AN009

AN009 AIC | Alldatasheet

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Tiny Step-Up DC/DC Converter Design for Portal Single Cell Applications Abstract Products such as walkman, pager, electronic dictionary, MP3 player and PDA , none of them can escape out of using one cell trend. This article introduces a high efficiency AIC1638/1639 step-up controller that can boost the voltage of single battery cell up to 3.0V, 3.3V, even to 5.0V. It resolves the problem intend using only one cell in the devices. Introduction AIC1638/1639 is a high efficiency volta ge step-up type control IC using PFM (Pulse Frequency Modulation) control technology. It consists of PFM control unit, high -speed error amplifier, precise reference voltage , singleswitching transistor for AIC1638, or push-pull driver output for AIC1639. Features of AIC1638/1639 : it provides low quiescent current, very high efficiency power conversion, and very low gate threshold voltage. The latter one can guarantee the output voltage up to the required working level even the input voltage is started up at 0.8 V. Therefore, AIC1638/1639 can use every bit of energy in the cell efficiently to stretch out the cell’s lifetime. More importantly, it can work with minimal external elements (such as inductor L, capacitor C and diode D) . It perfectly meets the smart requirements for the least number of elements and minimum space (see Figure 1). Figure 1 AIC1638/1639 Typical Application Principle The following simple diagram is used to illustrate the operating principle of AIC1638/1639.

It provides a precise reference voltage as the reference to Error Comparator input terminal. Driver and Output Switch In order to emphasize the requirement for system a pplication flexibility and high efficient conversion, Analog Integrations Corporation (AIC) introduces AIC1638 built -in type and AIC1639 external type models that user’s can choose properly according the demand s of application. The R DS(ON) of N-MOSFET built in AIC1638 is 1 ~ 1.5Ω only which can reduce the conduction loss effectively due to the current flow through the MOSFET . Besides, AIC1639 provides an excellent output driving powered by means of a Push -Pull design. it gives the external transistor a full play. Selection of elements For the reason of obtain ing the best performance of the system, it is necessary to consider the following peripheral elements: Inductor In order to reduce the loss caused by inductor DC resistance, it is wise to select an inductor with a small DC resistance (less than 1 Ω is commanded), and value of inductance between 22µH and 1mH. If large r inductor is used , the peak current flowing through the inductor Ipeak , will b e reduced because the maximum energy stored in inductor is expressed as peakL IL2 1? ×= Therefore, it is difficult to achieve voltage step up operation under a low input voltage condition. It also raises the minimum operating input voltage. However, the losses on inductor L and switching transistor Q will be reduced and the efficiency will be improved because a small inductor peak current IPEAK On the other hand, using a small value inductance can reduce the minimum input voltage under a loading effectively . But it will aggravate the inductor efficiency for using a too small inductor. It is a trade-off problem depends on user’s application. Besides, switching peak current flowing through inductor many times , this is also the output current. The relationship between input and output efficiency can be expressed as: Input Power × Efficiency Than (VIN × IIN) × Efficiency and )y)(V(Efficienc )(I(V)I IN(MIN) OUT(MAX)OUT(MA) IN(MAX = …..(7) Using the changing rate of current in inductor formula T I L d to obtain the actual increasing rate of current L ]t)V[(V ONSATIN(MIN) I , …..(9) where V SAT is the saturati on voltage across transistor CE terminals. Combining formula (6) with (9) ,we obtain i)0.5(II IN(MAX)PEAK Δ+= …………….. (10) For example, the required output is 3.3V, 100mA (max), and 80% efficiency. The input voltage of the cell is ranged from 0.9V to 1.8V, and L=33µH, transistor saturation voltage VSAT is 0.3 V. 1033 107.50.3)(0.9 0.90.8 101003.3I 6 PEAK ××−+× ××= − The inductor current at input terminal is much

operation of magnetic saturation on inductor. Maximum rated current >IPEAK,. Figure 5. External Switch Peripheral Circuit determines the transistors output capability. for the value of R B between 50 Ω and 5 00Ω . on/off switching loss to improve the efficiency.

Table 1. A quick table of selecting elements

• For the circuit implementation, the external elements should be as close to IC as possible.
• Make sure an effective grounding. If not so, increasing imp edance relative to the ground would occur when switching current flows through ground pin. This can cause instability of IC.
• The capacitor value should be larger than 10µF and behave good frequency characteristics. Additionally, a voltage spike usually oc curs when the transistor is switching on/off. It is suggested to use a capacitor with high voltage endurance, e.g., three times as the output voltage.
• Schottky diode with low forward voltage VF is recommended
• In switching regulator application, ripple voltage and spike noise is closely related to the magnitudes of coil and capacitor. Be sure the proper magnitude is used.
• Make sure the power loss of switching transistor is within safe operating area (SOA).
• The performance of AIC1638/1639 is strongly cor related to the selection of peripheral elements. The magnitudes of voltage, current and power loss should not exceed the safe nominal value.

AIC1638 Load Regulation and Efficiency AIC1638-30 Efficiency (L=100mH, CD54) Output voltage (VOUT) Output Current (IOUT, mA) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 1402.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 2.0V 1.8V1.5V 1.2V 0.9V 0 20 40 60 80 100 120 140 160 180 2.0V1.8V 1.5V 1.2VVIN=0.9V AIC1638-30 Efficiency (L=100mH, CD54) Efficiency (%) Output Current (mA) AIC1639 Load Regulation and Efficiency AIC1639 Load Regulation (L=33mH) Output voltage (V) Output Current (IOUT, mA) 0 50 100 150 200 250 300 350 400 3.10 3.14 3.18 3.22 3.26 3.30 3.34 L=33µH V=2.0V AIC1639-33 Efficiency（L=33mH） Efficiency (%) Output Current (mA) 0 50 100 150 200 250 300 350 400 VIN=2.0V H, Q1=2SD1803 L=33V Conclusion AIC1638/1639 accomplishes the step -up voltage conversion with a minimal external element addition. Its exquisite package (SOT-89) taking a small space proves to be cost effective. Furthermore, an extra-ordinary output current driving power and high efficiency over a wide range of output current. The device can be widely used in many applications, particularly, those portable produ cts based on cell as the power source, e.g., pager, walkman, electronic dictionary, MP3 player …etc.