The demand for storage capabilities in mobile phones is growing rapidly. This article explores the evolution of memory and several different ways to power SD flash memory.
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The capture, display, and sharing capabilities of video and image content are driving the need for memory. Capture capabilities are rapidly improving due to the advent of compact camera optics, highly integrated image processing, and advanced flash technology, with LED solutions with ever-increasing brightness and a shrinking twilight solution. Thin and efficient touch screen LCD, AMOLED and ultra-AMOLED solutions, combined with an intuitive and convenient user interface, make the display of images and video images simple and vivid. In addition, ubiquitous network connectivity, various social networks, and the existence of a large number of downloadable content sources have accelerated the sharing of features.
These technologies are driving handset manufacturers to design camera phones with image resolutions of 8 megapixels or even 12 megapixels. In terms of video, mobile phones with shooting performance of 720p@30fps are also available. These are memory intensive applications whose contents are typically stored in solid state memory.
Although digital cameras have historically required large storage capacity, for mobile phones, this requirement has only become important in recent years. Fortunately, the density and capacity of solid-state memory is gradually increasing, and its price ($/MB) and area (mm2/MB) are greatly reduced. As can be seen from Figure 1, in 2011, the manufacturing process geometry of NAND flash memory may exceed 25nm.
Figure 1: Trends in the manufacturing process geometry of NAND flash technology.
For consumer storage applications, the most common solid state memory format is SD flash. The current SD flash memory is based on NAND technology and has three external dimensions, as shown in Figure 2, which are SD card, mini SD card and micro SD card. Although the three form factor products are different in size, their interfaces are the same.
In addition to the form factor, SD memory can be classified by capacity. The standard SD card can provide up to 2GB of capacity, the large capacity (SDHC) card capacity is 2 to 32GB, and the eXtended capacity (SDXC) card is as high as 32GB to 2TB.
In addition, SD memory has two types of clock frequencies. In the default mode, the memory can operate at a clock frequency of 0 to 25 MHz with an interface speed of 12.5 MB/s (using 4 parallel data lines). In high-speed mode, the memory can operate from a clock frequency of 0 to 50 MHz with an interface speed of 25 MB/s (using four parallel data lines). System designers can optimize this parameter based on the required read and write speeds, which becomes important when discussing power considerations.
Figure 2: The size of the SD card, mini SD card and micro SD card.
The SD specification version 2.0 requires an operating voltage range of 2.7 to 3.6V. 2.7V is the minimum voltage required to ensure performance. The operating voltage is acceptable from 2.7V to 3.6V, but if it exceeds 3.6V, performance is not guaranteed. And the memory may be damaged.
Another consideration in power supply design is the power consumption of the memory. The memory has four main working modes: off, standby, read and write. It is in different states and consumes different power. For a given memory card, the power consumption of the read/write state is also related to the read and write speed. A normal card supports a clock frequency of up to 25 MHz, and a high speed card supports a clock frequency of up to 50 MHz. As storage capacity continues to increase, a faster clock frequency is required to ensure proper consumer usage patterns.
For low-capacity SD cards operating at low speeds, power consumption is often less than 100mA. Assuming that the lithium-ion battery is rated for 3.7V and the power rail of the memory is quite high, the linear regulator becomes the power supply for the SD memory. However, the selected linear regulator must be capable of operating in low dropout (LDO) mode because the effective voltage range for lithium ion batteries is between 3.2 and 4.2V.
With the introduction of 8, 16 and 32GB high-speed SD cards, the power consumption of 300 to 400 mA is not uncommon, and these currents are significantly larger than the 100 mA level of small-capacity cards. Due to these higher current demands, the power consumption of LDO power solutions has begun to increase significantly. Figure 3 shows an LDO regulator that provides a typical 2.9V voltage to the SD card. The 2.9V is chosen here because the LDO is assumed to ensure an output of no less than 2.7V under any line, load or temperature conditions.
Figure 3: LDO with 2.9V for the SD card.
Table 1 lists the conversion efficiencies of the LDO shown in Figure 3 at 300 mA and 400 mA output current. The calculated LDO conversion efficiency is 78%, so the power consumption is 240mW and 320mW, respectively.
Table 1: Power Efficiency vs. Power Comparison of LDO with 6MHz Buck Converter and 3MHz Buck Converter.
However, many system designers believe that 320mW or even 240mW of power consumption is unacceptable. Fortunately, switching converters as shown in Figure 4 can now be used to achieve higher power conversion efficiency.
Figure 4: The FAN5362 buck converter implements a 2.9V SD card power supply circuit.
According to the calculation in Table 1, the FAN5362 can reduce the power consumption to 55mW for a 300mA system, and the FAN5362 can reduce the power consumption to 101mW for a 400mA system. These efficiency values ​​were obtained based on the measured FAN5362 efficiency curve. Figure 5 shows the efficiency curves for AutoPFM (solid line) and ForcePWM (dashed line). While optimizing the efficiency of the FAN5362, 3MHz is chosen as the nominal switching frequency because it provides the best trade-off between size and efficiency. As can be seen from Table 1, for this power-sensitive application, the power consumption at 6MHz switching frequency is much greater than the power consumption at 3MHz switching frequency.
Figure 5: FAN5362 efficiency vs. load current, AutoPFM is solid and ForcePWM is dashed.
Although it makes little sense to choose a buck converter to replace the LDO, it is important to consider that the buck converter must be able to operate at very high duty cycles. If the output of the buck converter is set to 2.9V and the battery voltage is as low as 3.3V, the buck converter operates at 88% duty cycle. Under certain load and input voltage conditions, the buck converter will even be forced to stop switching and operate at 100% duty cycle. If the phone starts to emit GSM pulses, this situation will become more severe at low battery voltages (VVBAT). The GSM pulse can be as high as 2A, and during these pulses, the output impedance of the lithium-ion battery drops the battery voltage by 400mV. For LDOs, a sudden drop in VVBAT is beneficial because LDOs always work in linear regions. But for buck converters, the situation is different, because they must gradually change from the switching state to 100% conduction, once the battery voltage returns to 3.3V, it will return to the switching state again. During this time, the high-side device is fully turned on, and the output voltage of the buck converter is only VVBAT– RDS(ON) *I – DCR*I, where RDS(ON) is the on-resistance of the high-side FET and DCR is the inductor. The serial impedance, I is the memory load current.
The FAN5362 is specifically designed to handle the minimum overshoot/undershoot described above. In addition, the FET control mechanism and RDS(ON) have been carefully designed to ensure that the output voltage is never less than 2.7V, even including line and load transient response. This is critical for memory because the SD specification version 2.0 requires an operating voltage range of 2.7 to 3.6V.
While advances in process geometries have met the need for ultra-compact, low-cost SD memory, this high-capacity device also introduces power consumption issues. This problem can be solved by replacing the current LDO with products such as the buck converter FAN5362 designed specifically for this type of application. Figure 6 is a schematic diagram of the typical principle and PCB layout of the complete FAN5362 power solution. (Fairchild Semiconductor)
Figure 6: Schematic diagram of the typical principle of FAN5362 and PCB layout.
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