CM2030 Datasheet, PDF (4/14 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 cir­
cuitry. This is a potentially common occurrence in mul­
timedia 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 possibil­
ity 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 inter­
nal 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.
LOW VOLTAGE
HDMI ASIC
LV_SUPPLY
=OFF
+5V
ASIC
LV_SUPPLY
=OFF
LOW VOLTAGE
HDMI ASIC
+5V
ASIC
HDMISOURCE
HDMISINK
HDMI SOURCE
HDMI SINK
Figure 1. Backdrive Protection Diagram.
Display Data Channel (DDC) lines
The DDC interface is based on the I²C serial bus proto­
col for EDID configuration.
DYNAMIC PULLUPS
Based on the HDMI specification, the maximum capac­
itance 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 specifi­
cation even with very long and non-compliant cables.
The internal accelerator increases the positive slew
rate of the DDC_CLK_OUT and DDC_DAT_OUT lines
whenever the sensed voltage level exceeds
0.3*5V_SUPPLY (approximately 1.5V). This provides
faster overall risetime in heavily loaded situations with­
out overloading the multi-drop open drain I²C outputs
elsewhere.
© 2007 California Micro Devices Corp. All rights reserved.
4
490 N. McCarthy Blvd., Milpitas, CA 95035-5112 ●Tel: [phone] ●Fax: [phone] ●www.cmd.com
Issue A – 11/16/07