Integrated Load Detection for Video Drivers

AND9046/D
Integrated Load Detection
for Video Drivers
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APPLICATION NOTE
Introduction
One may think the load detection is a trivial function but
it’s only true with an amplifier driving a DC coupled line. In
general, a discrete current sense technique is used to fulfill
this requirement. Meanwhile video drivers are also required
to drive AC coupled output lines through a large capacitor,
generally more the 220 mF. In order to answer the widest
range of applications, the newest video drivers like the
NCS2584 are also able to detect the load when the outputs
are AC coupled. The main idea is put the driver in a very low
power consumption mode when the television is unplugged.
To have a good sense of the function, the focus needs to
be brought on the amplifier stage. Two cases need to be
demonstrated when the driver is OFF:
♦ AC coupled output
♦ DC coupled output
In this two parts application note illustrating the power
saving features on the new generation of video drivers, we
will continue with the output load detection explanation.
The idea of the first feature was to detect any incoming
analog video signals into the driver (application note
AND8473/D). The second step is now to check the status of
the load to save even more power to the application.
It’s costly and difficult for the chipset to detect the
presence of a TV on the RCA connectors dedicated to the
analog video channels, especially when the output of the
video drivers need to be AC coupled. This load detection
feature presented, answers the need by integrating the load
detection into the video driver. Therefore the chipset does
not need to manage the presence of the TV connection,
resulting in simplified chipset code and lower power
dissipation.
As a quick reminder, the video driver is build of a clamp
or bias circuitry. The second stage is generally a 6th order
Butterworth low pass filter (LPF) and last stage will be the
6 dB amplifier (see figure below).
Clamp/
Bias
LPF
6dB
Figure 1. Typical Video Driver Architecture
© Semiconductor Components Industries, LLC, 2012
May, 2012 − Rev. 0
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AND9046/D
AND9046/D
AC Coupled Output
Now, when the load is removed, the output voltage is
going the vary from 0 to the device supply voltage level VCC.
When the output of the driver is OFF and AC coupled, two
phases occur.
Phase 1:
Phase 1:
Small Current
Small Current
C
Vout
C
−
+
Vcap
Amplifier
OFF
Vout
−
+
Rload
Vcap
Amplifier
OFF
Vout
Vcap
Vout
VCC
time
Figure 2. Phase 1, AC Coupled and Loaded Output
Phase 2:
time
Figure 4. Phase 1, AC Coupled and Unloaded Output
Amplifier
OFF
Phase 2 :
C
Vout
Small Current
Amplifier
−
+
Vcap
OFF
C
Rload
Vout
−
+
Vout
Small Current
Vcap
Vcap
Vout
time
Figure 3. Phase 2, AC Coupled and Loaded Output
In the phase 1, a small current is pushed in the node of the
output buffer stage. The output voltage Vout becomes equal
to the voltage across the capacitor Vcap. Vcap is equal to the
initial capacitor voltage and we assume the drop across the
load due to the small current is negligible. The value of C
(the output capacitor) is important (220 mF). Consequently,
its charging time by the the small current can be considered
as negligible. During the second phase the current on the
output node is sunk and Vout is still equal to Vcap which
remains unchanged.
0
time
Figure 5. Phase 2, AC Coupled and Unloaded Output
In the phase 1, there is no more load to absorb the current
fed into the output node. That’s why the voltage at this point
goes to VCC. For the same reason , in phase 2, the output
voltage is brought to the ground.
In AC coupling configuration without load, the output
voltage before the capacitor varies from 0 to VCC
periodically.
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AND9046/D
DC Coupled Output
connected. The output voltage is consequently pulled close
to 0 V.
When the load is disconnected and the output is DC
coupled, the output voltage will swing between the rails. The
same two phases occur as described when the output is AC
coupled.
Phase 1:
Now, when the output of the driver is OFF and DC
coupled, two phases also occur.
Phase 1:
Small Current
Small Current
Vout
Vout
Amplifier
OFF
Rload
Amplifier
OFF
Vout
Vout
Vcc
Close
to 0 V
time
Figure 6. Phase 1, DC Coupled and Loaded Output
time
Figure 8. Phase 1, DC Coupled and Unloaded Output
Phase 2 :
Phase 2 :
Amplifier
OFF
Amplifier
OFF
Vout
Vout
Small Current
Rload
Small Current
Vout
Vout
Close
to 0V
time
Close
Figure 7. Phase 2, DC coupled and loaded output
to 0 V
time
Figure 9. Phase 2, DC Coupled and Unloaded Output
During phase 1 and phase 2, the output capacitor is not
present anymore to hold a voltage when the load is
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AND9046/D
Now understanding those phenomenons, a simple voltage
comparator is needed to operate the detection of the load.
It’s best to understand the general behavior of those two
types of detection through a state machine. The input video
signal detection being treated in the application note
AND8473/D “Integrated Input Auto-Detection Mode for
Video Drivers”.
Conclusion
Driver
ON
No Input
Signal
Input
Signal
&
or
Loaded
Output
Unloaded
Output
Driver
OFF
Figure 10. Transition from Loaded to Unloaded AC
Coupled Output
Figure 12. State Machine
The driver is only turned ON when both conditions are
met: receiving the proper video signal from the micro
controller and having the TV plugged into the player to
create a load for the driver. In any other case, the driver will
be turned OFF.
This latest generation of analog video drivers has been
designed to support the power saving challenges of the
consumer industry. Due to this embedded detection feature,
system designers will not have to worry about how to
improve their design for the analog video outputs. The
NCS2584 will help to simplify the control of the video driver
and will also be the best fit in an Energy Star® design
compliance environment.
The figure above illustrates a Cvbs type signal sent
through the driver until the TV is unplugged. At this point
of time, the output swings between the rails. The square
signal is centered on 0 because it is measured after the
capacitor.
The Figure 11 illustrates what is happening on the driver
output when the user plugs the TV RCA cable back in the
player. The device leaves its switching mode and directly
drives the video signal.
Figure 11. Transition from Unloaded to Loaded AC
Coupled Output
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AND9046/D
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