AND9046/D Integrated Load Detection for Video Drivers http://onsemi.com 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 1 Publication Order Number: 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. http://onsemi.com 2 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 http://onsemi.com 3 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 http://onsemi.com 4 AND9046/D ENERGY STAR and the ENERGY STAR mark are registered U.S. marks. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. 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