AN97-002 Two-Cell, Step-Up Converter Design for Portable Applications James Liu Introduction (4) Squeeze out as much of the battery energy 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. 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 The AIC1630/1631/1633 are very suitable for the consumption of the converter itself must also be application of using 2~4 batteries. This article will low. With regards to size and weight, an 8-pin IC, focus on the application of AIC1631-5 for 2 which does not need a heatsink, with a small batteries in the following description. inductor, and minimum number of external parts, would be an ideal choice. In the following, we use A set of two alkaline, NiMH, or NiCd battery cells the AIC1631 DC/DC converter as an example for accompanied by a switching converter with 3V, several different special products using two 3.3V, or 5V output voltage is frequently used as battery cells to demonstrate how the AIC1631 the power supply source for portable electronic products. Using two battery cells is usually a result of compromise between size and can be used to make an ideal power source. +VOUT +VIN battery 1K 33K operating life. Besides the batteries' own stored R1 R2 ILIM/SD energy, two other factors also influence batteries' operating life: the product's power consumption and conversion efficiency of the converter. In + 10mF 100mF C1 C2 output should meet the product's demands. VIN LBI SW LBO SGND converter for an electronic product, therefore, 5 (1) The current capacity and regulation of the VOUT + order to choose the appropriate switching key factors need to be considered: (+5V) CGND + C3 100mF AIC1631-5 47mH* *Sumida RCH-108 1N5819 D1 Fig. 1 A High-Efficiency 2-Cell to 5V Converter (2) High conversion efficiency. (3) Low power consumption. May 1997 1 AN97-002 Inductors of less inductance can output a larger 90 current. However, too low inductance would result Efficiency (%) VIN=3.3V in serious efficiency loss due to inductor core VIN=3V 85 VIN=1.8V 80 saturation. The most reliable method, therefore, is VIN=2.5V to test for the best conditions using a range of VIN=2.2V suggested values. The lower coil resistance (RDC) be chosen, the better VIN=2V 75 0 100 200 300 400 efficiency obtained. For products consuming about 20mA, the RDC should 500 600 Load Current (mA) Fig. 2 Conversion Efficiency for the 5V Converter be under 1W. For products with current consumption at or above 50mA, the RDC should be below 0.5W. The larger the consumption current, the smaller the RDC required. Conversion Efficiency Considerations Conversion efficiency is the main consideration for those devices which consume above 2mA most of Low Power Consumption Products the time. Fig. 1 shows a high efficiency converter The quiescent current of the AIC1631-5 in an no circuit suitable for producing 5V output from two load state is about 200mA. For special energy- battery cells. The components shown in Fig. 1 are conscious products, some circuit configurations easily obtained low cost common parts. If high- are presented here to illustrate how the AIC1631 quality parts (such as MMP core inductors) are can be used: used, efficiencies of 90% and above can be (1) If this DC/DC converter is not required to reached. However, if good output power quality is maintain output voltage, you may simply pull desired, such as in Fig. 2 or better, at least two pin 1 (shutdown pin) to ground by using a points need to be observed: control (1) A good power plan and a correct layout for functioning and the power consumption drop the circuit board is a must. The AIC1631 to less than 10mA. Normal operation can be evaluation boards are available for your recovered by pulling pin 1 high. Generally reference. speaking, this is ideal for systems with (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- signal. The AIC1631 will stop backup lithium batteries. Fig. 3 is an example circuit. While using the lowest possible amount of power, the circuit makes it possible to retain data in memory even when changing batteries. 108 47mH inductor, whose coil resistance (RDC) is 0.14W. When converting 2 alkaline battery cells to 5V, an inductor of 40-150mH is suggested. For NiCd batteries, a 20-100mH inductor is suggested. 2 AN97-002 VOUT (+5V) SDN* 33K 1K + R2 10mF C1 VIN LBI AIC 1631-5 SW LBO 1N4148 ** SGND 47mH 100mF C2 * 1M VOUT 47mH SDN=VIN or VOUT to Operate SDN=GND to Shutdown ** Backup Lithium Battery 100mF C3 CTL* 0.01mF C1 D1 VOUT D2 + CGND 1N5819 D1 1N4148 1K + C2 LBI VIN + AIC1631-5 SW D3 1N5819 + R3 1M 2N3904 Q1 LBO R4 150K CGND SGND * CTL =GND in Burst Mode CTL=VIN or VOUT in Normal Mode 100mF C3 Fig. 3 Step-Up converter with Shutdown Control 1M R2 VOUT ILIM/SO 10mF R1 ILIM/SO 1N5819 D2 VIN + R5 VIN (a) (2) If the output voltage requires the AIC1631, 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 VIN=1.8V Total Supply Current (mA) one could add a few external components to 800 VIN=2V 600 VIN=2.5V 400 200 VIN=3V VIN=3.3V 0 50 characteristics similar to the circuit in Fig.1. If 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 R3 and R4 in Fig. 4(a). Note that the voltage of normal operation will never be exceeded. 150 200 250 300 Load Current (mA) CTL is connected to GND, the circuit will enter Burst Mode with an output voltage of 4V, 100 (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 Fig. 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. 3 AN97-002 1K +VOUT R1 (+5V) 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 R2 33K + 10mF C3 a PNP transistor 2SA1244, while the other VOUT ILIM/SO components perform the job of a step-up VIN LBI SW LBO voltage of the switch regulator by 0.4V, allowing SGND CGND the linear low dropout regulator to filter out ripple +VIN switching regulator. The 1N4148 raises the output noise. This results in a 7% loss in conversion AIC1631-5 efficiency, a necessary price to pay for a clean 47mH + + 1N5819 D1 100mF C1 power source. The lower the output voltage, the 100mF C2 higher the conversion efficiency loss due to this 0.4V voltage drop. For 3.3V (the AIC1631) and 3V Fig. 5 1 Cell to 5V Step-Up Converter (the AIC1631-3) output voltages, it creates a conversion efficiency loss around 10%. 2~4 Cells to 5V Low Noise Power Supply The AIC1631-5 can be used to convert 2-4 2SA1244 Q1 1N4148 VIN 1K R5 100mH 1N5819 L2 + D1 47mF C1 10mF + C2 1 D2 ILIM/SD VOUT 2 VIN LBI SW LBO 4 R4 SGND CGND 62K 7 5 40mH 1M 8 6 3 L1 1.2K 20K VOUT (5V) R1 + C3 + C4 22mF 47mF R2 R3 AIC1631-5 Fig. 6 2~4 Cells to 5V Low Noise Power Supply Components Selection The usable range of values for R4 is very large. Ordinarily, 1MW is a good choice, R3 must match A few guidelines can be followed when using the with the PNP transistor. Since the current gain of LBI and LBO pins of the AIC1631 (also the the PNP transistor is usually only between 5 and AIC1633 and the AIC1630) to form a low dropout 30 when VCE=0.4V, the output current capability linear regulator. One can see from Fig. 6, that will be inadequate if the R3 value is too large. On output voltage is: the other hand, since the LB loop can adjust V OUT = (R1 + R2 ) 1. 22 V R2 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 4 AN97-002 perform current limiting function. Normally, when 10MHz on the oscilloscope. This is referred to as output current is below 300mA and the PNP ground noise. Ground noise is conducted by the transistor is selected according to the following PCB ground traces and closely related to the guidelines, the R3 value would be between 1KW layout of the ground plane. How do we distinguish and 5KW. 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 The first rule in selecting a PNP transistor is that changes significantly, then it is ground noise. In the rated current should be high enough. practical applications, we only need to apply a Secondly, current gain b of the PNP must be 0.1mF ceramic capacitor across the power input maintained at greater than 10 (the greater the terminals of the analog circuit, as shown in Fig. 7. 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. VIN Since under some circumstance, the use of a capacitor with too large capacitance value or too low ESR value (such as OSCON capacitors) can + VOUT AIC1631 CIRCUIT VDD 0.1mF ANALOG CIRCUIT GND GND easily shifts the dominant pole of the linear regulator and causes an oscillation situation. An electrolytic or a tantalum capacitor with capacitance lower than 100mF is suggested for the Fig. 7 Ground Noise Reduction by Using a 0.1mF Ceramic Capacitor 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 p 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 5