Principle and Application of Buck Dual Output Synchronous Controller LM5642

1 Introduction

AC/DC and DC/DC converters are responsible for the conversion and transmission of system power. Their role in the system is equivalent to the human heart. Once a problem occurs, the system will stop working. The power supply of communication products requires high reliability. Small, lightweight and high-efficiency power supplies are used in various terminal devices and communication equipment dominated by electronic computers. They are an indispensable power supply method for the rapid development of today's electronic information industry.

2 LM5642 Device Introduction

Figure 1 shows the pin configuration of the LM5642 device. The LM5642 switch controller has two operating modes to choose from. In addition to providing two voltage outputs, it can also use the dual-phase mode to provide a single voltage output with a load current that is 1 times larger than the dual output; the device can operate at a frequency of 200 kHz, and the two controllers contained in it can continue to work at 180° anti-phase; for high-current applications that require the device to perform a single-output voltage regulation function, this 180° anti-phase function can greatly reduce the input filter capacitor. The oscillator frequency of the LM5642 can be adjusted within 150 to 250 kHz to increase or decrease the voltage to ensure that it can operate synchronously with the system, thereby reducing the interference caused by the switching frequency. The built-in current mode control loop of the device can vary in a wide range of input voltage Vin of 4.5 to 36 V, and each channel output can be adjusted from 1.3 V to 90% Vin. Its conversion speed is extremely high, and its efficiency is as high as 95%, which is suitable for applications that provide high current and low voltage power to the system.

[page]3 Dual output high current application based on LM5642

Figure 2 is the schematic diagram of the LM5642 dual-output circuit. Its input is 15 V, and the outputs are +5 V/7 A and 3.3 V/7 A respectively. Taking channel 1 in Figure 2 as an example, the key parameters and peripheral device selection considered in the design are introduced.

3.1 Threshold Current Determination

LM5642 has two current threshold setting pins: pins ILIM1 and ILIM2 (Figure 2). Taking pin ILIM1 as an example, the device has a built-in constant current of 0.99μA, which is converted into a voltage through the external input resistor R7 connected to Vin. The voltage will be compared with the voltage of R9. R9 is connected to the input and is the main path for the current. The current enters the power field effect tube through R9. Here R8 is selected as a small 100 Ω, and its voltage division is ignored. By comparing the voltages of R7 and R9, it is determined whether the current is appropriate. If the voltage across R7 is greater than the voltage of R9, the device works normally, otherwise the internal overcurrent protection will be turned off. In the design, the parameters of R7 are reasonably set by comparing the voltage, and R9 is selected with a low resistance value, generally 10 mΩ.

3.2 Output resistance selection

According to the actual output voltage requirements, the relevant resistance values ​​are calculated by equations (1) and (2). In FIG2 , the two output resistors are R13, R14, R23, and R24 respectively.

Considering the accuracy of the output voltage, equation (2) is used to first determine the maximum resistance value of R14. Considering that the output voltage has a 3% deviation, here Vfb = 1.238 V, the maximum current flowing into the feedback pin FB1 is 200 nA.

3.3 Output Inductor Selection

The inductance is determined by the output ripple voltage, and the inductance value Lmin is calculated by equation (3).

In the formula, f is the switching frequency of the device, 200 Hz, Vin is the input voltage, Vo is the output voltage, Vr is the ripple voltage, which is set to 60 mV in power supply design, and Rc is approximately the equivalent impedance of the output capacitor, which is generally 20 mΩ. In practice, the ripple current must also be considered to be too high, and the ripple current must also be used as a basis when selecting the inductor value.

[page] In addition, the influence of the equivalent impedance of the output capacitor on the output ripple, the selection of the power field effect tube and its power consumption should also be considered. In the design, high voltage and large capacity capacitors are used to reduce the ripple. At the same time, low equivalent impedance capacitors are selected. The key components used in the power supply design are all industrial-grade products or military products to ensure the high efficiency of the power supply.

Therefore, the equivalent impedance of the power field effect tube fully meets the standard, and the diode is selected as a Schottky tube with a fast conversion speed to adapt to the fast conversion rate of 200 Hz switching frequency of LM5642.

4 Multi-channel output design based on LM5642

There are many types of controllers for switching power supplies, mainly current and voltage. The voltage control type only samples the output voltage as a feedback signal for closed-loop control, and uses PWM technology to adjust the output voltage. From the perspective of control theory, this is a single-loop control system; the current control type adds a current negative feedback link on the basis of the voltage control type, making it a dual-loop control system, thereby improving the power supply performance. Based on the many advantages of LM5642, the single and dual output channel characteristics of the device are fully utilized. Multiple output voltages are designed with other DC/DC converters to power the system. The power input comes from the 28 V voltage of the on-board battery, which can be used as the power supply for the optical communication system. The specific design block diagram is shown in Figure 3.

The circuit board design is based on the requirements of Figure 3. The components are domestically made LM5642, LM2596, and LM2679. Two LM5642s are used, namely U8 and U6, which output 15 V, +5 V/7 A, and 3.3 V/7 A respectively; two LM2679s are respectively U4 and U5, which output +5 V/3.5 A and VDD respectively. VDD is the power supply for the VDD pin of LM5642; three LM2596s are respectively U1, U2, and U3, which output two -5 V/1 A and +5V/3 A respectively.

[page]5 PCB layout

The physical design of the PCB board is the last link in the design of the switching power supply. If the design method is not appropriate, the PCB may radiate too much electromagnetic interference (EMI). Combining the characteristics of LM5642, a multi-output voltage source is designed with other devices (such as LM2679). The PCB uses a 4-layer board wiring, with components placed on the top and bottom layers, and the middle layers are the ground layer and the high-current wiring layer. The second layer is the wiring layer, and the third layer is the ground layer to ensure high reliability. For +5 V/7 A and 3.3 V/7 A high-current outputs, the wiring is mainly completed on the second layer with a line width of 200 mil. Considering electromagnetic interference. The wiring spacing is appropriately widened, and the input and output are filtered by a combination of tantalum capacitors and ceramic capacitors (the board line width is designed according to the thickness of 1.5 ounces).

6 Conclusion

Since the power supply design is to provide power for the system, the entire system must be considered comprehensively. The power board provides multiple outputs to each sub-board. When planning the layout, each output is separated by a ground line. Since there is a distinction between digital grounds in the sub-boards, in order to reduce interference, DC/DC conversion is also used in each sub-board to reduce digital and analog signal interference. Considering the system performance, input and output ground isolation technology can be used; and considering the transmission of high-speed signals between sub-boards, the input and output isolation ground is not used in the power board design, which can also make the system stable, so a unified ground is used as a reference surface. ■