AN002

AN002 AIC | Alldatasheet

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Two-Cell, Step-Up Converter Design for Portable Applications Introduction In recent years, the markets for portable electrical devices, such as the electronic dictionary, palmtop computers, notebook PCs, PDAs and cellular phones has grown rapidly. All of these devices use batteries as the source of power. Due to their limitations of small size and light weight and due to increasingly stringent environmental requirements in product development, the use of small numbers, small size , long -lasting batteries has become a trend. The AIC1630/1631/1633 are very suitable for the application of using 2~4 batteries. This article will focus on the application of AIC1631-5 for 2 batteries in the following description. A set of two alkaline, NiMH, or NiCd battery cells accompanied by a switching converter with 3V, 3.3V, or 5V output voltage is frequently used as the power supply source for portable electronic products. Using two battery cells is usually a result of compromis e between size and battery operating life. Besides the batteries’ own stored energy, two other factors also influence batteries’ operating life: the product’s power consumption and conversion efficiency of the converter. In order to choose the appropr iate switching converter for an electronic product, therefore, 5 key factors need to be considered: (1) The current capacity and regulation of the output should meet the product’s demands. (2) High conversion efficiency. (3) Low power consumption. (4) Squeeze out as much of the battery energy as possible. (5) Small size and light weight. The first factor is absolutely necessary, and the importance of the other four differs in priority depending on the character of the product. For products, which operate relatively high current for extended time periods, conversion efficiency is paramount. For products operate with low current for most of the time, the current consumption of the converter itself must also be low. With regards to size and weight, an 8-pin IC, which does not need a heatsink, with a small inductor, and minimum number of external parts, would be an ideal choice. In the following, we use the AIC1631 DC/DC converter as an example for several different special products usi ng two battery cells to demonstrate how the AIC1631 can be used to make an ideal power source. (+5V) +VOUT+VIN 1N5819 CGND LBO LBI VOUT SGND SW VIN ILIM/SD 100µF 47µH* *Sumida RCH-108 100µF 33K 10µF VIN AIC1631-5 Fig. 1 A High-Efficiency 2-Cell to 5V Converter

Load Current (mA) 0 100 200 300 400 500 600 VIN=1.8V VIN=2V VIN=2.2V VIN=2.5V VIN=3V VIN=3.3V Efficiency (%) Fig. 2 Conversion Efficiency for the 5V Converter Conversion Efficiency Considerations Conversion efficiency is the main consideration for those devices which consume above 2mA most of the time. Fig. 1 shows a high efficiency converter circuit suitable for producing 5V output from two battery cells. The components shown in Fig. 1 are easily obtained low cost common parts. If high-quality parts (such as MMP core inductors) are used, efficiencies of 90% and above can be reached. However, if good output power quality is desired, such as in Fig. 2 or better, at lea st two points need to be observed: (1) A good power plan and a correct layout for the circuit board is a must. The AIC1631 evaluation boards are available for your reference. (2) One must have the correct inductor values. The equivalent series resistance of the inductor must be low and the core must not be saturated under any operation condition . The above example uses the Sumida RCH-108 47 µH inductor, whose coil resistance (RDC) is 0.14Ω . When converting 2 alkaline battery cells to 5V, an inductor of 40-150µH is suggested. For NiCd batteries, a 20 -100µH inductor is suggested. Inductors of less inductance can output a larger current. However, too low inductance would result in serious efficiency loss due to inductor core saturation. The most reliable method, therefore, is to test for the best conditions using a range of suggested values. The lower coil resistance (RDC) be chosen , the better efficiency obtained . For products consuming about 20mA, the R DC should be under 1 Ω . For products with cu rrent consumption at or above 50mA, the R DC should be below 0.5 Ω . The larger the consumption current, the smaller the RDC required. Low Power Consumption Products The quiescent current of the AIC1631 -5 in an no load state is about 200 µA. For special energy-conscious products, some circuit configurations are presented here to illustrate how the AIC1631 can be used: (1) If this DC/DC converter is not required to maintain output voltage, you may simply pull pin 1 (shutdown pin) to ground by using a control s ignal. The AIC1631 will stop functioning and the power consumption drop to less than 10 µA. Normal operation can be recovered by pulling pin 1 high. Generally speaking, this is ideal for systems with backup lithium batteries. Fig. 3 is an example circuit. W hile using the lowest possible amount of power, the circuit makes it possible to retain data in memory even when changing batteries.

47µH 100µF +100µF + 10µF 33K SDN* VIN 1N5819 (+5V) VOUT AIC 1631-5 1N5819 VOUT LBI LBO CGNDSGND SW VIN ILIM/SO ** + * SDN=VIN or VOUT to Operate SDN=GND to Shutdown ** Backup Lithium Battery Fig. 3 Step-Up converter with Shutdown Control (2) If the output voltage requires the AIC16 31, one could add a few external components to make an oscillator to generate control signals for the AIC1631 to perform intermittent operations. This is called “Burst Mode Operation”, and is shown in Fig. 4(a). If CTL shown in Fig. 4(a) is connected to VIN or VOUT, the circuit will function normally with the characteristics similar to the circuit in Fig.1. If CTL is connected to GND, the circuit will enter Burst Mode with an output voltage of 4V, and the circuit itself consumes very low power, as shown in Fig. 4(b). To change the output voltage in the burst mode, simply change the divider ratio of R 3 and R4 in Fig. 4(a). Note that the voltage of normal operation will never be exceeded. 0.01µF 2N3904 1N4148 1N4148 R2 R3 150K 1K47µH 100µF 10µF VIN CTL* VOUT 1N5819 AIC1631-5 VOUT LBI LBO CGNDSGND SW VIN ILIM/SO * CTL =GND in Burst Mode CTL=VIN or VOUT in Normal Mode (a) Load Current (µA) Total Supply Current (µA) 0 50 100 150 200 250 300200 400 600 800 VIN=1.8V VIN=2.5V VIN=2V VIN=3V VIN=3.3V (b) Fig. 4. Low Supply Current, Burst Mode Step-Up Converter (a) Application Circuit (b) Total supply Current vs. Load Current (3) To get the most out of the batteries‘ energy, the DC/DC converter must be able to work at low battery voltage. The circuit shown in F ig. 5 still deliver 30mA at 5V with input voltage at 1.6V. The conversion efficiency, however, is somewhat lower than circuit in Fig. 1. These examples shown above use the AIC1631-5 to produce a 5V output. The AIC1631 and AIC1631 -3 can be used similarly to produce 3.3V and 3V, respectively.

47µH + 10µF + + (+5V) +VOUT +VIN AIC1631-5 1N5819 VOUT LBI LBO CGNDSGND SW VIN ILIM/SO 33K 100µF 100µF Fig. 5 1 Cell to 5V Step-Up Converter 2~4 Cells to 5V Low Noise Power Supply The AIC1631 -5 can be used to convert 2 -4 Alkaline cells to 5 volts. The low dropout linear regulator is composed of the low battery detector (LBI and LBO pins), resistors R1, R2, R3, R4, and a PNP transistor 2SA1244, while the other components perform the job of a step -up switching regulator. The 1N4148 raises the output voltage of the switch regulator by 0.4V, allo wing the linear low dropout regulator to filter out ripple noise. This results in a 7% loss in conversion efficiency, a necessary price to pay for a clean power source. The lower the output voltage, the higher the conversion efficiency loss due to this 0.4V voltage drop. For 3.3V (the AIC1631) and 3V (the AIC1631 -3) output voltages, it creates a conversion efficiency loss around 10%. 2SA1244 1N4148 1N5819 47µF 22µF 47µF 10µF R162K R220K 1.2K 40µH 100µH VOUT (5V) VIN SGND CGND SW LBO VIN LBI ILIM/SD VOUT AIC1631-5 Fig. 6 2~4 Cells to 5V Low Noise Power Supply Components Selection A few guidelines can be followed when using the LBI and LBO pins of the AIC1631 (also the AIC1633 and the AIC1630) to form a low dropout linear regulator. One can see from Fig. 6, that output voltage is: V (R R ) R 1.22VOUT 1 2 = + × The usable range of values for R4 is very large. Ordinarily, 1MΩ is a good choice, R3 must match with the PNP transistor. Since the current gain of the PNP transistor is usually only between 5 and 30 when V CE=0.4V, the output current capability will be inadequate if the R3 value is too large. On the othe r hand, since the LB loop can adjust automatically, it will be acceptable if R3 value is a little smaller. However, if we use the right value for R3, in addition to its normal function, it can also

perform current limiting function. Normally, when output c urrent is below 300mA and the PNP transistor is selected according to the following guidelines, the R3 value would be between 1K Ω and 5KΩ . The first rule in selecting a PNP transistor is that the rated current should be high enough. Secondly, current gain β of the PNP must be maintained at greater than 10 (the greater the value the better) when VCE=0.4V and IC is equal to or larger than the required output current. The transistor 2SA1244 in Fig. 6 is a good example. Since under some circumstance , t he use of a capacitor with too large capacitance value or too low ESR value (such as OSCON capacitors) can easily shift s the dominant pole of the linear regulator and cause s an oscillation situation . An electrolytic or a tantalum capacitor w ith capacitance lower than 100µF is suggested for the capacitor C3. Due to the limited bandwidth of the linear regulator and the ESR effect of C3, high frequency spikes and noises may pass the linear regulator and appear at the output terminal. L1 and C4 are therefore added at the output node to form a π filter to filter out high frequency noises and obtain a truly clean power source. PCB Layout Hints With a correct PCB layout, the output voltage noise of the circuit, shown in Fig. 1 should be below 20mV (Peak to Peak). Under most circumstances, the noise cannot even be seen. However, sometimes one can see ringing of about 10MHz on the oscilloscope. This is referred to as ground noise. Ground noise is conducted by the PCB ground traces and closely related to the layout of the ground plane. How do we distinguish ground noise? Simply connect the ground end of the oscilloscope probe to different spots of the ground plane. If the noise‘s magnitude or shape changes significantly, then it is ground no ise. In practical applications, we only need to apply a 0.1µF ceramic capacitor across the power input terminals of the analog circuit, as shown in Fig. 7. ANALOG CIRCUIT GND VDDVOUTVIN GND + AIC1631 CIRCUIT 0.1µF Fig. 7 Ground Noise Reduction by Using a 0.1mF Ceramic Capacitor