AN020 The Power Management of PDA—The Application of SEPIC Circuit Introduction The PDA (Personal Digital Assistant) appeals to an increasing number of users because of its multifunction such as: Wireless Communication, Organizer, Mobile Phone, Handwriting Recognition, Web Access, Flash Memory, and Data/fax Modem. The users can choose their favorites among various brands according to their described above. For example, there would be an increase of the number of elements and space, higher cost, reduced reliability, and low efficiency of power transfer. This article introduces a better approach to achieve a regulated voltage. The benefits of the simplified circuit with low cost and high efficiency may result from this approach. individual requirement. And the efficiency and the duration of the battery used in the products are critical to the users. From the designers’ point of view, the circuit for power management becomes obviously substantial. Here goes the block diagram of circuit in PDA. Referred to Fig.1, it is easily seen that there are two possible combinations for input. One way is to combine 2 Ni-MH cells and a 6V adapter. The combination causes the input voltage ranging from 1.8V to 2.6V. The other way is to put a Li-Ion battery and an adapter together. That results in a range from 2.4V to 4.3V for the input voltage. To have a regulated 3.3V input voltage for the controller, the voltage obtained from battery needs another treatment. The conventional method is to boost the battery voltage and then reduce it to what we expect. In this manner, regulated voltages are obtained from the battery steadily, regardless of the original level of the battery. Operation Principle A. The Description of the Circuit Referred to Fig. 2, the SEPIC (Single End Primary Inductor Circuit) meets the requirement for the output voltage to tolerate any levels of voltage from input. You might have heard of ”SEPIC”, yet the corresponding operation theory, design guide, and application are not often employed in the literature. We provide insight of the circuit for your design. As shown in Fig.2, L1 and L2 are chokes. They can be coupled or uncoupled. C1 and C2 are aluminum electrolytic capacitors. M1 is MOSFET and D1 is the power diode. When M1 turns on, D1 is off and VIN and C1 provide energy to L1, L2, respectively. In turn, as M1 turns off, D1 is on. L1 charges C1. L1 and L2 provide electric energy to C2 and load from magnetic energy stored before. In steady state, the average voltage of L1 and L2 is zero and that of C1 is VIN. Nevertheless, there are some defects in the method April, 2001 1 AN020 Mic Adapter Controller SEPIC Converter VIN VB1 PA VCORE Main Battery 1-Cell Li-Ion or 2-Cell Ni-Mh Speaker Tabl Memory VIN IrDA or Wireless RS-232 Fig.1 PDA Power Distribution Operation Principle C1 VIN D1 VOUT L1 M1 L2 + C2 LOAD Fig.2 The Topology of SEPIC Circuit B. Analysis To have a small current ripple, the circuit has to be now, the readers might be puzzled about equality of VC1 and VIN. In steady state, the average voltage of operated in the continuous conduction mode (CCM). inductor is zero, so VIN is directly across capacitor Besides, there would be less electromagnetic C1. That makes VC1 equal to VIN. The plot of interference in CCM. Therefore, the circuit is to be currents with respect to switching signal is shown in analyzed in this mode. Fig.5 (a). Mode. 1 (tON< t ≤T) Refer to Fig.3, when M1 is on, the diode D1 is off and VIN is across the inductor L1. The current of L1 increases in linear proportion. Meanwhile, the voltage of C1 is across L2 and, when L1 is the same as L2, the current of IC1 and IL2 is identical. Until 2 AN020 +VL1- +VC1- VOUT +VD1IL1 - IC1 VIN IM1 + -VL1+ + IL2 LOAD Fig.3 The Equivalent Circuit of Fig.2 when M1 is ON and D1 is OFF Mode. 2 (0< t ≤ tON) C2 and the “load” as in fig. 3, which is a power plant. According to Kirchhoff’s current law, ID1 is the sum As shown in Fig.4, when M1 turns off, the diode D1 of IL1 and IL2. If we neglect the forward drop voltage is on and the magnetic energy stored in L1 is released to charge C1. The current declines in of diode D1, VL2 is equal to minus VOUT. The plot of voltages with respect to switching signal is shown in linear proportion. The voltage across L1 is equal to Fig.5 (b). minus VOUT. Similarly, the magnetic energy in L2 is transferred to +VL1- +VC1- IL1 IC1 VIN Fig.4 M1 +VD- IL2 VL1+ VOUT + + LOAD The Equivalent Circuit of Fig.2 when M1 is OFF and D1 ON. M1-ON VGS M1-OFF 0 IL1 or IL2 0 IC1 VD1 0 0 IM1 IL1-IC1 0 Fig.5 (a) The Plots of Currents 3 AN020 M1-ON M1-OFF VGS 0 IL1 0 VIN -VO IL2 0 VIN -VO VC1 VIN 0 VD1 0 -(VIN+VOUT) Fig.5 (b) The Plots of Voltages In steady state, the characteristic of inductor is the There are some specifications in this design example: voltage-second balance. Therefore, we can obtain The range of input voltage: 2.9V~4.5V the relationship between VIN and VOUT in (1). If The desired output voltage: 3.3V we neglect the power loss in the converting circuit, The maximum current: IOUT=500mA PINPUT equals POUTPUT. And the relationship of current between input and output is shown in (3), Step 1: Selection of L1 and L2. where D is the duty cycle. VIN × D × TS = VOUT × (1 − D) × TS AIC1630A, one of products for power management VOUT D = VIN 1− D IIN IOUT = D 1− D …..…….… (1) ……………………..…………(2) ……………………….………….(3) from AIC, is the switching controller whose switching frequency is from 90kHz to 150KHz. Ts=1/FS.MIN=1/90k=11.1µS, VOUT VIN D MAX = MIN 1 − D MAX DMAX 3.3 = 2.9 1 − D MAX Design Guide From the description above, here is a typical design IOUT-BOUNDARY=IOUT-MAX=0.5A, VOUT × TS × (1 − D) 2 × IOUT −BOUNDARY L1 > In MP3 or PDA, the battery is the power source to the L1>17.2µH change of battery capacity. To obtain regulated voltage from battery source regardless the level of the voltage, the SEPIC circuit is preferred. , DMAX=0.53……the maximum duty ratio example. DC/DC converter. The voltage fluctuates due to the , Let L1 be 25µH. IIN = POUT PIN 3.3 × 0.5 = = = 0.71A VIN EFF. × VIN 0.8 × 2.9 Step 2: Selection of C1 4 AN020 C1= C1= D × TS × IIN D VC1 M1 is chosen to be CEM4410（30V/10A） , D1: voltage stress>VIN+VOUT=4.5+3.3=7.8V, Current stress equals to M1 0.53 × 11.1× 0.71 =25.3µF, 3.3 × 0.05 D1 is SB220 (20V/2A) The whole circuit is shown below. Let C1 be 47µF/10V/Low ESR Step 3: Selection of M1 and D1 M1: voltage stress> VIN + VOUT = 4.5 + 3.3 = 7.8V , Current stress> Current stress> IIN × = 0.71 × 2 DMAX × 1 1+ K 2 1 × =1.78A 0.53 1 + 0.5 I Where k= P1 , IP2 1>k>0 Let it approximate 0.5. IP2 IP1 Fig.6 The Current of M1 R3 VIN 10 + C1 220µF D1 VOUT C6 C5 + 0.1µF 220µF 581 C7 0.1µF L2 L1 68µH 105 C8 U1 1 2 Q1 3 CET6030L 4 SD VOUT VIN LBI EXT LBO GND FB 8 7 R1 510K 6 5 R2 120K AIC1630A 68µH Fig.7 The SEPIC Circuit of AIC1630A 5 AN020 Experiment Results The results of experiment are shown below. Fig.8 (a) The plot of currents with respect to switching signal Fig.8 (b) The plot of voltages with respect to switching signal. 6 AN020 Summary Based on the calculation and description above, the SEPIC can be accurately designed. It is recommended especially in the applications where the battery is the power source in the appliances or the regulated voltage is demanded from the power source regardless the level of the voltage. We sincerely hope that this circuit could be of some help to engineers in related field. Other topics, e.g. the power management of portable appliances, the circuit combined to the charger, the boost mode circuit and some problems encountered in the design process, will be presented in the near future. Reference: Although the efficiency of SEPIC circuit is lower than [1]. AIC1630A BUCK converter or BOOST converter, it beats the Corporation 2000. Datasheet, Analog Integrations conventional method, that is, boosting the source voltage first and reducing it afterwards. For the low power-consumption portable appliances, SEPIC is a good option with benefits of simple circuit and low ripple current. 7