CM2020-00TR
Ordering Information
PART NUMBERING INFORMATION
Pins
Package
Ordering Part Number1
Part Marking
38
TSSOP-38
CM2020-00TR
CM2020-00TR
Note 1: Parts are shipped in Tape and Reel form unless otherwise specified.
Backdrive protection
Below, two scenarios are discussed to illustrate what can happen when a powered device is connected to an
unpowered device via a 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, then 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 CM2020-00TR 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 CM2020-00TR supply voltage.
Figure 1. Backdrive Protection Diagram.
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