CM2030 Datasheet, PDF (4/17 Pages) California Micro Devices Corp – HDMI Transmitter Port Protection and Interface Device

CM2030
Backdrive Protection and Isolation
Backdrive current is defined as the undesirable current flow through an I/O pin when that I/O pin’s voltage
exceeds the related local supply voltage for that circuitry. This is a potentially common occurrence in multimedia
entertainment systems with multiple components and several power plane domains in each system.
For example, if a DVD player is switched off and an HDMI connected TV is powered on, there is a possibility of
reverse current flow back into the main power supply rail of the DVD player from pull-ups in the TV. As little as a
few milliamps of backdrive current flowing back into the power rail can charge the DVD player’s bulk bypass
capacitance on the power rail 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, then these devices may not reset properly
when the DVD player is turned back on.
If any SOC devices are incorporated in the design which have built-in level shifter and/or ESD protection
structures, there can be a risk of permanent damage due to backdrive. In this case, backdrive current can
forward bias the on-chip ESD protection structure. If the 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 CM2030 was designed to block backdrive current, guaranteeing less
than 5µA into any I/O pin when the I/O pin voltage exceeds its related operating CM2030 supply voltage.
Figure 1. Backdrive Protection Diagram.
Display Data Channel (DDC) lines
The DDC interface is based on the I²C serial bus protocol for EDID configuration.
DYNAMIC PULLUPS
Based on the HDMI specification, the maximum capacitance of the DDC line can approach 800pF (50pF from
source, 50pF from sink, and 700pF from cable). At the upper range of capacitance values (i.e. long cables), it
becomes impossible for the DDC lines to meet the I²C timing specifications with the minimum pull-up resistor of
1.5kΩ.
For this reason, the CM2030 was designed with an internal I²C accelerator to meet the AC timing specification
even with very long and non-compliant cables.
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