above the power-on-reset (POR) voltage level of some of the integrated circuits in the DVD player, these
devices may not reset properly when the DVD player is turned back on.
In a more serious scenario, if any SOC devices are incorporated in the design which have built-in level shifter
and DRC diodes for ESD protection, there is even a risk for permanent damage. In this case, if there is a pullup
resistor (such as with DDC) on the other end of the cable, that resistance will pull the SOC chips “output” up to a
high level. This will forward bias the upper ESD diode in the DRC and charge the bulk capacitance in a similar
fashion as described in the first example. If this current flow is high enough, even as little as a few milliamps, it
could destroy one of the SOC chip’s internal DRC diodes as they are not designed for passing DC.
To avoid either of these situations, the CM2020-01TR was designed to block backdrive current, guaranteeing no
more than 5mA on any I/O pin when the I/O pin voltage is greater than the CM2020-01TR supply voltage.
Figure 1. Backdrive Protection Diagram
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