USB power charging design tips and considerations

Almost all current and future handheld devices require both USB power and AC wall adapter charging, which presents many challenges for battery charging. In this article, we will discuss some of the features that simplify USB charging. Specifically, what features can help users design an application that complies with the USB specification? How can the design support both USB and AC adapter input? We will discuss quiescent current, input voltage dynamic power management (VIN-DPM), and input current limit. The following uses the bq2407x family of devices as an example to illustrate these features. Introduction

Consumers want fewer cords and wires for their devices and want the flexibility of charging from a computer or AC outlet, requiring that nearly all current and future handheld devices be able to charge from both a USB power source and an AC wall adapter. USB requirements present many challenges for battery charging. The requirement to use the same cable and the resulting single input for USB and AC adapter means that the system must be able to recognize and use both power sources. In this article, we use the bq2407x family of devices as an example to introduce some of the features and specifications that simplify the design of lithium-ion (Li-Ion) battery charging systems for portable applications. Specifically, we will discuss input current limit, quiescent current, input voltage dynamic power management (VIN-DPM), and the impact of the Chinese charger specification. These features simplify the design of chargers for portable applications. Input Current Limit

The most widely recognized USB specification is the current consumption of USB devices. USB power supplies are divided into low power and high power supplies. Low power supplies are defined as non-self-powered USB hubs. These devices use 500mA of current from the USB power supply and must share the current between all devices connected to the hub. Low power supplies provide a minimum operating voltage of 4.4V, and their current limit is set at one unit load, which is 100mA under USB1.x and 2.0. For SuperSpeed ​​USB, the unit load is increased to 150mA. High power supplies are some self-powered hubs or computer USB ports that provide a minimum operating voltage of 4.75V and have a current of up to 5 unit loads, which is 500mA per port under USB1.x and 2.0, or 6 unit loads (900mA) under SuperSpeed ​​USB. All devices default to low power mode, but will request high power mode when enumerating with the host. In addition to the high-power and low-power ports, many devices operate from an AC adapter using the same connector. These power sources can provide up to 1.5A of current (USB cable capability). The final mode is suspend mode when the device cannot use more than 2.5mA of current. USB devices must support all four modes to meet the USB specification. The input current limit of the charging device cannot exceed the USB specification. This means that you must find the technical description specifications in the data sheet to verify that the current limit is guaranteed to not exceed the USB limit over all process variations and temperatures. Figure 1 is a portion of the bq2407x data sheet that clearly shows the input current limit specification. Note that the maximum value of the specification is the USB limit. This ensures that all bq2407x devices are within the USB specification over temperature, process, and input voltage. The typical value listed for the maximum USB specification limit is not allowed because some parts will exceed this value. If the maximum value is greater than the USB specification, there is a possibility of violating the USB specification.

Figure 1. Input current limit excerpt from the bq2407x data sheet. When the host requires the system to enter USB suspend mode, the specification limit is 2.5mA.

Driving EN1 and EN2 high puts the bq2407x into USB suspend mode. In this mode, the quiescent current is reduced to 50μA (max). This gives the designer more margin if other circuits are running from the VBUS supply. As for the standby mode current specification, it usually appears in the quiescent current section of the electrical characteristics table. Figure 2 shows the quiescent current of the bq2407x. Note that in USB suspend mode (EN1=EN2=HI), the maximum quiescent current is 50μA (VBUS = 6V), which is much lower than the 2.5mA requirement for USB suspend mode.

Figure 2. Excerpt from the quiescent current section of the bq2407x data sheet USB startup problem

The USB 2.0 specification states that "the maximum load allowed downstream of the cable (CRPB) is 10μF when connected in parallel with a unit load (100mA). The 10μF capacitor represents a bypass capacitor directly connected between the VBUS lines, plus the effect of any capacitance seen through the device regulator." [1]. To test this, the USB-IF Test Procedure Document Version 1.3 states that "The USB specification allows for hard starts of up to 10uF, which results in a maximum inrush current value of 50.0uC." [2] 50μC is calculated from Equation 1: How do we convert this into a current limit? Amps are in coulombs per second. This means that during startup, the maximum inrush current above the current limit multiplied by the time above the current limit must be less than 50μC. This is shown in Equation 2: Note that IIN_AVG represents the average input current during the inrush current. The input current limit of the charger device must prevent the current from the USB power source from exceeding 50μC. Figure 3 shows the startup of the bq2407x in 100mA mode. Note that the highlighted area represents the allowed overcharge region. The input capacitance requirement of the bq2407x is 1μF. This exceeds the input capacitance specification. The input current limit allows the designer to not worry about the capacitance value of the system because the USB power source will never see this current.

Figure 3 Input current startup waveform, EN1 = EN2 = GND, VBAT < VWEAKBATT The input capacitance requirement of bq2407x is 1μF.

A hard start of 1μF requires 5μC. The system capacitance for this start is 47μF, which cannot be started directly from the USB port. In terms of input current limit, hard starting the system capacitance and starting to charge is not a problem. The input current threshold is less than the 100mA specification, so after the first start, the USB100 specification will not be violated. Weak Battery Threshold

The USB specification requires that the host be enumerated before the VBUS supply current is greater than 2.5 mA. However, there is a provision in the specification regarding zero, weak, or no battery conditions. The provision is as follows: "A device with zero, weak, or no battery condition needs to provide 100 mA of current approximately 100 ms after connection to determine whether it can connect." [3] If the device cannot start up with 100 mA within 100 ms, then there may be problems. To address this problem, the USB specification adds a specific provision for battery charging. It states that "If a portable device cannot power up and connect with less than 100 mA of current, the zero or weak battery device may use 100 mA provided by the host to first charge its battery to its weak battery threshold. Once its weak battery threshold is reached, the device is immediately required to power up and connect." [4] Above the weak battery threshold, it is assumed that the battery is sufficient to power the host, so the host turns on. Each application defines its own weak battery threshold. The bq2407x's hardware enable and a simple voltage detector allow designers to easily meet this requirement.

Figure 4 shows a simple solution for the weak battery threshold case.

Figure 4. Weak battery detection implementation Weak battery threshold settings must be applied to the voltage detector.

For example, the TPS3836 has several valid thresholds. In addition, for maximum flexibility, some voltage detectors offer adjustable thresholds. For this application, the important voltage detector feature is an active low RESET (low when VIN < VTHRESHOLD) push/pull output so that it can be isolated from the host output. Once the host appears, it turns off the voltage detector, or stops pulling up. The pull-down force of the host output must be greater than some resistance isolating the voltage detector output from EN1 and EN2. Figure 5 shows the implementation waveform. The weak battery threshold is set to 3.3V. When a 3.5V battery is inserted, it is recognized as a good battery and EN1 and EN2 are pulled high by the TPS3836. After enumerating the host, the host pulls EN2 low to set the bq24072 battery charger to USB500 mode. This method assumes that the HOST GPIO is high impedance when the HOST is off.

Figure 5: Example of weak battery implementation Input voltage-based dynamic power management (VIN-DPM)

The USB specification states that the output of low-power port devices can go as low as 4.4V under full load conditions after passing through all hubs and cables. Some devices implement input voltage-based dynamic power management (VIN-DPM). This loop reduces the input current limit to prevent input collapse. Figure 6 shows the consequences of overloading a USB port without VIN-DPM protection. Note that the charger shuts down when the input voltage drops below the “power good” threshold. This turns off the load from the power supply and allows the input voltage to recover, turning the charger back on. This on/off pulse is redundant.

Figure 6 Input Collapse without VIN-DPM VIN-DPM prevents input supply collapse by limiting the input current to stop the pulses from occurring.

Figure 7 shows the result of USB port overload when using the bq2407x charger. The VIN-DPM function takes effect to reduce the input current limit and prevent the power supply from collapsing.

Figure 7 Input overload protection using VIN-DPM Chinese charger standard

Looking ahead, China has developed a set of standards for charger adapters sold in China, aiming to reduce the number of discarded adapters for more than 100 million retired handheld devices each year. This new standard uses many USB specifications. In this regard, it requires that the adapter power cord has a standard USB Type A connector to plug into an AC adapter or a standard USB port. The charger must be able to provide between 300mA and 1.8A. The nominal adapter voltage is 5V +/–5%, and the charger must operate from a power supply below 6V. Downstream circuitry must be protected when the power supply exceeds 6V. Many battery charger ICs have features that help meet the Chinese charger standard. As the term "universal charger" implies, the input of the charger IC must be robust enough to handle many different power supplies and no longer be designed for a single specific adapter. They must be able to withstand the accidental connection of high-voltage power supplies (such as: 12V car power supply, etc.). The wide input voltage range of devices such as the bq2407x can protect downstream equipment from input transient voltage conditions up to 28V. This high input voltage range and overvoltage protection (OVP) function protects the battery charger and downstream devices from damage caused by erroneous or potentially harmful input power. The VIN-DPM feature also helps to meet Chinese charger standards. Universal adapters can provide currents between 300mA and 1.8A. When using a device without VIN-DPM, a 300mA adapter will collapse if the IC is programmed for a 500mA input current limit. The VIN-DPM feature prevents the input from collapsing when a weak adapter is connected, and still allows the current limit to be set for a typical adapter to maximize charging time.

in conclusion

Consumers want fewer power cords and connections for their devices, and want the flexibility of charging from a computer or AC outlet, so many current and future handheld devices are required to be charged from both USB power and AC wall adapters. As a result, handheld devices must comply with USB specifications. These requirements bring many new challenges to battery charging. In this article, we use the bq2407x family of devices to introduce some examples of input current limit specifications, quiescent current, and input voltage dynamic power management (VIN-DPM), which simplify the design of battery chargers. In addition, we also explore the impact of China's charger standards on charger design. These specifications and features we briefly introduce simplify charger design.