AN004 Universal Step-Down DC/DC Converter Design Using AIC1563 Abstract driving signal to turn on or turn off on the power Voltage required in the modern electronic systems are single or multiple regulated voltages such as 3.3 V, 5V, 12V, -5V, or –12V, etc. It can be supplied by using DC/DC converters. The operations of Step-Down DC/DC converter is introduced in the application note. A practical constant voltage, DC/DC step-down converter design based on the AIC1563 is also illustrated to help the readers in the circuit designs and applications. switch (SW). The power switch can be controlled by using different methods, such as pulse width modulation (PWM), pulse frequency modulation (PFM), or pulse skipping modulation (PSM), depends on the applications. As for the output voltage stability, it is maintained by controlling the duty cycle without losing energy. This may be illustrated by examining the energy stored in the inductor. VG Operations of Step-Down DC/DC Converter Driving signal on SW Figure 1 shows a basic circuit configuration of step-down DC/DC Converter. ISW SW current SW + VL IL IOUT VG D t V OUT - L V IN t Inductor current + C t LOAD VOUT Output voltage CONTROL CIRCUIT t Fig. 2 Voltage and Current Waveforms of Fig. 1 Step-Down DC/DC Converter Converter The input electric energy is delivered to the load The voltage and current waveforms in the circuit and partially stored in an inductor during the power are shown in Fig. 2. The control circuit is used to switch turned on. The stored energy will be also detect the output voltage level and generates a transferred to the load through a free-wheel diode May 1997 1 AN004 during the power switch turned off. Figure 3 shows the voltage waveform of the inductor in a cycle. in detail. VL V IN-V OUT Operations of AIC1563 Basically AIC1563 is a universal DC/DC converter t -V OUT TOFF TON T Fig.3 Voltage Waveform of The Inductor IC that may be applied to the design of step-down DC/DC converter, step-up DC/DC converter, and inverting DC/DC converter. It has an ultimate performance when it is applied to a step-down DC/DC converter. Figure 4 is a function block diagram of AIC1563 that consists of a frequency generator, a voltage comparator, a current limiting Here PWM control method is used to illustrate the controller, a Darlington switching transistor, and relationship between input and output voltage. It bootstrap driver. can also be applied to both PFM and PSM. To prevent the inductor from saturation, the inductor SC 1 8 must release the same amount of energy as is Q2 stored under the stable condition. If not so, the Q1 inductor will enter a saturation region and lose its capability of storing energy due to unbalanced BST QS R 80 SE 2 7 IS energy. This can be expressed as 1 T ∫ T 0 VL × IOUT × Is CT Oscillator dt = 0 ..............................(1) TC VCC Comparator In which T is the period, VL is the inductor voltage, 1.25V Reference Voltage and IOUT is the output current (DC). From Fig.3 we can derive 6 3 GND 4 + 5 FB (VIN -VOUT)×TON=VOUT×TOFF.................(2) VOUT TON TON = = = DUTY ...........(3) VIN TON + TOFF T Fig. 4 AIC1563 Function Block Diagram Where DUTY is the duty cycle ratio. Eq. 3 indicates that the ratio of output voltage to the input voltage The working voltage of AIC1563 is between 3V and is the duty cycle ratio. Therefore, by sensing the 30V. It is recommended to operate it between output voltage to control the duty cycle, the output 50KHz and 100KHz to obtain an optimum voltage can be well stabilized. performance, although its operating frequency is In the following two sections, the operations and a practical design example of AIC1563 are explained ranging from 10 Hz to 100 kHz. AIC1563 is controlled by pulse skipping modulation (PSM). When the output voltage at FB pin of voltage May 1997 2 AN97-004 comparator is below 1.25V, it drives the Darlington pulse skipping modulation control method. As it is switching transistor with a constant frequency and shown, this type of control can attain the stable duty cycle to elevate the output voltage. On the state naturally without a compensation circuit. In other hand, the Darlington switching transistor addition, this control method requires a simpler stops switching when the output voltage at the FB circuit with low quiescent current and it can achieve pin is higher than 1.25V, and the output voltage will very high voltage conversion efficiency under light be decreased to stabilize the output voltage. load. Figure 5 is the time sequence waveforms for the TOFF TON Oscillator Output Sensed Output Error Comparator Input V REF Error Comparator Output Driver Output Fig. 5 Time Sequence Waveforms of Pulse Skipping Modulation (PSM) In a switching type DC/DC converter, the switching add extra heat sink fins to an 8-pin DIP package loss of the switch can affect the converter’s with a 1.5A output current. If larger output current is efficiency directly. Since a high side switch is required, an inexpensive NPN transistor can be configured in a step-down DC/DC converter, a PNP added externally. The bootstrap driving circuit can transistor is normally used. also drives the external NPN transistor into the characteristics, cost and However, the suppliers of PNP transistors are not as good as that of NPN transistors. The problem with using saturation region (VCE=0.4 V) to achieve a high efficiency. NPN transistors is that they cannot be driven to the This leads to a lower converting efficiency. Constant Voltage DC/DC Converter Design AIC1563 not only uses a 2A peak current NPN The main voltage source required for the transistor as a switching switch, but also includes a electronic system products such as video CD, bootstrap driver to drive the transistor into the DVD, modem, and scanner is either 5 V or 12 V, saturation region. This has improved the efficiency with a maximum current of 1 A. We take this as an dramatically. Running it in a continuous output example to design a 5V/1A constant voltage current below 1.5A is the best working condition. Its DC/DC converter. saturation region if they are switched on high side. efficiency can be as high as 90% depending on the external components used. There is no need to Table 1 is the electrical specification for the design 3 AN97-004 example. We select the operating frequency of 50 KHz Table 1 Specifications for Constant Voltage DC/DC Converter Application Example Items Symbol MIN. VIN 8 Output voltage VOUT 4.75 Output current IOUT 0.1 Input voltage Output ripple voltage VRIPPLE TYP. 5 MAX. UNIT 15 V 5.25 V 1 A 50 mV The Figure 6 is an application circuit diagram of Since we have assumed the output voltage to be 5 DC/DC converter. The voltage divider resistors RA V, the ratio of RB/RA is 3. The sum of RA and RB and RB for output voltage feedback can be is best selected between 10 KΩ and 50 KΩ. obtained from the formula Therefore, we choose RA to be 10 KΩ, and RB to VOUT be 30 KΩ. RB = 1.25 × (1 + ) … … (4) RA D1 1N4148 D2 1N4148 C2 R1 680 8 RS 0.22 7 6 VIN 8∼15V C1 100µF + 5 BST SC IS SE VCC TC FB 220µH VOUT *L1 + C4 D3 1N5819 220µF 5V/1A 1µF + GND 1 2 3 4 AIC1563 RB 30K C5 5nF 390K RS1 1nF CT RA 10K R2 2.2M * L1: Iron Powder Core Fig. 6 Application Circuit for 5V/1A Step-Down Type Constant Voltage DC/DC Converter Combing equation (2) and Figure 2 with voltage saturation voltage approximate 0.4 V. The voltage drops on the switching transistor and free-wheel drop on the inductor is (VOUT+VF) if the transistor diode, the voltage difference on the inductor would is turning off, where VSAT is the forward voltage be (VIN–VOUT-VSAT) if the transistor is switching on, where VSAT is the switching transistor (0.2 V) of the free-wheel diode. Taking all of these into account, equation (2) can be modified as 4 AN97-004 (VIN -VOUT -VSAT)×TON=(VOUT+VF)×TOFF ......(5) To and maintain the converter in a continuous-conduction mode under a minimum TON(MAX) DUTY(MAX)= T = loading current, the minimum inductance can be obtained from Lenz’s law VOUT + VF = 66.67% ......(6) VIN(MIN) − VSAT + VF VL = L If we choose the switching frequency as 50 KHz, dIL ........................… … … … .............(12) dt Based on the inductor voltage waveform in Fig. 3, the minimum inductance is then the maximum turn-on time of the switching L(MIN) = transistor is TON(MAX)=DUTY(MAX) x T=66.67% x 1 50K ∆VL × ∆T ∆IP VIN − VOUT − VSAT × TON ∆IP ..(13) (8 − 5 − 0.4) -6 = × 13.34 × 10 = 173µH 200 × 10 −3 = =13.34µS......(7) For the oscillator in AIC1563, the nominal charging current is 25 µA and its voltage amplitude is 0.6 V. The formula ICT= CT We select a 220µH inductor for our design. For the dVCT ......(8) dt consideration of the inductor diameter, a conductor with a cross-sectional area of 1 mm can be simplified into CT= can take a 3A to 5A current, which depends on the operating temperature. MPP core or inexpensive ICT × TON(MAX) ......(9) ∆VCT iron powder core is recommended as the core material of inductor. It is not recommended to use if the charging current is a DC. It turns out type I inductors because they are only good for −6 CT= 2 25 × 10 A × 13.34µS = 556pF ......(10) 0.6V low loading current. They can deteriorate the converter efficiency considerably. Thus, we select CT=470 pF with an operating frequency of 50 KHz. The limiting current resistor can be estimated by From the inductor current waveform as shown in RS= Fig. 2, the converter continuous-conduction mode works in (CCM, a the advantage of which is beyond the scope of this note) when the minimum loading current is 100 mA. Then the peak-to-peak current amplitude of the inductor is ΔIP=2IOUT(MIN)=2×100mA=200mA.......(11) 0.3V .........................................(14) ILIMIT If we set the short circuit protection current to be 1.3 A, then Rs would be 0.25 Ω. The minimum capacitance of the output capacitor is IP(MAX) (1 + 0.1) = 8VRIPPLE F 8 × 50 × 10-3 × 50 × 103 ...(15) = 55µF COUT = where F is the switching frequency. The output 5 AN97-004 voltage ripple is ΔIL ×ESR , in which 50 × 10 −3 200 × 10 −3 80 = 0.25 Ω .............(16). V IN= 8V In order to reduce the output voltage ripple below to 50 mV, we select a 200 µF capacitor. If a smaller output voltage ripple is required, a capacitor with a smaller ESR should be considered. Explanation of other circuit elements in Fig. 6 is as follows. C1 is the input filter capacitor. If the changing rate of Efficiency ESR= V IN= 12V 85 70 65 60 55 0.1 0.3 can increase the dynamic response of converters. 0.5 0.7 0.9 1.0 IOUT (A) input voltage is fast, then C1 needs a larger capacitance. C5 is the feedback capacitor, which V IN= 15V 75 Fig. 7 Efficiency Characteristic Curves of 5V/1A DC/DC Converter RS1 and RS2 are compensation resistors which can stabilize operating frequency to make it work under PWM control method. RS1 provides CT an extra charging path to increase switching Line Regulator (VIN=8V∼15V, IOUT=1A) frequency. To maintain a switching frequency of 50 Load Regulation KHz, CT is modified to 1nF. R1 and D1 construct a (VIN=12V, IOUT=0.1A∼1A) starting circuit to provide AIC 1563 a starting Short current. Under the normal working condition, the (VIN=12V) driving current of IC is provided from the output Circuit Current 50mV 10mV 1.38A Table 2 Output characteristics of converter voltage 5 V via the diode D2. Therefore, R1 must limit the voltage between VIN and BST pin to Conclusion below 5 V. There are many power supplies to provide steady If the output voltage is 12V, one can modify the circuit elements’values as fore-mentioned. The experimental results of designing on the relationship between the efficiency and output current and input voltage are plotted in Fig. 7. Other characteristics of the design are listed in table 2. voltage sources for electronic elements in most electronic system products. Only a few electronic systems use dry cell batteries as voltage sources without voltage converter because they consist of a regulator circuit allowing a larger input voltage variation in electronic elements. Based on this, it is important to include a DC/DC voltage converter in an electronic system design. However, most circuit design engineers are unfamiliar with this design technique and often overlook this section of design because it is a small portion of design in electronic system products. In addition to what is described in this application note, it is also critical to consider the layout on a printed circuit board (PCB) when designing a DC/DC voltage converter. 6 AN97-004 Since large current, high voltage and high with PCB layout have an enormous impact on frequency signals are used in voltage converter, converter characteristics. parasitic inductance and capacitance associated 7