CM1223
Backdrive protection
Backdrive protection is needed to block against back-
drive current flowing from a high potential voltage node
toward a lower potential voltage node through the inter-
face cable.
For example, consider a DVD player 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 imped-
ance on the power rail node when "off", if there is very
little parasitic shunt resistance and as little as a few
milliamps of "backdrive" current flowing back 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, the DVD player
may not reset properly when the DVD player is turned
back on.
To avoid this situation, the CM1223 with integrated
backdrive protection diode was designed to block
backdrive current, guaranteeing no more than 5
μA
on
any I/O pin when the I/O pin voltage is greater than the
CM1223 supply voltage.
VDD_SOURCE
Discrete Backdrive
Protection Diode
“Off
One Channel
CM1213
VDD_SOURCE
VDD_SINK
Integrated Backdrive
Protection Diode
“Off
One Channel
CM1223
VDD_SINK
C
BULK
C
BULK
+
+
HOST
ASIC
D
1
I/O
D
2
I
BACKDRIVE
= 0
Peripheral
ASIC
HOST
ASIC
D
1
I/O
D
2
I
BACKDRIVE
= 0
Peripheral
ASIC
Figure 1. Backdrive Protection Diagram.
© 2006 California Micro Devices Corp. All rights reserved.
08/02/06
490 N. McCarthy Blvd., Milpitas, CA 95035-5112
●
Tel: 408.263.3214
●
Fax: 408.263.7846
●
www.cmd.com
3
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
