AN020

AN020 AIC | Alldatasheet

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April, 2001 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 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. Nevertheless, there are some defects in the method 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. 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, L 1 and L2 are chokes. They can be coupled or uncoupled. C 1 and C 2 are aluminum electrolytic capacitors. M1 is MOSFET and D1 is the power diode. When M 1 turns on, D 1 is off and V IN and C 1 provide energy to L 1, L 2, respectively. In turn, as M 1 turns off, D 1 is on. L 1 charges C 1. L 1 and L2 provide electric energy to C 2 and load from magnetic energy stored before. In steady state, the average voltage of L 1 and L2 is zero and that of C 1 is VIN.

Fig.1 PDA Power Distribution Operation Principle C2 LOAD VIN VOUT Fig.2 The Topology of SEPIC Circuit B. Analysis To have a small current ripple, the circuit has to be operated in the continuous conduction mode (CCM). Besides, there would be less electromagnetic interference in CCM. Therefore, the circuit is to be analyzed in this mode. Mode. 1 (t ON< t ≤T) Refer to Fig.3, when M1 is on, the diode D1 is off and V IN is across the inductor L 1. The current of L 1 increases in linear proportion. Meanwhile, the voltage of C1 is across L2 and, when L1 is the same as L 2, the current of I C1 and I L2 is identical. Until now, the readers might be puzzled about equality of VC1 and VIN. In steady state, the average voltage of inductor is zero, so V IN is directly across capacitor C1. That makes V C1 equal to V IN. The plot of currents with respect to switching signal is shown in Fig.5 (a).

+VL1- IM1 IC1 -VL1+ IL2 +VD1- +VC1- Fig.3 The Equivalent Circuit of Fig.2 when M1 is ON and D1 is OFF Mode. 2 (0< t ≤ tON) As shown in Fig.4, when M1 turns off, the diode D 1 is on and the magnetic energy stored in L 1 is released to charge C 1. The current declines in linear proportion. The voltage across L1 is equal to minus V OUT. Similarly, the magnetic energy in L2 is transferred to C2 and the “load” as in fig. 3, which is a power plant. According to Kirchhoff’s current law, ID1 is the sum of IL1 and IL2. If we neglect the forward drop voltage of diode D1, VL2 is equal to minus V OUT. The plot of voltages with respect to switching signal is shown in Fig.5 (b). LOAD VIN VOUT IL1 +VC1- IC1 VL1+ IL2 +VD-+VL1- Fig.4 The Equivalent Circuit of Fig.2 when M1 is OFF and D1 ON. VGS M1-ON M1-OFF IL1 or IL2 IC1 VD1 IM1 IL1-IC1 Fig.5 (a) The Plots of Currents

-(VIN+VOUT) IL2 VIN VIN -VO -VO Fig.5 (b) The Plots of Voltages In steady state, the characteristic of inductor is the voltage-second balance. Therefore, we can obtain the relationship between V IN and V OUT in (1). If we neglect the power loss in the converting circuit, PINPUT equals P OUTPUT. And the relationship of current between input and output is shown in (3), where D is the duty cycle. D V V IN OUT D I I OUT IN Design Guide From the description above, here is a typical design example. In MP3 or PDA, the battery is the power source to the DC/DC converter. The voltage fluctuates due to the change of battery capacity. To obtain regulated voltage from battery source regardless the level of the voltage, the SEPIC circuit is preferred. There are some specifications in this design example: The range of input voltage: 2.9V~4.5V The desired output voltage: 3.3V The maximum current: I OUT=500mA Step 1: Selection of L 1 and L2. AIC1630A, one of products for power management from AIC, is the switching controller whose switching frequency is from 90kHz to 150KHz. Ts=1/FS.MIN=1/90k=11.1µS, MAX MAX MININ OUT D V V −=  , MAX MAX D 9.2 3.3 −= , DMAX=0.53……the maximum duty ratio IOUT-BOUNDARY=IOUT-MAX=0.5A, L1 > ( ) BOUNDARYOUT SOUT D1TV −×× L1>17.2µH Let L1 be 25µH. 0.71A2.90.8 0.53.3 VEFF. P V PI IN OUT IN IN IN =× ×=×== Step 2: Selection of C1

C1= VC1 INS D ITD ×× , C1= 0.053.3 71.011.10.53 ×× =25.3µF, Let C1 be 47µF/10V/Low ESR Step 3: Selection of M1 and D1 M1: voltage stress> VVV OUTIN 8.73.35.4 =+=+ , Current stress> Current stress> K1 D MAX IN +×× = 0.51 0.53 20.71 +×× =1.78A Where k= I I , 1>k>0 Let it approximate 0.5. IP1 IP2 Fig.6 The Current of M1 M1 is chosen to be CEM4410（30V/10A） D1: voltage stress>VIN+VOUT=4.5+3.3=7.8V, Current stress equals to M1 D1 is SB220 (20V/2A) The whole circuit is shown below. SD1 VIN2 EXT GND

4 FB 5

220µF VIN 68µH CET6030L 0.1µF 68µH 581+C5 220µF 0.1µF VOUT 510K 120K C8105 Fig.7 The SEPIC Circuit of AIC1630A

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.

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. Although the efficiency of SEPIC circuit is lower than BUCK converter or BOOST converter, it beats the 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. 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: [1]. AIC1630A Datasheet, Analog Integrations Corporation 2000.