Abstract: Voltage dropping switch rectifier chip’s handling characteristics has carried on the analysis to HSOP the seal EL7558BC, gave has used this rectifier chip to design the DC/DC converter’s peripheral circuit and the design method. And through experimental verification this design method.
Key word: DC/DC converter; EL7558BC; Switch rectifier
Introduction
The EL7558BCDC/DC converter chip was the interior which Elantec Corporation produced integrated the MOSFETs low input voltage (4.5~5.5V), high output current (8A) the PWM rectifier, the efficiency might reach 94%. The output voltage deviation is smaller than 1.5%. The highest turn-on frequency may reach 1MHz, may establish the fixed voltage to output (3.5V) or the variable voltage outputs (1.0~3.8V). EL7558BC has reduces the periphery primary device’s high integrated characteristic as far as possible, only needs the few periphery primary device then to work, thus reduced the circuit wafer area and the designed cost greatly, was the power source design has provided one kind fast and the simple solution. EL7558BC simultaneously has the overheated instruction and the overheated closure load protection function, uses in the logical/processor replacement and the control power supply order voltage feedback PWRGD output signal and so on. Its seal form to have the good heat dispersion 28 foot HSOP seal. These merits cause the EL7558BC power source chip to be possible widely to apply in the high performance DSPs/FPGAs/ASICs/microprocessor, the PC motherboard, portable electronic instrumentation, portable computer and so on many electronic installations.
1 base pin function and handling characteristics
EL7558BC seal form as shown in Figure 1, various base pins function is as follows:
Foot 1 (FB1) voltage feedback input end 1, when the chip establishes as the variable voltage output (VCC2DET is low) effective;
The foot 2 (CREF) the reference voltage shunt capacitor input end, generally connects with 0.1μF the porcelain piece electric capacity and the place;
Foot 3 (CSLOPE) pitch compensation electric capacity input end;
The foot 4 (COSC) the internal neterodyne electric capacity input end, electric capacity CSLOPE and the COSC proportion usually is 1:1.5;
The foot 5 (VDD) the PWM control circuit supply voltage input end, usually and the VIN voltage is the same;
Foot 6 and foot 8 (VIN) voltage dropping rectifier supply voltage input end;
The foot 7, the foot 9-12, the foot 18-19 (VSSP) the voltage dropping rectifier returns to the place, namely power source place;
The foot 13 (VCC2DET) the interface logic input end, the logical 1:00 chip is the 3.5V fixed voltage output, the logical 0:00 chips is the 1.0~3.8V variable voltage output;
The foot 14 (OUTEN) the switch rectifier outputs enables the end effectively, logic 1;
Foot 15 (OT) chip overheated indicated output, usually for high, when the temperature surpasses 135℃ pulls lowly, the temperature drops to 100℃ below when restores changes high;
Foot 16 (PWRGD) Powergood output signal, when output voltage’s error is smaller than predetermined value ±10% for high, otherwise for low;
The foot 17 (TEST) tests the foot, usually must with the VSSP connection;
The foot 20-23 (LX) the voltage out-port, actuates the exterior inductance;
The foot 24 (VHI) the internal high-end gate actuation end, passes a 0.1μF shunt capacitor and LX is connected;
The foot 25 (VSS) the control circuit returns to the place, namely signal place;
The foot 26 (C2V) connects the voltage-doubling circuit output, takes the internal low end gate actuation end;
The foot 27 (CP) the electric charge pump electric capacity negative side actuates the end;
Foot 28 (FB2) voltage feedback input end 2, when the chip establishes as the fixed voltage output (VCC2DET is low) effective, this time the output voltage is 3.5V.
The EL7558BCDC/DC converter chip has the soft start function, moreover does not need the exterior capacitor, when the chip adds the electricity will complete the soft start. EL7558BC has the VCC2DET function, was IntelP54 and the P55 microprocessor has provided the direct connection. EL7558BC has the built-in electric charge pump voltage-doubling circuit, uses in opening internal MOSFET, C5 (to see Figure 1) namely for the electric charge pump electric capacity, D2 and D3 is the electric charge pump diode. If has the 12V analog input, then D2 and D3 may abbreviate.
Figure 1 EL7558BC DC/DC converter chip seal form and typical circuit
2 DC/DC converter’s design
Below take the EL7558BCDC/DC converter chip as an example, carries on the detailed explanation to the DC/DC converter’s design process. Its model designa circuit as shown in Figure 1.
2.1 choice output voltage
The EL7558BCDC/DC converter chip may (3.5V) output or the variable voltage (1.0~3.8V) output through the VCC2DET foot establishment fixed voltage. When VCC2DET is Gao Shiwei fixed voltage output; When VCC2DET is low for the variable voltage output, if this time wants to obtain the different voltage output, may adjusts through feedback resistance R3 and R4, the adjustable output voltage scope is 1.0V to 3.8V. R3 and between the R4 resistance number and output voltage’s corresponding relationships may (1) indicate approximately with the type, under this kind of pattern, the VCC2DET base pin must for low.
Output voltage Vo=1 (R3/R4)×1V (1)
2.2 selection switch frequency
The turn-on frequency as well as needs the external connection inductance to the EL7558BC chip’s transfer efficiency the size to have the very tremendous influence. The frequency is lower, the efficiency is higher, but needs the inductance the value to be also bigger. May connect COSC through the adjustment foot’s electric capacity C8 to establish the turn-on frequency, the adjustable frequency is highest may reach between the 1MHz, C8 capacitance value and the turn-on frequency corresponding relationships may (2) express approximately with the type.
Turn-on frequency fsw=0.0001/Cs (Hz) (2)
In the formula: The C8 unit is farad F.
Changes the turn-on frequency when through adjustment electric capacity C8, connects CSLOPE foot’s pitch compensation electric capacity C7 also to make the corresponding adjustment, electric capacity C7 and the C8 proportion usually is 1:1.5.
2.3 selection input filter element
The EL7558BC chip’s input end usually needs a decoupling electric capacity and a large capacity input capacity. The decoupling electric capacity C12 leading role reduces the chip input end’s high frequency noise, generally uses 1~10 μF porcelain piece electric capacity, this electric capacity when layout must approach the EL7558BC chip to obtain the desired effect as far as possible. The large capacity input capacity C9 leading role reduces the input ripple voltage, a 10μF decoupling electric capacity was already enough in certain applications the filter not to need the large capacity input capacity. As for whether to need the large capacity input capacity, first is decided by the permission biggest input ripple voltage. Usually must make the EL7558BC normal work, the input ripple voltage not to be possible to surpass 300mV. Available -like (3) calculates when only uses 10μF the electric capacity, possibly appears biggest input ripple voltage, if calculates the value which obtains to surpass the permissible value, must use the large capacity input capacity.
δvin=iout (MAX) 0.25/(10μF) (3)
In the formula: δvin to not have time the large capacity electric capacity’s input ripple voltage
Most Feng Feng value;
IOUT (MAX) is the biggest DC load electric current.
The large capacity input capacity’s value is bigger is more advantageous in reduces the ripple voltage, but its equivalent series resistance (ESR) bigger actually will increase the ripple voltage, therefore, must choose the capacity and greatly ESR low electric capacity. The type (4) has given the large capacity input capacity and the input ripple voltage approximate pass
Department. If the ripple voltage is too big, may use the many electric capacity parallel methods. Moreover the large capacity input capacity’s rated voltage and the electric current must be appropriate.
δvin ‘ = (IOUT (MAX) 0.25) /CBULKfSW IOUT(MAX)ESRMAX (4)
In the formula: δvin ‘ to have time the large capacity input capacity’s input ripple voltage most Feng Feng value;
IOUT (MAX) is the biggest DC load electric current; CBULK is the large capacity input capacity which uses is C9;
ESRMAX is large capacity input capacity’s biggest ESR.
2.4 choice output filter element
The output filter element’s choice is in the DC/DC converter design a most essential link, output the filter element to decide power source’s stability. The key point is must choose two parts, one outputs inductance L1, another is output capacity C10. The influence power source stable most key parameter is output capacity’s ESR, electric capacity’s data book can give electric capacity’s biggest ESR generally, but smallest ESR usually for biggest ESR 40%~60%. In addition, in chooses electric capacity’s time, electric capacity ESR floats warm also wants the due consideration.
Output inductance L1 has the memory energy and filters out the ripple two big functions, the inductance choice is mainly by the input, the output voltage, as well as turn-on frequency decision. The inductance nominal current must be bigger than the maximum output electric current (8A), the inductance value’s selection may (5) calculate based on the type obtains.
LOUT=(VIN(MAX)-VOUT) (1/δil) (VOUT/VIN (MAX) (1/fsW) (5)
In the formula: LOUT is output inductance, namely L1;
VIN (MAX) is the biggest input voltage;
VOUT is the output voltage;
δil for the permission maximum output inductance ripple magnitude of current, this value usually must be smaller than 0.8A.
Electric capacity’s choice needs from the electric capacity cocurrent rated voltage, electric capacity’s fixed ripple electric current, power source’s maximum output ripple voltage, the power source stable and so on four factors to consider. The electric capacity rated voltage must be bigger than the output voltage, generally must be higher than 10% at least compared to the output voltage, controls the ripple and the transient response. The biggest electric capacity ripple electric current (i.e. the electric capacity RMS electric current) may use the type (6) to calculate, chooses electric capacity’s fixed ripple electric current must be bigger than the type (6) the computed result.
IC(RMS, MAX)=1/root number 12 (VIN (MAX) - VOUT) VOUT) /VIN (MAX) LOUTsW (6)
In the formula: IC (RMS, MAX) is the biggest electric capacity RMS electric current.
Regarding power source’s maximum output ripple voltage, first, must determine that the concrete application to outputs the ripple voltage the request, the EL7558BC chip output ripple voltage must limit in output voltage 2%. Then, the use type (7) calculates the permission electric capacity biggest ESR, the choice most large quantity decides ESR is smaller than the type (7) the predicted value, guaranteed that the output ripple voltage meets the application requirement. moreover, electric capacity ESR floats warm must consider.
ESRMAX ‘ =ΔV (MAX)/ΔIL (MAX) (7)
In the formula: ESRMAX is permission maximum output electric capacity ESR;
δv (MAX) is the permission maximum output ripple voltage value;
δil (MAX) is the permission maximum output ripple magnitude of current, this value usually must be smaller than 0.8A.
May reduce ESR through the many electric capacity parallel methods, enhances electric circuit’s transient response, total ESR must be bigger than 10mΩ, the total capacitance value must be bigger than 330μF.
2.5 layout wiring matters needing attention
When layout wiring, the principle is all periphery primary device must as far as possible approach the EL7558BC power source chip, particularly the decoupling electric capacity and the shunt capacitor must the cloth nearby the corresponding base pin. The EL7558BC component has two places (simulation and power source), simulation connection all noise sensitive signal, but the power source connection has the noise signal. Between two places will introduce the noise to reduce the chip the performance, especially in big electric current output situation. But, the simulation place’s noise oversized will affect the control signal, therefore, will recommend the simulation and the power source separates, and two places (usually under chip or in input or output capacity negative side) the direct connection will reduce between two places in a spot the noises. Connection feedback foot (foot 1 and foot 28) walks the line to be most sensitive to the noise, must as far as possible short, best cloth among two groundings.
The EL7558BC chip’s radiation mainly disperses the thermit welding plate depending on the VSSP pin as well as the chip base. In order to achieve the good heat dispersion, disperses the thermit welding plate to weld completely on PCB, if has time the middle stratum, must with disperse through many holes the stratum the thermit welding to scrutinize enhances continually the radiation effect.
3 conclusions
We use the above method, designed with two EL7558BC chips (has input 4.5~5.5V based on the FPGA MPEG4 decoder chip design * demonstration development board power source, output 3.3~1.5V). And 3.3V output’s designa circuit as shown in Figure 1, each target is as follows:
1) the output voltage calibration when the input voltage from 4.5V to 5.5V and the load current from changes 0 to the 8.0A scope, the output voltage change does not surpass 1.0%;
2) the load transient response load current in 15μs in changes suddenly from 0A to 8A or from 8A to 0A, the output voltage instant fluctuation does not surpass 120mV, the fluctuation time does not surpass 25μs;
3) the output voltage ripple when the input voltage is 4.5~5.5V, the output voltage ripple peak peak value is lower than 22mV.
4) the input voltage ripple when the load is 8A, the input voltage is 4.5~5.5V, the input voltage ripple peak peak value probably is 230mV,
Increases the input capacitance value, from 220μF changes into C9 470μF, the input voltage ripple peak peak value falls to 180mV about.
Has manifested new fast based on the EL7558BC DC/DC module design, the simple power source solution, its design method in the present DC/DC converter design is very typical, has the suitable reference value.
