Analysis on Reducing Video System Power Consumption and Prolonging Battery Life in Portable Devices

1 Introduction

Low power consumption has always been a top priority in portable devices. Considering the increasing energy costs and global warming issues, wall-adapter-powered devices are becoming more and more power-conscious. Therefore, the trend is to integrate more intelligent power management circuits in analog chips. For video filter amplifiers, not only should the power consumption be low, but also video load detection, video input detection and control circuits should be provided to control the corresponding operating mode.

More and more portable devices, such as digital cameras, cell phones and portable media players, are gradually adding composite video output connectivity. In these devices, the video filter amplifier connected after the video digital-to-analog converter (DAC) generates the video signal. For example, the current 3.3V video filter amplifier consumes 45W when processing the video signal.

Video output function is becoming a common function in many portable electronic products today. Since this function is usually an auxiliary function, the video driver is usually turned off during normal operation, so the operating current and standby current will be the key parameters when evaluating the video amplifier or video driver. Battery life is therefore the key in portable devices, and the first consideration here is low power consumption of the video system IC. For this reason, the new generation of video filter amplifiers can operate at 1.8V and consume only 12mW, which is nearly 70% less power.

Based on this, we will discuss the issue of reducing the working and standby power consumption of the video system in order to extend the battery life of the handheld video system, and introduce the characteristics of low-power video system chips. To this end, we first introduce the basic characteristics of the battery used in the video system, which is the premise of this article.

2 Basic Concepts of Battery Characteristics and Energy Consumption in Handheld Video System Applications

2.1 Battery Characteristics in Handheld-Video System Applications

⑴ Battery Circuit Model

Lithium-ion battery is a high-density energy storage device. Its circuit model can be simplified as follows: it can only work in the temperature range of 0℃-50℃, has an effective series resistance (ESR), and has a nonlinear time-varying capacitor with huge capacitance. Its capacitance and ESR values ​​depend on the voltage, current and temperature of the battery. The breakdown voltage is 4.3V, and overcharging will shorten its life.

⑵ The internal electronic circuit of the lithium-ion battery pack is shown in Figure 1(a)

The lithium-ion battery pack contains at least 1 security chip and 1 dual MOSFET tube, at least 2 pins, and generally has 3 to 4 pins. Many of them have other functions, such as a fuel gauge, an IC for identifying genuine batteries, an NTC thermistor, and an ID resistor.

(a)

(b)

Figure 1 (a) Schematic diagram of the internal electronic circuit of the lithium-ion battery pack; (b) Lithium-ion battery charging curve

⑶ The lithium-ion battery charging curve is shown in Figure 1 (b)

Start with constant current charging (CC-Constant Current), then charge the battery with constant voltage (CV-ConstantVoltaqe), and end the charging (EOC-End of Charge) when the charging current drops to 3% to 10% of the CC current. The typical CC current is set between 0.5C and 1.0C, not higher than 1.5C. A higher constant current charging current will shorten the constant current charging period and extend the constant voltage charging period.

⑷ Other requirements for lithium-ion battery charging

Charging temperature range 0℃ to 50℃; Charging time, if the constant current charging rate is 1C, the total charging time should not exceed 3 hours. If the time required is longer than expected, stop (charging cycle time limit); high-precision 4.1V or 4.2V ± 50mV, do not overcharge; charging of over-discharged batteries, - VBAT < VMIN when trickle charging / pre-charging; automatic restart charging cycle, indication signal.

⑸ Battery capacity / battery life and charging voltage

If the charging voltage rises by 1%, the initial capacity increases by 5%; if the battery is under high voltage for a long time, the aging speed will be faster; the battery life of 4.2V charging is 500 times; overcharging will shorten the battery life, cause malfunctions, and cause safety hazards; undercharging can extend the battery life.

⑹ Hazards of overcharging and over-discharging

Overcharging can increase the initial capacity, but shorten the battery life; overcharging or over-discharging will cause the irreversible decrease of the battery cell capacity; if overcharged, it may cause leakage or fire, or even explosion.

⑺ Storage time

Capacity changes at different storage temperatures:

At 0℃, the battery capacity decreases by 2% after a full charge of 40% for one year; the battery capacity decreases by 6% after a full charge of 100% for one year;

At 25℃, the battery capacity decreases by 4% after a full charge of 40% for one year; the battery capacity decreases by 20% after a full charge of

100% for one year; At 40℃, the battery capacity decreases by 15% after a full charge of 40% for one year; the battery capacity decreases by 35% after a full charge of 100% for one year;

At 60℃, the battery capacity decreases by 25% after a full charge of 40% for one year; the battery capacity decreases by 40% after a full charge of 100% for one year.

For long-term storage of more than two months, the battery should be placed in a cool place and half-charged.

⑻ Summary of lithium battery characteristics

It should be said that battery life is the most important user demand in portable devices. Even if users desire advanced multimedia functions, they are unwilling to give up long usage time (such as mobile phone call time) and standby time to obtain these functions. However, even if designers extend battery life, they are facing "new demands" and will increase power consumption. Although battery technology has continued to advance in recent years, the technology to improve efficiency is a new task facing IC designers, that is, lower power consumption and better power management.

[page]2.2 Basic Concepts of Battery Energy Consumption

Simply put, the power consumption of each circuit includes the loss of its own operation and the loss of driving the load. In Figure 1, the power supply provides a total current (IT) to the circuit, where IQ is the quiescent current of the operational amplifier and IL is the load current. The current is multiplied by the power supply voltage to obtain the power. First, the quiescent power consumption (PQ), load power consumption (PL), and total power consumption (PT) are calculated according to the following formulas:

To reduce the actual power consumption, PQ and PL must be reduced at the same time. Reducing VDD, IQ, and IL can achieve this goal. Usually, IC data sheets will give IQ or PQ parameters, but rarely mention the average power consumption under typical signal and typical load conditions. For portable video filter amplifiers, PQ is almost useless information because the circuit is either in the off state or fully turned on (that is, defined as when the video filter amplifier provides video signal driving to the load).

When there is no video load, the video filter amplifier should be turned off to save battery energy; if the video filter amplifier is turned on when there is no video load, it will waste battery energy.

3 3.3V video filter amplifier low power consumption technology and typical chip features example

Typical video driver architectures use AC coupling or DC coupling schemes. By using input and output capacitors, AC coupling can eliminate DC voltage on the transmission line and isolate the grounding points of the transmitting and receiving systems, thereby simplifying circuit design. However, these capacitors will reduce signal quality. To minimize the impact on signal quality, the output capacitor must be in the hundreds of μF.

Order of magnitude. DC coupling does not require output capacitors, which is particularly attractive for price-sensitive high-volume products, but when the input signal has positive and negative swings, an additional negative power supply is required to provide a common input voltage range that can accommodate the positive and negative swings of the signal.

For example, Linear Technology's LT6557, a 3-way video amplifier for single-supply applications, has a wide output swing of 0.8V power rail. It is a broadband RGB amplifier that can provide full video swing when operating with a 5V single power supply, with 500MHz 3dB bandwidth, 2200V/s conversion rate, and 4ns settling time.

3.1 The power consumption of 3.3V video filter amplifier increases when providing video signal driving to the load The problem of increased power consumption can be seen

through the comparison of the average power consumption and static power consumption of the three video filter amplifiers MAX9502, OPA360 and MAX9503. Their indicators are as follows:

① Average current: MAX9502 is 13.5mA, OPA360 is 12.2mA, MAX9503 is 13.2mA;

② Average power consumption: MAX9502 is 44.6mW, OPA360 is 40.1mW, MAX9503 is 43.4mW;

③ IQ: MAX9502 is 5.3mA, OPA360 is 6mA, MAX9503 is 12mA;

④ PQ: MAX9502 is 17.5mW, OPA360 is 19.8mW, MAX9503 is 39.6mW;

⑤ Output mode: MAX9502 is positive DC bias, OPA360 is negative DC bias, MAX9503 is DirectDrive technology.

The average power consumption is defined as the power consumption of the video filter amplifier when driving a 150Ω load to ground with a 50% average video signal. The 50% average signal is a typical video signal, which is displayed as a gray screen on a TV (PL depends on the image content, with the lowest power consumption when the screen is black and the highest power consumption when the screen is white). Note that although the PQ of the components varies greatly, the average power consumption is very similar.

Driving the video signal into the video load causes increased power consumption, which depends largely on the output of the video amplifier. The MAX9502 outputs the video signal with a forward DC bias. Maintaining the forward DC bias of the output signal will increase the total power consumption. Therefore, the MAX9502 must supply about 8.7mA of current.

The output of the OPA360 can be operated with a SAG network, which consists of two AC coupling capacitors, as shown in Figure 2. These capacitors block the DC connection between the output and the load. Therefore, the amplifier does not need to source or sink current to maintain the output bias, thereby reducing power consumption. This can be seen from the technical characteristics of the OPA360.

Figure 2 Schematic diagram of OPA360 output working with SAG network

3.2 Technical features of OPA360 chip that effectively reduce power consumption

The new 3V video amplifier can enhance video output performance and reduce board space. This video amplifier series provides a high level of features/integration in its compact SC70 package. Relying on its integrated shutdown function, 6db double-pole low-pass filter and SAG correction, the low-power video device improves its video performance while reducing cost and board space. The use of SAG correction enables the output coupling capacitor to be replaced from a large 470μF capacitor to two smaller capacitors, as shown in Figure 2, which greatly reduces the size and cost. The 50mV level converter allows the output to be DC coupled without clipping distortion, thereby achieving the best video performance within the total solution area of ​​5mm2. As

can be seen from Figure 2, for a 50% average signal, the OPA360 application circuit can effectively reduce power consumption because the capacitor blocks the DC connection between the output and the load. The main features of its OPA360 chip are:
①Excellent video performance: 0.5dB gain flatness at 35MHz, differential gain of 0.02%, differential phase of 0.05º;
②Unit gain bandwidth of 5MHz;
③High slew rate: 100V/μs;
④Input range includes ground;
⑤Rail-to-rail output;
⑥Low power consumption 6mA (when turned on), 2.5μA (when shut down);
⑦Single power supply operating range: 2.7Vto3.3V;
⑧Package type: micro-package SC70.

OPA360 can be used in digital cameras, mobile phones with video functions, portable media players, set-top box video filters, and digital TVs.

3.3 Maxim's DirectDrive technology

allows the MAX9503 to output video signals with near-zero DC bias without any AC coupling capacitors. This technology enables the MAX9503 to output signals below ground level because the on-chip reverse charge pump can generate negative voltages. Although DirectDrive increases PQ, the average power consumption of the MAX9503 can be kept at the same level as the MAX9502 and OPA360 due to the reduced PL. Since the DC bias is close to ground level, the MAX9503 only needs to source a smaller current.

3.4 THS7353 three-channel video power amplifier

This amplifier has selectable filters, 2:1 input multiplexing and external gain control.

High-performance streaming media technology covers a wide range of digital media and emerging media-based technologies, including the integration of video, voice and data content into many new and diverse applications, thus completely changing the way digital media content is delivered.

Among the many challenges facing streaming media are real-time performance, higher channel density and software programming flexibility for simultaneous processing of video, voice and data streams across wired and wireless networks. Typical digital media processing functions include encoding and decoding of media streams, code conversion (from one format to another), and rate conversion of data streams (from higher bit rates to lower bit rates), designed to accommodate a variety of system-level dependencies. Other processing functions include compression, decompression, encryption, packetization and transmission of media streams.

For example, the video amplifier THS7327 integrates three analog video channels and two digital channels for HV synchronization, greatly simplifying system design and reducing the number of components. Programmable filters and input bias modes can meet the general analog signal conditioning requirements required by all signal standards. The typical chip THS7353 is introduced.

THS7353 is a low-power, three-channel integrated video buffer, which is constructed using BCOM-111 processing. The chip integrates a selectable 5th-order Butterworth anti-aliasing/DAC reconstruction filter to eliminate data conversion imaging. Each channel can be configured individually. Its rail-to-rail output stage allows AC and DC coupling applications, and the external control gain adjustment pin allows precise gain adjustment, such as linear drive, compensation for cable loss or Sin-X/X compensation.

The main features of THS7353 are: three-channel video amplification for CVBS, S-Video, YUV, SD/ED/H-P-B-R and RGB; control of all functions; integrated low-pass filter; selectable input bias mode; 2:1 input multiplexing allows multiple input sources; external gain control range 0dB to 14dB; single power supply 2.7V to 5V; low quiescent current 16.2mA; differential gain/phase 0.15%/0.3°.

THS7353 applications: can be used in HDTV video buffering, PVR/DVD day output buffering, projector video buffering and USB/portable video buffering, etc.

[page]4 New generation of low power video filter/buffer products

As the video content in mobile phones or PMPs increases, these devices require composite video output and/or S-video output. Video buffers used in these devices can benefit from the analog standard definition video filter MAX9508. The MAX9508 uses signal payload detection technology, which can automatically disable the video output when there is no driving signal. Smart Sleep technology can maximize battery life and provide high-frequency EMI filtering, as shown in Figure 3.

Figure 3 Smart Sleep technology can maximize battery life and provide high-frequency EMI filtering diagram

In the design of video applications, design engineers must pay attention to the cost of the entire product solution, not just the cost of the chip. Fairchild Semiconductor's ultra-portable video filter driver FMS6151 can drive mobile phone video images to TVs and computer monitors. For applications with higher performance requirements, the device's 5th-order 8MHz SD filter can improve image quality, while the low 3.8mA supply current (only 25nA when turned off) can extend battery life.

4.1 Low-Power Video Switch TS5V330

Portable multimedia automatic music selectors, sometimes called video jukeboxes, portable media players or portable video players, are a market that consumers and consumer electronics manufacturers are paying attention to. These devices, usually based on hard drives, can store thousands of hours of program content and provide consumers with entertainment that is suitable for today's "mobile" lifestyle. Although the market for these products is not very large at present, it is a consensus in the industry that this market will gradually become a trend. Here we introduce its video switch TS5V330.

The video switches in the TS switch product series provide low differential gain and phase. This makes them ideal for composite and RGB video applications. TS video switches also provide the large bandwidth and low crosstalk required to support high-frequency video applications. Figure 4 is a schematic diagram of the external video port where multiple video signals are transmitted from a video graphics processor to a VSA.

Figure 4 Schematic diagram of multi-channel video signal transmission from video graphics processor to external video port of VSA

Main features: low differential gain and phase (3V DG=0.82％. Dp=0.1*typical value), (5VDG=0.64％, Dp=0.1*typical value); wide bandwidth (BW=300MHz minimum value); low crosstalk (3VXTALK=-80dB typical value), (5VXTAL=-63dB typical value); low power consumption (Icc=3μA maximum value); bidirectional data flow with near-zero propagation delay; low on-state resistance (rON=3Ω typical value); rail-to-rail switching operation on data I/O port (0V to Vcc); Ioff supports partial power-off mode operation; suitable for RGB and composite video switching, can be applied on composite and RGB video.

4.2 The 1.8V video filter amplifier MAX9509 with DirectDrive technology

is the first device in Maxim's new generation of video filter amplifiers, which greatly reduces average power consumption and PQ, as shown in Figure 5. The MAX95091.8 application circuit in Figure 5 processes 50% average signals and greatly reduces power consumption. Its power supply voltage (VDD) is reduced from 3.3V to 1.8V, which is the digital I/O voltage that mobile phones are gradually using; the static power supply current (IQ) is also reduced from 12mA to 3.1mA.

Figure 5 The first device in a new generation of video filter amplifiers significantly reduces average power consumption and PQ

When video filter amplifiers are operated from a 1.8V supply voltage, DirectDrive technology must be used. Amplifiers with voltage-mode output stages must provide at least 2Vp-p swing to output composite video signals. Conventional amplifiers do not have enough headroom to produce a 2Vp-p output signal when powered from a single 1.8V supply. With DirectDrive, the integrated inverting charge pump generates a noisy 1.8V voltage; the negative linear regulator stabilizes the -1.8V voltage to -1V, reducing charge pump noise. Therefore, the MAX9509 has just enough headroom to output a 2Vp-p video signal when actually powered from a 1V to +1.8V supply. The MAX9509

uses a low-voltage, low-IQ DirectDrive output stage, and the average power consumption of the device is significantly lower than that of a 3.3V device. More importantly, the average power consumption of the MAX9509 is lower than the PQ of a 3.3V video filter amplifier. It is important to note that when the circuit is running at high speed at such low voltage, the noise will be greatly increased because the circuit is now operating at a lower current than normal. The MAX9509 takes the noise issue into account. The device has an excellent peak signal-to-noise ratio (SNR) of 64dB, which is sufficient for consumer products. In order to display a clear image on a TV screen, the peak SNR should be around 40dB. Placing the noisy charge pump on the same chip as the filter and amplifier is an important new technology. The charge pump has the potential to introduce switching noise into the sensitive video signal. Isolating the MAX9509's charge pump from the video signal path effectively solves this problem, resulting in extremely low charge pump noise in the frequency domain and almost no noise in the time domain.

When consumers observe the output signal of the MAX9509 on the screen, they will not see either broadband noise or charge pump noise.

5 Conclusion From

the above analysis, we can see that although some progress has been made in the development of low-power video filter amplifiers, IC designers still have a lot of work to do, such as video load detection. If the video filter amplifier has load electronic detection function and provides load status to the microcontroller system, the video output circuit will only be turned on when there is a valid video load, which can further enhance the system's intelligent video power management. ■