Preface: The LTC1709 is a power converter from Linear Technology for Intel Pentium 4 microprocessors, featuring dynamically adjustable output, ultra-fast transient response, high precision, and high efficiency. The LTC1709 complies with Intel's new IMVP IV specification and supports three operating modes for the Pentium 4 CPU: high-performance mode, power-saving mode, and sleep power-saving mode. The LTC1709 uses a dual-phase power supply to provide a maximum current output of 40A; current-mode control ensures stable current output; a wide input voltage range of 4V to 36V allows it to directly convert high voltage from the power supply to the CPU core voltage output, dynamically adjusted via a 5-bit DAC signal; output voltage range: 0.925V-2.0V 1%. The LTC1709 uses a 36-pin SSOP flat package. The LTC1709 has three operating modes: Burst mode, Skip frequency hopping mode, and PWM mode. When the CPU is lightly loaded, the power converter enters the Skip frequency hopping mode. The power converter in the frequency hopping mode has a high working efficiency. When the load current is less than 1A, the converter efficiency can reach 75%. However, when the CPU is heavily loaded and the load current exceeds 1A, the power converter enters the PWM mode to provide a strong current supply to the CPU. The LTC1709 power converter operates at a frequency of 150k-300kHz. It uses inductors and resistors externally and controls the energy transfer of each conversion cycle by adjusting the pulse width to generate a stable voltage output. Its working principle is as follows: (1) In the PWM working mode, the internal clock triggers the N-channel MOSFET Q1 to turn on at the rising edge. The power supply VIN flows through Q1 and L1, and R1 generates the output voltage VOUT. Since the current on the inductor L cannot change abruptly, the load current Iload will not change at the instant Q1 is turned on. However, when the voltage stabilizes, the load current will gradually increase. When the load current Iload is equal to or exceeds the threshold current Ith set by the power converter, the power converter will automatically turn off the top MOSFET and turn on the bottom MOSFET at the same time to complete one cycle. (2) When the CPU changes from light load to heavy load instantly, the power converter does not react, and the CPU load current is provided by the output capacitor COUT. At this time: VOUT (output voltage) = I (load current change) ESR (COUT equivalent series resistance). After that, the CPU will change the voltage VID value and output it to the power converter. (3) When the power converter receives the voltage VID signal change, adjusts the switching time of the MOSFET, changes the pulse width and starts to send energy from the power input terminal VIN to the output terminal. Because the current on the inductor L1 cannot change instantaneously, the output voltage VOUT continues to drop. The degree of voltage drop is related to the total output capacitance COUT. The larger the capacitance of Cout, the smaller the drop in output voltage VOUT. (4) When the current IL on the inductor L1 can provide the load current Iload and there is excess current to provide to the output capacitor, the output voltage VOUT starts to rise and returns to the regulated point. The energy stored in the inductor L1 is transferred to the output capacitor COUT to charge the capacitor and to reserve energy for the next cycle. The above analysis shows that the selection of peripheral circuit components is crucial to ensuring the efficient and stable operation of the power converter. The application circuit design of the LTC1709 typical circuit, under normal circumstances, the DC power supply VIN is the adapter or battery voltage input, the voltage input range is 4-36V, Q1-Q4 are voltage switching MOSFETs, and the output voltage VOUT is dynamically adjusted through the voltage feedback VID signal between LTC1709 and CPU. The circuit design process and selection of peripheral components are introduced below. (1) Selection of POWER MOSFET Each LTC1709 converter controller requires two MOSFETs, namely one N-channel MOSFET at the top and one at the bottom. The operating parameters of MOSFETs are as follows: MOSFET on-resistance RDS(ON), reverse conduction capacitance CRSS, maximum input voltage, maximum output current, etc. The power consumed by the top and bottom MOSFETs when they are working is as follows: (Top MOSFET) (Bottom MOSFET) Where: =0.005/℃; K=1.7 Substituting the parameters and the conventional maximum current and voltage values, Q1-Q4 should be selected as N-channel MOSFETs with a maximum voltage of 30V and a maximum current of 20A as switching MOSFETs. (2) Inductor Selection The selection of inductor L is intrinsically related to the operating frequency f. The higher the operating frequency of the power converter, the smaller the value of the external inductor. Therefore, it is thought that the operating frequency f can be increased to reduce the requirement for the value of the external inductor L. In fact, this approach is not advisable. The reason is that for the sake of working efficiency, the top MOSFET and inductor L will consume more energy as the operating frequency f increases, which will greatly reduce the efficiency of the system. The inductor value is also directly related to the surge current: the larger the inductor value and the higher the operating frequency, the smaller the surge current. Conversely, the larger VIN or VOUT is, the larger the surge current is. The specific formula is as follows: Surge current Iripple is an important aspect to consider in CPU power supply design. At present, CPU power supplies are developing towards the design direction of low voltage and high current. Here, a 1.5 H, 20A inductor with a ferrite core is selected. (3) Select the internal current monitoring pins SENSE+ and SENSE- of Rsense LTC1709 and connect them to the positive and negative terminals of Rsense to monitor the surge current Iripple and the maximum conduction current IMAX of the load circuit. The current comparator limits the maximum current: IMAX = 75mV/Rsense. According to the measured surge current Iripple, it is slightly less than twice the maximum conduction current IMAX, that is: Rsense = 2 (50mV/IMAX). Here IMAX is 20A. After calculation, Rsense is selected as a 5mΩ, 1% precision resistor. (4) Select diodes. Diodes D1 and D2 should be Schottky diodes used in the dead time period of MOSFET to form a discharge loop between inductor L and load circuit, prevent the bottom MOSFET from reverse conducting, protect the MOSFET, and improve working efficiency. Usually, diodes D1 and D2 are selected as Schottky diodes with a rated current of 1A-3A and a reverse breakdown voltage of 30V. (5) Selecting the Output Capacitor COUT: COUT should be a capacitor with a small ESR (Equivalent Series Impedance). This is crucial for reducing the VOUT step voltage when the circuit changes from heavy load to light load, preventing excessive step voltage from burning out the CPU, and suppressing surge current Iripple. Based on the above formula and considering cost, four 180F tantalum capacitors with a maximum voltage of 4V are selected for the output capacitor COUT. Conclusion: The LTC1709 power converter provides stable voltage and current output with a small number of external components, making it particularly suitable for use in the design of portable computer CPU power systems. This chip has been successfully applied in the research and development of several portable computers, and after rigorous testing, it has demonstrated superior performance, strong stability, and excellent results.