Abstract :On HSOP package EL7558BC buck switching characteristics of rectifier chips were analyzed using the given chip design using the rectifier DC / DC converter and external circuit design . Through experiments to verify the design . Keywords :DC / DC converters ; EL7558BC; switch rectifier Introduction EL7558BCDC/DC converter chip is Elantec company 's internal integrated MOSFETs , low input voltage (4.5 ~ 5.5V), high output current (8A) of the PWM rectifier, the efficiency of up to 94%. Output voltage deviation is less than 1.5%. The maximum switching frequency up to 1MHz, can be set to a fixed voltage output (3.5V) or adjustable output voltage (1.0 ~ 3.8V). EL7558BC minimize external components with highly integrated features , can work only a small amount of external components , thus significantly reducing the circuit Ban Mian Ji and design costs , as Dianyuan design provides a quick and easy solution. EL7558BC both direct and thermal overheat protection cut-off load for the logic / processor reset and control supply sequence voltage feedback PWRGD output signals . The package has good thermal performance for the 28- foot HSOP package . These advantages make EL7558BC power chip can be widely used in high-performance DSPs / FPGAs / ASICs / microprocessors , PC motherboards, portable electronic devices , laptop computers and many other electronic devices . 1 pin functions and use characteristics EL7558BC package shown in Figure 1 , the pin function as follows : Pin 1 (FB1) voltage feedback input terminal 1 , when the chip is set to adjustable voltage output (VCC2DET to low) effective; Pin 2 (CREF) input reference voltage bypass capacitor is generally used to connect 0.1µF capacitor and tiles ; Pin 3 (CSLOPE) slope compensation capacitor input ; Pin 4 (COSC) internal oscillator input capacitor , capacitor COSC CSLOPE and the ratio is usually 1:1.5 ; Pin 5 (VDD) PWM control circuit supply voltage input voltage VIN is usually with the same ; Pin 6 and pin 8 (VIN) buck rectifier power supply voltage input ; Feet 7 feet 9-12 feet 18-19 (VSSP) buck rectifier to return to , or power to ; Pin 13 (VCC2DET) interface logic input is 3.5V logic chip 1, a fixed voltage output , logic 0, the chip is 1.0 ~ 3.8V adjustable output voltage ; Pin 14 (OUTEN) switch rectifier output enable terminal , the logic 1 valid; Pin 15 (OT) chip overheating instructions output , normally high, when pulled low when the temperature exceeds 135 ? , the temperature dropped to below 100 ? recovery becomes higher ; Pin 16 (PWRGD) Powergood output signal , when the output voltage error is less than the default value of ± 10% when high or low ; Pin 17 (TEST) test pin , normally connected to and VSSP ; Foot 20-23 (LX) voltage output to drive external inductor ; Pin 24 (VHI) internal high- side gate driver through a 0.1µF bypass capacitor connected with the LX ; Pin 25 (VSS) control circuit to return to , or signal ground ; Pin 26 (C2V) times the voltage circuit connecting the output , as an internal low- side gate driver ; Pin 27 (CP) charge pump capacitor negative side of the drive end ; Pin 28 (FB2) voltage feedback input 2, when the chip is set to a fixed voltage output (VCC2DET low ) effective , then the output voltage is 3.5V. EL7558BCDC/DC converter chip with soft-start function , and does not require an external capacitor , when the chip will be completed when power soft-start . EL7558BC with VCC2DET function, IntelP54 and P55 provide a direct interface to the microprocessor . EL7558BC with built-in charge pump voltage doubler circuit for opening the internal MOSFET, C5 ( see Figure 1 ) is the charge pump capacitors , D2 and D3 for the charge-pump diode . If there is 12V voltage input, the D2 and D3 can be omitted . Figure 1 EL7558BC DC / DC converter chip package and a typical circuit Click to enlarge 2 DC / DC converter design EL7558BCDC/DC converter below to chip , for example, on the DC / DC converter design process in detail. The typical design of the circuit shown in Figure 1 . 2.1 Select the output voltage EL7558BCDC/DC converter chip set can be fixed by VCC2DET pin voltage (3.5V) or adjustable output voltage (1.0 ~ 3.8V) output . When VCC2DET as high as the fixed voltage output ; when VCC2DET as low as adjustable voltage output , this time to obtain a different voltage output by feedback resistors R3 and R4 to adjust , adjustable output voltage range of 1.0V to 3.8 V. R3 and R4 and the resistance of the correspondence between the output voltage can be approximated by equation ( 1 ) that , in this mode , VCC2DET pin must be low . Output voltage Vo = 1 + (R3/R4) × 1V (1) Select switching frequency of 2.2 Switching frequency conversion efficiency of the EL7558BC chip and the required size of an external inductor has a great influence . Lower frequency , the higher the efficiency , but the greater value of inductance required . COSC pin can be connected by adjusting the capacitance of C8 to set the switching frequency , adjustable frequency up to 1MHz, C8 capacitor value and the corresponding relationship between switching frequency can be approximated by equation ( 2 ) said . Switching frequency fsw = 0.0001/Cs (Hz) (2) Type in : C8 unit Farah F. By adjusting the capacitor C8 to change the switching frequency , the connection CSLOPE foot slope compensation capacitor C7 also do the appropriate adjustments , capacitors C7 and C8 ratio is usually 1:1.5. 2.3 to select the input filter components EL7558BC chip, usually require the input of a decoupling capacitor and a bulk input capacitor . Decoupling capacitor C12 primary role is to reduce the chip input high-frequency noise , generally 1 ~ 10µF of porcelain capacitors , the capacitance in the layout must be as close as possible to get the best results EL7558BC chips . Bulk input capacitor C9 's main role is to reduce input ripple voltage and, in some applications, a 10µF decoupling capacitor has enough filtering capacity without a large input capacitance . As the need for large -capacity input capacitance , first of all depends on the maximum allowable input ripple voltage . Usually make EL7558BC work, input ripple voltage should not exceed 300mV. Free style (3 ) Calculate the only 10µF capacitor , the potential maximum input ripple voltage , if the calculated value exceeds allowable value , it is necessary to use large -capacity input capacitance . ?VIN=IOUT(MAX)0.25/(10µF)(3) Type in : ?VIN to have no input bulk capacitor ripple voltage The maximum peak to peak ; IOUT (MAX) for the maximum DC load current . Bulk input capacitance of the bigger and more beneficial to lower the value of ripple voltage , while its equivalent series resistance (ESR) will increase the ripple voltage is greater , so , to select large capacity and low ESR capacitors . Type (4 ) gives the bulk input capacitor and input ripple voltage less relevant System . If the ripple voltage is too large , you can use multiple capacitors in parallel ways . Another bulk input capacitor rated voltage and current is suitable. ?VIN'=(IOUT(MAX)0.25)/CBULKfSW+IOUT(MAX)ESRMAX(4) Type in : ?VIN ' for the bulk input capacitor are the input ripple voltage when the maximum peak to peak ; IOUT (MAX) for the maximum DC load current ; CBULK used for the bulk input capacitance of the C9; ESRMAX for the bulk input capacitance of the maximum ESR. 2.4 Select the output filter components The choice of output filter components are DC / DC Converter with the most critical part of the output filter components determine the power stability. The key is to select two components, one is the output inductor L1, the other is the output capacitor C10. Affect the power of the most critical parameter is the stability of the output capacitor 's ESR, capacitance of the capacitor data sheet will usually give the maximum ESR, the minimum ESR ESR is usually the maximum of 40 % to 60% . In addition , the choice of capacitance , when the drift ESR capacitor should also be considered appropriate . The output inductor L1 has a filtered ripple stored energy and the two functions , the inductor selection is mainly input and output voltage, and switching frequency decision. Inductor current must be greater than the maximum rated output current (8A), the selection of inductor value can be from ( 5) calculated . LOUT=(VIN(MAX)-VOUT)(1/?IL)(VOUT/VIN(MAX)(1/fsW) (5) Type in : LOUT is the output inductor , that is, L1; VIN (MAX) for the maximum input voltage ; The output voltage VOUT ; ?IL to allow the maximum output inductor ripple current value , this value must usually be less than 0.8A. Select from the capacitance of the capacitor DC rated voltage , capacitor ripple current , the maximum output ripple voltage power supply , power supply stability of the four factors to consider . Capacitor rated voltage must be greater than the output voltage , usually at least 10% higher than the output voltage to control the ripple and transient response. Maximum capacitor ripple current (that is, capacitor RMS current) can type (6 ) calculation , the selected capacitor ripple current must be greater than equation ( 6) were calculated. IC(RMS,MAX)=1/??12(VIN(MAX)-VOUT)VOUT)/VIN(MAX)LOUTsW (6) The formula : IC (RMS, MAX) for the largest capacitor RMS current . For maximum output ripple voltage power supply , first , to determine the specific application requirements on the output ripple voltage , EL7558BC chip output ripple voltage must be limited to less than 2% of the output voltage . Then, using equation (7) calculate the maximum allowed capacitor ESR, select the maximum ESR rating is less than equation ( 7), calculated to ensure that the output ripple voltage is consistent with application requirements . In addition, the capacitor ESR of drift must also be taken into account. ESRMAX '= ?V ( MAX ) / ?IL ( MAX ) (7) Type in : ESRMAX for the maximum allowable output capacitor ESR; ?V (MAX) for the maximum allowable output ripple voltage ; ?IL (MAX) for the maximum allowable output ripple current value , this value must usually be less than 0.8A. Capacitor in parallel by multiple methods to reduce the ESR, improve the circuit 's transient response , but must be greater than the total ESR 10mO, the total capacitance value must be greater than 330µF. 2.5 Notes route In the route , in principle, to all the external components as close as possible EL7558BC power chips , in particular decoupling capacitor and bypass capacitor to the corresponding pin in the vicinity of cloth . EL7558BC device has two ground ( analog ground and power ground ), connect all the noise sensitive analog signals, connect a noise power signal . The introduction of noise between the two places will reduce the chip 's performance , especially in the case of high current output . However , analog to the high noise will affect the control signal, therefore , recommended Ba analog ground and power and separately , and two to one point ( a chip, the following Huozhe in the input or output capacitor negative side ) to reduce the two directly connected One -way noise. Connect feedback pin ( pin 1 and pin 28) the alignment of the most sensitive to noise , should be kept as short , the best fabric in both the middle ground . EL7558BC chip heat mainly by VSSP pin and chip at the bottom of thermal pad . In order to achieve good thermal performance , thermal pad must be fully welded on the PCB , if there is middle ground must be through multiple vias connected to ground and the thermal pad to improve heat dissipation. 3 Conclusion We use the above method , using two EL7558BC chip design FPGA- based MPEG ? 4 decoder chip design * demonstrate the power development board (input 4.5 ~ 5.5V, output 3.3 ~ 1.5V). 3.3V output circuit of which the design shown in Figure 1 , the indicators are as follows: 1 ) output voltage calibration input voltage from 4.5V to 5.5V and from 0 to 8.0A load current varies in the range , the output voltage does not exceed 1.0% ; 2 ) Load transient response in load current in 15µs or from 0A to 8A 0A from 8A to mutation , the output voltage transient fluctuation of not more than 120mV, fluctuations in less than 25µs; 3) The output voltage ripple in the input voltage of 4.5 ~ 5.5V , the output voltage ripple peak value less than 22mV. 4) The input voltage ripple in load 8A, input voltage of 4.5 ~ 5.5V , the input voltage peak to peak ripple is about 230mV, Increase the input capacitance value , the C9 replaced from 220µF 470µF, the input voltage down to 180mV peak to peak ripple is about . Based EL7558BC of DC / DC module design embodies a new and quick , simple power solutions , the design method in the current DC / DC converter design is very typical , with considerable reference value .