Power Management for Video Doorbell

Abstract

IoT has become more and more important in our life. The doorbell is one of the IoT equipment. Traditional doorbells are only available for communication or video display, but the modern doorbell can provide many functions including video recording, RF connection (WiFi, BT). These functions bring our life more convenience than before. This document introduces the system overview, different power source with pros and cons, and power management selections for video doorbells.

About the power management, this document introduces the power delivery of USB Type-C, battery charger, and gauge for battery management, and buck/boost DC-DC converters for increasing the system efficiency. These power management solutions can help increasing the reliability and lifetime of doorbells.

1. Introduction of Video Doorbell

The traditional doorbell is only available for communication and video display when someone presses the doorbell. However, as an IoT equipment, the modern video doorbells have many features, such as video recording, monitoring the outdoor environment all day, and recording the video in memory cards or a cloud drive, as shown in Figure 1. Moreover, the doorbell can be remote controlled by mobile devices through RF circuits. When users are not at home, they can talk to visitors via mobile phone.

Generally, there are two considerations before installing a video doorbell. The first is the power source and the other is the internet. Due to the popularity of Wi-Fi and BT, the internet connection of a video doorbell is not a problem. As for the power source, if the power outlet is not designed in advance, the battery-powered solution is the most convenient way to install because no extra cable required.

At normal usage condition, the battery life of battery powered smart doorbell is about six months. How to extend the battery life becomes an important consideration.

With so many functions in the modern doorbell, the power consumption is more than traditional doorbell. Too much heat in the doorbell will cause the reliability reduction and shorter lifetime, so the power management is very essential.

Figure 1. Function of Video Doorbell

Figure 2 is the function block of a video doorbell. The power source of doorbells can be an AC power source, battery only, or both. The camera sensor is for video recording. Video recording can be set recording all day or recording after this function enabled. The PIR sensor can detect the human motion. When a stranger stands in front of a doorbell, the PIR sensor detects the motion and then enable the system. This function can help decrease system power consumption. Flash memory is for the data storage. it can store the video recording data and overwrite the old data to continuously record the new video without interrupt. Video data can be uploaded to the cloud drive too. Speaker and microphone provide communication in the doorbell. WiFi is the key function of this product. WiFi helps users to connect to the video doorbell by mobile phone. Thus, we can know the outdoor situation, or communicate with visitors in front of the doorbell though mobile phones. Furthermore, it can notify us when something is abnormal or out of order. WiFi is the key function of IoT equipment.

Figure 2. Power Tree of Battery Powered Video Doorbell

2. The Power Source of Doorbell

There are three kinds of power sources for the video doorbell, one is 12VAC to 24VAC source, another is the battery, and the other is the combination of AC source and battery. 12VAC to 24VAC source is from the distribution board of houses and it is a permanent power source. The most important design consideration of AC source is the system efficiency. We can get the better thermal performance and save electricity by increasing system efficiency.

The other source is the battery. The main advantage of battery powered doorbells is easy install, no extra cables necessary to connect. However, the quiescent and shutdown current have to be very small to extend the usage time. Otherwise, the battery will run out quickly and users have to charge the battery frequently. Battery power only will also limit the function due to the need for low power design.

The last kind of power source is the combination of AC source and battery. During normal operation, the doorbell is supplied by AC source. The doorbell will change to battery supply mode in case the power outage happens. The back-up battery means a battery charger is necessary, which means more cost.

Table 1. Comparison between Three Kinds of Power Source

Power Type	Pros	Cons
AC	Permanent power High system performance	Need AC-DC adapter Install needs cable connection
Battery	Free location Easy installment	Need to change the battery Performance would be limited due to low power design
AC + Battery	Battery can be backup during power outage	Need AC-DC converter Install needs cable connection High cost

 

 

 

 

 

3. Power Management

3.1. Switching Charger

A switching charger is necessary for the doorbell with combined power source. When the AC source is present, the switching charger can charge the battery and provide power to system. If the battery fully charged, the power management feature can supply the system load from the AC source. The battery will not be charged or discharged frequently, which extend the battery life cycle. Once the power outage happens, the doorbell can be supplied by the battery. The doorbell can work normally all the time.

The switching charger, RT9471 can achieve the goals above. Moreover, the RT9471 can support 3.9V to 13.5V input voltage, maximum 3A fast-charge to reduce the charge time, minimum 50mA pre-charge current when the battery is deeply discharged.

The RT9471 Efficiency is up to 92% at 5V input to 4.2V battery with 2A charge current to prevent too much heat in the doorbell, as shown in Figure3 and Figure 4.

Figure 3. RT9471 Charger Efficiency

Figure 4. RT9471 Power Loss vs. Charging Current

RT9471 features battery charging with high regulation voltage accuracy, only ±0.5% tolerance. This improves the battery charging safety and the fully charged battery capacity (SOC) will be about 2% higher than that with charge voltage accuracy of ±1%, as shown in Figure 5.

Figure 5. SOC Compared with Competitor

3.2. Battery Gauge

The smart doorbell is usually in standby mode and consumes low quiescent. Occasionally, when a guests visits and presses the doorbell, the smart doorbell switches to normal operation and suddenly consumes large current. Monitoring the battery status during light and heavy load and reporting battery capacity is very important.

The smart doorbell is an outdoor product. In tropical countries, the ambient temperature is always hot; in cold countries, ambient temperature can be quite low; and in countries with desert climate, the ambient temperature fluctuates considerably day and night. No matter which ambient condition, the battery status and capacity all have to be monitored accurately by the battery gauge IC.

Employing Fuel Gauge IC for Battery Management

In view of the above mentioned application challenges, Richtek’s fuel gauge RT9426A, is the solution. RT9426A is a Li-Ion and Li-Polymer battery fuel gauge IC for single-cell applications. It provides accurate voltage measurement with error within ±7.5mV and current measurement with error within ±1%. The best capacity estimation error is within 1%. The RT9426A power consumption is 14µA for normal mode and 5µA for sleep mode. Figure 6 is a simplified application circuit for RT9426A.

Figure 6. RT9426A Simplified Application Circuit

Figure 7 shows the performance of capacity estimation for a wearable device with 7-days usage situation. (4.4V/230mAh) Figure 8 shows the corresponding battery voltage & current profile of Figure 7. There is no accumulation error on SOC estimation during long system suspend and light load period.

Figure 7. Performance of Capacity Estimation for Wearable Device with 7-Days Usage

Figure 8. Battery Voltage & Current Profile of Wearable Device with 7-Days Usage

Figure 9. RT9426A Power Mode vs. System Loading

Monitoring Battery Status and Capacity with Ultra-Low Power

The common battery capacity for smart doorbell is around 5200mAh. Considering a battery life of 6 months, the RT9426A could save 3% system power consumption and extend about 139 hours compared with similar competitor solutions which have Iq in the range of 50uA. Figure 9 shows RT9426A will change its power mode according to system loading.

Accurate Capacity Estimation under Dynamic Load

As mentioned above, smart doorbells usually operate in standby mode with low quiescent. When the doorbell is activated, a large dynamic load will required by the system. The RT9426A is equipped with VGCS (VoltaicGauge^TM with Current Sensing) algorithm. It provides accurate battery capacity report by battery current sensing in short-term. While the smart doorbell is in standby mode, light load is required by the system, RT9426A estimates the battery capacity according to the difference and trend of battery voltage in long-term.

RT9426A enters sleep mode automatically while detecting the system is in standby mode. It helps extending the battery life. When the system switches to normal operation and the load increases, RT9426A switches to active mode automatically. This feature satisfies the system behavior of smart doorbell.

Compact and Min BOM

The RT9426A uses a tiny package of WL-CSP. Minimal BOM application includes only one sense resistor and one capacitor, so it’s easy to integrate to system main board.

Supporting Temperature Compensation and Battery Aging Correction

The RT9426A supports ambient temperature compensation when estimating battery capacity. By temperature compensation, battery capacity under different ambient temperature will be smoothly and continuously reported. Figure 10~13 show the SOC performance at 25°C/40°C/5°C. Moreover, the RT9426A monitors battery health and corrects battery capacity when battery aged. As a result, the RT9426A always keeps high accuracy of battery capacity report.

RT9426A compensates battery capacity estimation for the degradation of battery. It controls the capacity error within 2%. Figure 14 to 15 show FCC & SOH performance of 450 cycles usage. Referring to the Figure 16 below shows an example of 4.4V/2700mAh battery. The SOC error stays within 2% after 450 cycles of usage.

Figure 16. SOC Performance of 450 cycles usage

3.3. DC-DC Converter

In this chapter, the DC-DC solutions for the video doorbell application are proposed, and different cases of power solution are discussed. It shows an overview of recommended power solutions and their key features for the video doorbell application.

• RT4813 : High Efficiency Boost Converter

Figure 17. RT4813 Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent :120µA

      - Small Size : UQFN-9L 2x2mm²

      - High Efficeincy : 93.9%@5V/6mA

      - Wide VIN Range from 1.8V to 5.5V

• RT5707 : Ultra-Low Quiescent Current HCOT Buck Converter

Figure 18. RT5707 Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent : 0.46µA

      - Small Size : WL-CSP-8B 0.9x1.6mm²

      - High Efficeincy : 94.6%@1.8V/180mA

      - High Efficeincy : 87.8%@1.2V/120mA

      - Internal Compensation for Optimized Transient Response

• RT5713 : Tiny, Ultra-Low Quiescent Current HCOT Buck Converter

Figure 19. RT5713 Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent : 0.46µA

      - Small Size : WL-CSP-6B 1.415x0.885mm², WDFN-6L 2x2mm²

      - High Efficeincy : 85.4%@1.1V/280mA

      - High Efficeincy : 82.4%@0.8V/40mA

      - Internal Compensation for Optimized Transient Response

• RT6160A : Low Quiescent, High Efficiency 3A Buck-Boost Converter with I²C Interface

Figure 20. RT6160A Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent : 2μA

      - Small Size : WL-CSP-15B 1.4x2.3mm²

      - High Efficeincy : 95.2%@3.3V/1.3A

      - 1μA Non-Switching Low Quiescent Current

      - Ultra-Sonic Mode Operation to Avoid Acoustic Noise

      - Automatic PFM Mode and Forced PWM Mode Selection

• RT9086 : 250mA, Ultra-Low Noise, Low Quiescent Current, LDO Regulator

Figure 21. RT9086 Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent : 16μA

      - Small Size : WL-CSP-4B 0.67x0.67mm², ZQFN-4L 1x1mm², SOT-23-5

      - High PSRR : 85dB@1kHz

      - Very Low Dropout : 120mV

      - Excellent Noise Immunity

      - Stable with a 1μF Input and Output Ceramic Capacitors

• RT8092 : 3MHz 4A High Efficiency Step-Down Converter with I²C Interface

Figure 22. RT8092 Block Diagram, Simplified Circuit and Package

♦Feature

      - Low Quiescent : 15μA (Low Power Mode)

      - Small Size : WL-CSP-15B 1.2x2mm², WQFN-14L 3.5x3.5mm²

      - High Efficeincy : 82.28%@0.75V/1A

      - Auto-PSM/PWM or Force-PWM Selectable

      - Support Remote Ground Sensing for Accurate Output Voltage

Section 1 : How to Improve System Efficiency by Choosing Proper Converter

In this section, we introduce the system power design to increase more efficiency. The power source is the battery with typical voltage 3.8V, and the capacity is about 5200mAh. The power tree is as shown in Figure 23. Due to the use of some LDO’s, this topology is not the best structure to get high system efficiency. RT4813, RT6160A, RT9086, RT5707, RT5713 and RT8092 are proposed in this power solution.

• Topology 1

Topology 1 is the power tree and the system efficiency is 86.8%.

Power converter : 1Boost + 1Buck-Boost + 4Buck + 3LDO

Figure 23. Power Tree of Topology1

Table 2. Efficiency of Topology 1

• Topology 2

In Topology 2, we replace RT9086 with RT5707 to improve the efficiency. This power rail’s efficiency increases from 66.6% to 87.8%. Furthermore, the WiFi power is supplied by RT6160A, which is introduced in section 2. By adjusting the power converter, the system efficiency is about 90.8%, which improves 4% of the system efficiency.

Power converter : 1Boost + 1Buck-Boost + 4Buck + 2LDO

Figure 24. Power Tree for Topology 1 Changing to Topology 2

Table 3. Efficiency of Topology 2

• Topology 3

In topology 2, the efficiency of power rail for 0.8V is only 21.1%. To improve the efficiency, we replace RT9085A with RT5713 in topology 3, and the efficiency increases to 82.4%. As a result, the system efficiency increases to 92.5%.

Power converter : 1Boost + 1Buck-Boost + 5Buck + 1LDO

Figure 25. Power Tree for Topology 2 Changing to Topology 3

Table 4. Efficiency of topology 3

We can estimate that how much the battery lifetime can be extended after the efficiency increases. With the battery with 5200mAh capacity as power source, the watt hour is 19.8Wh. The result is shown in Figure 26. The topology 3 has the longest battery life. Assuming that the doorbell is activated for 10s each time and 5 times a day, the battery life with topology 1 is about 180 days. With topology 3, the battery life can be extended with more than 10 more days.

Figure 26. Battery Run Time Comparison between Different Power Topologies

Table 5. System Comparison

Section 2 : How to Select a Suitable Converter for 3.3V WiFi Application

In this section, the power converter selection for the 3.3V battery-powered WiFi application is introduced. Choosing a converter that is able to support wider battery voltage range is very important to extend the battery lifetime. Here are three types of converters for regulating 3.3V. One is a buck converter, another is a boost converter series with a LDO, and the last is a buck-boost converter.

• Topology 1

Generally, the Li-Ion battery's useful voltage range is from 3V to 4.35V, which depends on the battery type. Selecting a buck converter to regulate 3.3V for WiFi application, if the battery voltage is lower than 3.3V, the stable WiFi function cannot be maintained.

Figure 27. Buck Converter for WiFi

• Topology 2

If a boost + LDO is chosen to regulate 3.3V, full range of battery voltage can be utilized. This structure can help extending the battery lifetime because it supports a wide battery range. However, this topology would suffer from low efficiency at Vin higher than 3.6V. When Vin is higher than 3.3V, the boost is operated in bypass mode, and the LDO regulates 3.3V. However, the higher voltage drop on LDO will cause lower efficiency. Therefore, this structure is less preferred.

Figure 28. Boost Converter + LDO for WiFi

• Topology 3

If a buck-boost converter is chosen to regulate 3.3V, it has the advantage of wide input range and high efficiency at high Vin. Thus, the buck-boost converter is the most optimal solution for 3.3V application.

Figure 29. Buck-Boost Converter for WiFi

The Curves in Figure 30 are the battery run time estimation versus Vin. Buck converter has good battery run time at high Vin, but it can not work when Vin is lower than 3.3V. Boost + LDO can support whole Vin range, but its battery run time is shorter because of low efficiency at high voltage drop on LDO. Buck-boost converter has the longest battery run time and supports the whole Vin range. So this is the best topology for WiFi application.

Figure 30. Battery Run Time vs. Input Voltage

Section 3 : Why We Need High PSRR LDO for Camera System

Figure 31 shows the power rail of ISP (Image sensor processor) for camera system. There are three rails, one is AVDD, another is DVDD and the other is VIO. Among them, AVDD is related to the image quality. The lower ripple on AVDD, the better image quality.

Figure 31. Block Diagram of Camera System

The following Diagram is the detailed analysis for the AVDD power rail. The image sensor gets the signal and then sends the signal to the ADC through the CDS and the Amplifier. The key factor of image quality is ADC, so the input ripple of ADC has to be as small as possible. Assuming that LDO input ripple is 5mV, and the LDO’s PSRR is 40dB, the LDO output ripple is 50µV. The PSRR of ISP is around 18dB, (a value which is based on different ISP vendors). Through the CDS, the output ripple decreases to 6.3µV. This ripple would be amplifed 30dB to the ADC by the internal amplifier of the ISP. Finally, the ADC’s input ripple will be 200µV, which just meets the ADC’s LSB resolution. Assuming that ADC’s reference voltage is 0.8V and the resolution is 12bit, if the voltage ripple for the ADC is larger than 200µV, it would result in increased ADC error.

Figure 32. Power Requirement Deduction for AVDD

To achieve low noise, RT9086 with high PSRR and low noise is proposed for the AVDD power rail. RT9086 PSRR is 80dB@10kHz. According to the previous deduction of camera system, RT9086 can meet the ADC’s LSB voltage requirement with 500mV input ripple.

In other words, if buck converter output ripple becomes larger because of the component aging, temperature variation or load transient on this rail, the image quality would not be affected due to high PSRR of RT9086. In addition, RT9086 also has low quiescent design, which is suitable for battery application.

Figure 33. Power Requirement Deduction with RT9086 for AVDD

Figure 34. PSRR and Noise Density of RT9086

However, PSRR becomes worse at frequencies higher than 1MHz. If the LDO input ripple is coming from a buck converter with switching frequency 1.5MHz the ripple of buck converter will affect LDO output noise due to insufficienct PSRR at high frequency. In this case, it is recommended to add a small HF bead between the buck and the LDO. The bead + LDO input capacitor will act as a low pass filter to reduce HF ripple. Figure 35 is test result of adding HF bead to filiter out noise, after adding HF bead, LDO input ripple can reduce to 30mV, which is helpful for LDO output noise result when frequency exceeds 1MHz.

Figure 35. HF Bead Filter out HF Ripple

3.4. Audio Solution

Boost Converter with Low Battery Voltage

A smart amplifier is recommended for the doorbell product because this kind of product uses 1-cell battery, which limits the audio output power level with normal Audio amplifiers. Richtek recommends using RT5509 smart amplifier, which includes a Class-G boost converter as shown in Figure 36. It can supply 4.3W output power @ boosted 9.5V supply, in 8Ω Load, THD < 1% and it can provide several boost voltage levels based on the input signal level to increase the efficiency.