Power MOSFET and Schottky diode

Since the grid drive of the device is powered by the 5V bus, the use of logic level MOSFET is very recommended, especially in large current applications. Their breakdown V (BR) DSS must be greater than VINMAX, so they choose to target 20V or 30V devices. Once the allowable power consumption is established, you can choose RDS (ON). By selecting the same power MOSFET as the main switch and synchronous rectifier, the total power consumption does not depend on the duty cycle. Therefore, if PON is this power loss (a few percent of the rated output power), RDS (@25 ° C) is required from the following formulas:

Among them, iout is ITOT5 or IOUT3. According to the chapters considers, A is the temperature coefficient RDS (usually) (usually, for these low-voltage levels, A 5 · 10-3 ° C 1) and #8710; T; T Allow Wen Sheng. However, it is worth noting that the lower the RDS (on), the higher the door charge QG, which will cause higher gate driver consumption. In fact, each switching cycle, a charge QG moves from the input source to grounding, generate an equivalent driving current: IG QG · FSW

This will affect the efficiency of low load current.此外，该电流来自于PREG5线，其来源

容量，ISRC（25mA最小值）不得超过，因此对MOSFET总栅极有进一步的限制充电（@VGS 5V）：

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Assuming four the same MOSFET.

The reverse voltage VRM of the Schottky diode in parallel to the synchronous rectifier must be greater than Vinmax. Because its turnover time is 5%of the switching cycle, the current rated value can be much lower

instead of love. The selection criteria should be: vt (Shawki) lt; VT (body diode)@i ILPK

sensor

Select the inductive resistance of each part according to the respective largest output current Essence The current influenza comparator limits the peak of the inductance, so the maximum DC output current is half the peak value is less than half of the ripples between the peak. The intervention thresholds of the two parts are set to 50mV, so the resistance value should be:

Because the comparison device threshold of the trigger pulse jump mode is 11mV, the system enters this operation. The output current is about a quarter of the maximum output current. The sensing resistance number is low, so it is very important to correct the current signal. Ensure that the Kaervin connection between the IC current detection pin and the sensor does not transmit the output current.

Input a capacitor

Pulse current (average is zero) flowing through BUCK converter input capacitors. The AC component of this current is quite high, and consumes a lot of power on the ESR of the capacitor:

It is easy to find that the maximum value of PCIN is equal to iOut/2 (@vin 2 × VOUT, which is a 50%duty ratio). Therefore, the input capacitor of each part should be selected by the balance -rooted ripple current rated as high as half of the maximum output current. The capacitance value is not very important, but the minimum value must ensure stability. In fact, the switching regulator displays negative input impedance, at a low frequency, that is,:

Therefore, if the impedance of the power supply is not far lower than the absolute value of Zin (DC), the frequency does not exceed the regulator control loop The bandwidth, there is a possibility of oscillation. In order to ensure stability, the following conditions must be met:

, leq is the inductance of the switching regulator input upstream circuit, ESRIN and input capacitors themselves. It is recommended to use high -performance electrolytic capacitors. If the higher cost is ESR-OS capacitor is a good choice because they do not provide a minimum size or rated current.受 The capacitor cannot withstand the pulse current, so it is not recommended.

Output capacitor

The selection of output capacitors is based on the requirements of output voltage ripples. This ripple and current ripples are exactly due to the induction and ESR that is almost due to the output capacitance. Therefore, the goal is to achieve an ESR below a certain value, regardless of the actual capacitance value. +3.3V's maximum current lines are: #8710; IL3 2 · (IL3PK-IOUT3)

Considering the value obtained in the +3.3V induction . For+5.1V, the maximum ripple is:

Among them, VIN refers to the minimum or maximum value selected in the section of the +5.1 transformer section. In any case, the maximum ESR is:

Among the bid x indicating any section. In the pulse jump operation, the capacitance component and resistance component of the output ripples are equivalent to the bidding x indicating any section. In the jumping pulse operation, the capacitance component of the output ripple is equivalent to the resistance. Therefore Waves also stipulate that COUTX and ESRX must be in line with it. Further restrictions on the minimum output capacitance can be arrears from the specifications of the maximum output capacitance, #8710; v out, or super -adjustment, #8710; V+ ]

The amount related to the maximum working cycle and DMAX+3.3V or+5.1V segment. High -performance capacitors should be used to reduce the required capacitance for the given ESR, in order to avoidMultiple components are wasting a lot of space. Although there are good electrolytic capacitors, the OS-CON or 钽钽 capacitors may be the first choice, especially when the design is very compact, or the surface installation component. Multi -layer ceramic capacitors with extremely low ESRs but their capacitors have greatly expanded, so they should be stable with another, high ESR capacitor.

Other components

The bandwidth of the feedback circuit is almost infinite, so a filter is required to make the system unsightly, in order to prevent the correct operation error and comparator of noise interference switch. In short, the cut -off frequency of this filter can be very high, so that the lines and loads can have a very fast transient response. This filter is a simple R-C type. Resistance and capacitors can choose a typical 3DB cut-off frequency of 60kHz. For the self -raising diode, although the small signal diode can be effectively used, it is best to use a low -power Schott Faculture, because it slightly increases the grid driving voltage, which is conducive to efficiency. Boot with a capacitor can be 100NF film capacitors. The soft startup capacitor determines the time limit circuit to gradually move the set point from zero to 50mv to limit the current flow in the startup. The slope lasted about 1 millisecond -to -NF soft startup capacitor (a typical value of 10 to 100 NF), but the actual time required the output voltage to reach the steady state value depends on the load current and the output filter capacitor. Some key points of IC may require bypass electrical containers to prevent noise interference circuits. These points are reference voltage VREF, IC power pins VIN, Preg5 lines and other selected power pins V5SW. Use a thin film capacitor suitable for AC. The output of the external PNP bipolar transistor considers that the output end of the auxiliary voltage is 5.13V. The breakdown voltage of the transformer at 5.1V is also the same for the base of the collector. A small signal transistor is enough for the application.

The transformer capture the diode (only

l5994

a)

control+5.1V transformer second winding of the current. :

There is a certain degree of safety. The diode must withstand the peak value: its equal square root value is given by the following formula:

The value of the DC is obviously low.

Transformer filter capacitor

The most stringent requirement for input filter capacitors (connected between V13In and ground) is its valid value ripple current rated value, at least:

[[] [

123]

The working voltage should be higher than the voltage generated during the light load of the regulator. It is also to recommend the use of high qualityElectrolytic container or OS-CON capacitor.

Arranging and grounding

Electric design is just the first step in the development of switching converters. Because the current range from MA to A, DC and Switched are on the same circuit board. The PCB layout is important to the correct operating circuit for A, but it is not an easy task. The correct layout process usually includes carefully placing components, correct grounding, correct wiring, and proper trace line width. Fortunately, because this application involves low voltage, isolation requirements are not important. References analyze this matter in detail, and only a few points will be reminded here.

(1) All current circuits (signal grounding, power ground ground, etc.) should be isolated and connected at only one place. The horizon is a very useful current to return and minimize radiation (see 2 o'clock), even if they cannot solve all problems

(2) noise coupling between adjacent circuits can be reduced To minimize the area of u200bu200bthe current circuit. This is particularly important when there is a high pulse current, that is, the circuit includes input filter capacitors, power switches, synchronous rectifies and output capacitors. The next priority is the grid drive circuit.

(3) Magnetic field radiation (and mixed inductance) can be as short as possible by retaining all portable switch currents.

(4) The Kelvin connection trace of the current should remain short and close.

(5) For the high current path, as long as it is possible, the trace line on the other side of the PCB can be doubled: this will reduce the resistance and inductance of the line.

(6) Generally speaking, a record channel carrying signal current should be far from carrying pulse current or fast swing voltage. From this point of view, special attention should be paid to high impedance paths (feedback input, current detection tracking ...). The wiring signal trajectory on a PCB may be a good idea on the other side.

(7) Use heavy copper traces: This will reduce its resistance, improve overall efficiency, and improve their cooling capacity.

L5994 evaluation kit

L5994 evaluation kit is a demonstration board that is fully assembled and tested. It implements a standard application circuit, according to the following specifications:

Input Voltage range: 5V ～ 25V

3.3V output: iOUT3 3A

5.1v Output: iOUT5 3A

12V output: iOUT12 50MA

Switch frequency: fsw 300kHz.

The electrical schematic diagram shown in FIG. 7 shows that some upper pull/lower resistors are added to the components that are strictly required in actual applications. Together with the four DIP switches, they allow manually setting the control chip operationLogic signal. In this example, these signals are:

Switch 1: RUN1 (0 5.1V, 1 5.1V open)

Switch 2: OSC (0 200kHz, 1 300kHz )

Switch 3: Noskip (0 pulse jump away, 1 jump pulse level)

Switch 4: RUN2 (0 3.3V, 1 3.3V open)

Please note that as long as each regulator is disabled, the relevant low -end MOSFET is in a connected state. Therefore, if the load can generate current, it will be short -circuited to the ground by the stifling ring and the low -voltage side. Although the default switching frequency is 300kHz (Settings 2 set to 1), the passive element selects this frequency, and the demonstration board will work satisfactory at 200kHz and. In fact, the efficiency of the 200KHz regulator is the highest, and the input voltage range is expanded to the largest. On the other hand, the output ripples are large and the dynamic characteristics are slightly worse. In fact, the demonstration board does not provide synchronous interfaces. In any case, it is possible to synchronize an oscillator (using appropriate signal: 5V amplitude pulse, at least 400ns), and only need to set the switch to 1 in the middle of the signal input frequency divider R8-R9. In this way, synchronization can be achieved at the frequency of higher than 300kHz. Make the oscillator synchronize to 200kHz and 300kHz, which requires greater intervention on the board. In order to maximize the efficiency within the low load current range, the pulse skip operation was enabled by default. The conversion between PWM and pulse jump occurs below 1A, but coexist in the two operating models rather than a clear boundary. The waveforms can be seen on the oscilloscope, but it has little effect on the output ripples and efficiency. People who do not like the asynchronous operation of the pulse skip mode can disable the two regulators by setting the switch 3 to 1. It can keep the PWM run to a very low output current, but the adjustment is not compatible with the switching frequency. This means that the high side MOSFET is too long for penetrating energy levels at the operating frequency. Therefore, the control system began to skip the conduction cycle to avoid drifting upwards.