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As the power center of electronic products, the endurance of power supply directly determines the service life of electronic products. With the continuous progress of integrated circuit manufacturing process, the power supply voltage of digital circuits has been declining, but the power supply of the system is still at a high potential, so it is necessary to rely on the step-down power supply to provide a lower power supply. Before the advent of switching power supply technology, linear power supply, as the main power supply of various electronic products, can realize one-way conversion from DC high voltage to DC low voltage, and is suitable for low-voltage difference voltage conversion and low load current applications. To improve the performance of electronic products and save energy, the key is to solve the performance problem of power supply. Because of its low power consumption, high conversion efficiency and low production cost, switching power supply has gradually replaced the linear power supply and is widely used in the electronic industry.
At the beginning of the development of switching power supply, the power level mostly adopts discrete devices and simple asynchronous rectification technology, as shown in Figure (1). Fig. (1) Asynchronous rectification DCDC BUCK
The synchronous rectification technology uses MOSFET instead of rectifier diode. Because the conduction resistance of MOSFET is very low, the conduction loss of rectifier devices is greatly reduced, and the conversion efficiency is improved. The synchronous rectification technology is especially suitable for low voltage and large current applications. Synchronous rectification BUCK is shown in Figure (2). Fig. (2) Synchronous rectification DCDC BUCK
In the middle and late 1990s, with the development of integrated circuits, MOS discrete components were integrated into the chip, which greatly improved the overall performance of DCDC BUCK, reduced the cost, and showed strong vitality. For BUCKs with small current, PMOS is mostly used for power level High Side MOS, which makes the control circuit simple. For high current BUCK, the economical NMOS will be used, and the gate voltage of NMOS will be raised through the bootstrap circuit, as shown in Figure (3). Figure (3) Synchronous Rectification DCDC BUCK with Bootstrap Circuit
DCDC can be divided into voltage mode control (Figure 4) and current mode control (Figure 5) according to the control loop* Figure (4) Voltage Mode DCDC BUCK Control Schematic Diagram * Figure (5) Current Mode DCDC BUCK Control Schematic Diagram
The structure of the voltage control mode system is simple, because it has only one loop of voltage feedback, slow dynamic response, double poles and complex compensation. On the basis of retaining voltage control mode, current mode control adds a current feedback loop, that is, a double loop control system with voltage feedback outer loop and current feedback inner loop. The closed-loop response of current mode control is fast, and the single pole system is easy to compensate. However, when the duty cycle (D) is greater than 50%, subharmonic oscillation is easy to occur, and various harmonic compensation circuits emerge as the times require to make up for the shortcomings. For a long time, current mode control DCDC has been the mainstream of power supply.
Guided by Moore's Law, the line width of semiconductor manufacturing process is shrinking, and portable devices in the market such as smart phones, tablet computers and digital cameras are becoming thinner and thinner, with increasingly powerful functions. However, the power supply voltage required by digital products is declining, the current is increasing, and the requirements for power supply performance are increasing. The traditional PWM mode DCDC can no longer meet the market demand.
In recent years, COT (Constant On Time) control architecture has been widely used. The DCDC of COT architecture has several advantages:
1. The control circuit is simple, without error amplifier and current sampling resistor.
2. Fast response to load changes.
3. It still has high efficiency under light load. The ESR (series equivalent resistance) of the output stage capacitor has its own inductance current information. As long as its "information" is sufficient (the generated ripple can be compared with the capacitance ripple), it can be used as a current detection resistor to achieve current mode control [1] [2] [3] using only the output voltage. In applications where the output voltage ripple requirement is not high, a resistor can be superimposed on the capacitor to generate such a ripple signal, as shown in Figure (6) R3. Figure (6) COT application circuit
Capacitors with high ESR (electrolytic capacitor, solid state capacitor (OSCON), polymer organic semiconductor solid capacitor (POSCAP)) are usually used to achieve this ripple. Due to the strict limitation of output adjustment voltage specification and the need for cost and size compression, power designers turned to ceramic capacitors with lower cost, smaller size and lower ESR [1]. Using a COT architecture with ceramic capacitors, it is necessary to "create" a ripple with sufficient amplitude with inductive current information. Figure (7) shows a ripple generation circuit, and the generated ripple can be calculated by Formula (1) [1]. Figure (7) DCDC BUCK Ripple Generation Circuit of COT Architecture
Formula (1): V_ (CX(PP))=(I_(L(PP)) ¡Á L)/(R_X ¡Á C_ X )
Another application of the same chip as Figure (6) is shown in Figure (8): to obtain smaller output ripple, instead of R3, RA and CA generate ripple with enough inductance current "information", which is loaded on the feedback voltage signal. Figure (8) Minimum ripple output application circuit with ripple injection
TPP2020 DCDC BUCK developed by SAP adopts COT technology. Its input voltage is up to 20V, output voltage is 5V to 1V, output current is up to 3A, and efficiency is up to 93%. Its application circuit is shown in Figure (9).
Figure (9) TPP2020 Application Circuit
With the development of microelectronic technology and the need for power supply of electronic products, DCDC BUCK has been innovating constantly, from asynchronous rectification to synchronous rectification, from off chip discrete MOS to on-chip integrated high-power MOS, from single loop voltage mode to dual loop current mode, from complex loops and compensation circuits to simple COT architecture (error amplifiers, compensation circuits, and even oscillators can be omitted), From PWM to PFM operation... Now COT architecture has been vigorously developed in the field of power supply with its incomparable advantages. In the future, new technologies will continue to be created and integrated to make our DCDC BUCK performance more outstanding
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