Consumers want fewer power cords and cables 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 adapters 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 that uses the same connector. These power supplies can provide up to 1.5A (the USB cable capability). When the device cannot use more than 2.5mA, the final mode is suspend mode. 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 specification in the data sheet to verify that the current limit is guaranteed to not exceed the USB limit under all process variations and temperatures. Figure 1 is a portion of the bq2407x data sheet, which clearly marks 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 at 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. Excerpt from the input current limit of the bq2407x data sheet

When the host requests 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 boot problem

The USB 2.0 specification states that "the maximum load allowed at the downstream end of the cable (CRPB) is 10μF in parallel with a unit load (100mA). The 10μF represents a bypass capacitor directly connected between the VBUS lines, plus the effect of any capacitance seen through the device regulator." [1]. To test for this, the USB-IF Test Procedures document version 1.3 states that "The USB specification allows for up to 10uF hard start, which results in a maximum surge current value of 50.0uC." [2] 50μC is calculated using Equation 1:

How do we convert this to a current limit? Amps are measured 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 source from exceeding 50μC. Figure 3 shows the startup of the bq2407x in 100mA mode. Note that the highlighted area represents the allowed overshoot charging 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 source will never see this current.

Figure 3 Input current startup waveform, EN1 = EN2 = GND, VBAT < VWEAKBATT

The input capacitance requirement of the bq2407x is 1μF. A hard start of 1μF requires 5μC. The system capacitance for this startup 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 it is not a problem. The input current threshold is less than the 100mA specification, so after the first startup, 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 2.5mA. However, there is a provision in the specification regarding zero, weak, or no battery conditions. It states: "A device in a zero, weak, or no battery condition needs to provide 100mA of current approximately 100ms after connection to determine if it can be connected." [3] If the device cannot start with 100mA within 100ms, this may cause problems.

To address this problem, the USB specification added a specific provision for battery charging. It states that "If a portable device cannot power up and connect with less than 100 mA, the device with a zero or weak battery may first charge its battery to its weak battery threshold using 100 mA provided by the host. Once its weak battery threshold is reached, the device is required to power up and connect." [4] Above the weak battery threshold, the host is assumed to be able to power the battery sufficiently, so it turns on. Each application defines its own weak battery threshold. The hardware enable of the bq2407x and a simple voltage detector make it easy for designers to meet this requirement. Figure 4 shows a simple solution for the weak battery threshold case.

Figure 4 Weak battery detection implementation

The voltage detector must be set for weak battery thresholds. 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 strength of the host output must be greater than some resistor that isolates the voltage detector output from EN1 and EN2.

Figure 5 shows the implemented waveforms. 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 turned off.

Figure 5 Example of low-capacity 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 the input supply from collapsing by limiting the input current to prevent pulses from occurring. Figure 7 shows the result of an overloaded USB port when using a bq2407x charger. The VIN-DPM function takes effect to reduce the input current limit and prevent the supply from collapsing.

Figure 7 Input overload protection using VIN-DPM

China charger standard

Looking ahead, China has developed a set of standards for charger adapters sold in China, aimed at reducing 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 rated adapter voltage is 5V +/–5%, and the charger must operate from a power supply less than 6V. Downstream circuitry must be protected when the power supply exceeds 6V.

Many battery charger ICs have features that help meet Chinese charger standards. As the term "universal charger" implies, the inputs of the charger ICs must be robust enough to handle many different power sources and are no longer designed for a single specific adapter. They must be able to survive the accidental connection of high voltage power sources (e.g., 12V car power). The wide input voltage range of devices such as the bq2407x protects downstream equipment from input transient voltage conditions up to 28V. This high input voltage range and overvoltage protection (OVP) protect the battery charger and downstream equipment from damage caused by erroneous or potentially harmful input power supplies.

The VIN-DPM feature also helps meet Chinese charger standards. Universal adapters can provide between 300mA and 1.8A. Using a device without VIN-DPM will crash a 300mA adapter 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.