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Last year, consumers bought more than a billion mobile phones, 220 million laptops, 140 million MP3s, 90 million digital cameras (DSCs) and 10 million personal navigation devices (PNDs). According to the internal system architecture, all of these devices have certain commonalities. First, they are all battery-powered, usually using a lithium-ion battery (Li-Ion) as the main power source, and using another input power source as a backup or for charging. Second, they all have built-in storage devices, usually including some kind of ROM, RAM or NAND flash, and in many cases there is a hard disk (HDD) or SDIO card. According to the latest research by IDC, a technical research institute, 161 billion GB of digital information was produced worldwide last year. This is equivalent to the need to use 2 billion iPods to store this information.
However, another type of product not mentioned above is a product in which two or even three of the above product functions are combined, such as a portable media player (PMP) or a digital media broadcasting (DMB) product. These products also use lithium-ion batteries as the main power source and have a large storage capacity. They are becoming important playback devices in the consumer electronics arena.
A key advantage of PMP or DMB products is that they can play MP3 and MP4 formats. Therefore, you can enjoy music and movies downloaded from DVD-CD or downloaded from the website with one device. Typically, the device's storage medium can store more than 150 hours of video or 1200 hours of music. But like any other battery-powered handheld device, manufacturers of these PMP devices are under unprecedented pressure to integrate many functions into a size and shape-constrained structure while still providing longer work. time.
Since most PMPs have the functions of video playback and MP3 playback, the internal circuit requires multiple low voltage rails with different power levels. The reason is clear because most digital LSIs operate at 1.5V or less. At the same time, the required voltage for memory and I/O is 2.5 to 3.3V. Therefore, it is unrealistic to use a multi-point-of-load (POL) DC/DC converter to directly convert the voltage from a lithium-ion battery. System designers must adopt more integration schemes.
Most battery-powered handheld devices use an integrated integrated circuit (ASIC) to handle battery charging, power channel control, providing multiple supply voltages, and protection features such as actual output open and precision USB current limits. The purpose of this approach is clear, that is, one device can be used to meet all power management requirements. However, this solution also has some shortcomings. First, ASICs are manufactured using a specific wafer fabrication process, and it is very difficult to achieve optimal performance for each function. Second, the design of the short dynamic design cycle becomes more important because the definition of the ASIC and the lead time caused by the development of the ASIC are too long. In general, a power management ASIC takes more than a year and a half from concept to delivery. During this cycle, a particular product design may have changed three or more times.
Customized standard products for power management applications
Most battery-powered handheld devices can usually be powered by an AC adapter, a universal serial bus (USB), or a lithium/polymer battery, but how to achieve power channel control between these power supplies is a big deal. Technical challenges. Until recently, designers were trying to use a separate approach, using a set of MOSFETs and operational amplifiers to achieve this, but they faced big problems such as hot swapping and large transient currents. It will cause a lot of system problems.
There are certain commonalities in the function and performance of various battery-powered handheld devices, which can use Application Specific Standard Products (ASSP) without the performance trade-offs associated with IC manufacturing in single-wafer manufacturing processes. . Linear has recently developed a new generation of this product, the LTC3555, which represents a new level of performance and functionality in this class of applications.
The LTC3555 seamlessly manages the AC adapter, power flow between USB and Li-Ion batteries, is USB compliant, and all parts are packaged in a 4 x 5mm QFN. It seems that this is not enough. It also comes with a full-featured Li-Ion/Polymer battery charger that can deliver up to 1.2A of charging current, plus three low levels needed to produce the vast majority of USB peripherals. High efficiency synchronous buck converter for voltage rails. In addition, the LTC3555 provides a constant 25mA low dropout linear regulator to power real time clock (RTC) and low power logic. The entire device can be controlled via a simple I 2 C interface or a simple I/O port.
Figure 1: Simplified block diagram and schematic of the LTC3555.
The application circuit diagram of LTC3555 is shown in Figure 1. The figure shows the implementation principle of multi-function. The DC/DC converter is a relatively simple step-down converter. The LTC3555's three on-chip buck converters operate in current mode control with up to 95% efficiency, with I 2 C or core select trigger burst mode or automatic trigger burst mode. The DC/DC converter has a switching frequency of 2.25MHz, allowing the use of very small external capacitors and inductors. The output currents of these buck converters are 1A, 400mA and 400mA, respectively, and the output voltage is programmable from 0.8 to 3.6V.
The power supply method of the LTC3555 is different from the existing battery and power management ICs, and is actually a charge-fed system. In a typical power management IC, the external power supply does not directly power the load. Instead, the AC adapter or USB port charges the battery and then powers the load. There is a delay in powering the load if the battery is over-discharged or there is no power at all. This is because the power cannot be removed directly from the battery until the battery has the minimum amount of charge required. With the LTC3555, this delay can be eliminated and the handheld device can be powered as soon as the wall adapter or USB is plugged in. In addition, the chip can take away unused power from the load and use it to charge the battery.
These two advantages (ie, eliminating the charge delay and simultaneously charging and powering the load) extend the effective working time and speed up charging when connected to the USB. Another advantage of this power management technology is increased efficiency, as long as there is AC or USB power. In this case, unwanted conversion stages (for battery charging) can be eliminated.
High efficiency switching power supply channel controller
Unlike the previous generation LTC3455, which has a linear power channel controller, the LTC3555 has a high efficiency switch mode power channel controller. Designed specifically for USB applications, the LTC3555's power channel controller incorporates a precision average input-down-switching regulator that maximizes the available USB power. Because the power is saved, the LTC3555 allows the load current on VOUT to exceed the current drawn by the USB port, but does not exceed the USB load specification. The power channel switching regulator communicates with the battery charger to ensure that the input current does not exceed the USB specification limits. Furthermore, the ideal diode from BAT to VOUT ensures that power can always be delivered to VOUT, even if there is not enough power or there is no power at all on VBUS.
Figure 2: Block diagram of the LTC3555 power channel.
When VBUS is available and the power channel switching regulator is activated, the power can be sent from VBUS to VOUT via SW (see Figure 2). VOUT drives the mixed load of the external load (switching regulators 1, 2 and 3 in Figure 1) and the battery charger. If the mixed load does not exceed the programmed input current limit of the power channel switching regulator, VOUT will track 0.3V (above the battery voltage). By keeping the voltage on the battery charger low, efficiency is optimized because the power lost to the linear battery charger is minimized, with the result that the available power to the load is optimized.
If the mixed load of VOUT is large enough to cause the switching power supply to reach the limit of the programmed input current, the battery charger will reduce the charging current by the amount required to meet the external load demand. Even if the battery current is set to exceed the allowable USB current, it will not exceed the USB specification because the switching regulator will always limit the average input current to ensure that this does not happen. Further, the load current on VOUT is always prioritized, and only the remaining power is used to charge the battery.
If the battery voltage is lower than 3.3V, or the battery does not exist, and the load demand does not cause the switching regulator to exceed the USB specification, VOUT will drop to a value between 3.6V and the battery voltage. If the battery does not exist and the load exceeds the available USB power, VOUT will fall to ground.
The LTC3555 contains an ideal diode (see Figure 2) and a controller for an optional external ideal diode. This ideal diode controller is always on and will respond quickly when VOUT is below the battery voltage. If the load current increases beyond the power allowed by the switching regulator, the power is drawn from the battery through the ideal diode. In addition, if the power to VBUS (USB or wall adapter) is removed, all application power will be supplied by the battery via the ideal diode. The conversion from input power to battery power at VOUT is very fast, allowing only one 3uF capacitor to be used to avoid VOUT drop. This is possible because the ideal diode includes a precision amplifier that activates a high-power, on-chip P-channel MOSFET transistor when the voltage on VOUT is about 15mV (VFWD) lower than the battery voltage. The internal ideal diode has a resistance of approximately 180mΩ, which can be reduced to 50mΩ with an external resistor.
It is clear that for designers of battery-powered handheld devices, there are many options to ensure that battery life is optimized for their particular application. A performance-optimized, versatile ASSP provides the voltage or power level needed to achieve optimal system functionality while ensuring that battery leakage is minimized during normal operation.
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