Research on the Design of Power Supply Solution for DSP Processor

Designing a good power supply for complex DSP processors is very important. A good power supply should be able to handle dynamic load switching and control noise and crosstalk present in high-speed processor designs. Constant transients in DSP processors are caused by high switching frequencies and transitions in and out of low-power modes. Depending on the bandwidth and layout of the power supply design, these fast transients can cause high voltage drops. The power supply should also be able to handle large surge currents caused by bus contention and decoupling capacitor discharge . Without the ability to manage large currents, the output voltage may drop outside the maximum allowable range of the processor voltage.

The first thing designers need to do when selecting a DSP power supply is to choose the type of regulator. Regulators can be divided into two categories, linear regulators and switching regulators. Linear regulators are easy to use due to their simple topology consisting of a pass element and an error amplifier. The main advantages of linear regulators are low output noise and good transient performance due to the generally higher loop bandwidth. The main disadvantage is low efficiency at heavy loads and large voltage differences between input and output. The formula for calculating the power dissipation of a linear regulator is:

The input voltage is typically 5V or 3.3V, and the output voltage is reduced to 1.0V to 1.2V. This voltage difference, multiplied by a load current of 5A or more, can produce power dissipation that is beyond the capabilities of a linear regulator. Therefore, a switching regulator is often used for processor power supplies. A switching regulator uses an inductor and capacitor to store and transfer energy from the input to the output. This structure is more efficient than a linear regulator because the pass element is not always on and is delivering power to the output. Switching regulators can use pulse frequency modulation (PFM) and pulse width modulation (PWM). The advantage of a PFM-type switching regulator is high light load efficiency, which is a very important feature because DSPs frequently switch in and out of low power modes. The disadvantage of this technique is that it is usually noisier than a PWM regulator because a large amount of current is delivered to the output at the beginning of each cycle. This noise can be reduced by adding additional capacitance at the output. A PWM regulator operates at a fixed frequency and varies the pulse width to maintain the correct output voltage. Generally, the advantages of a PWM regulator are low noise and smaller components when running at higher frequencies. However, they do have the disadvantage of low efficiency at light loads, which can be problematic for processors running in low-power modes.

Supply voltage tolerance is an important specification in any DSP processor data sheet. The power supply that powers the processor must never drop outside of this specification. To meet this specification, the power supply faces many challenges that must be overcome, so careful consideration is required when selecting a power supply. The output voltage accuracy of the power supply is a large part of this tolerance. For example, a typical DSP processor may require a 1.2V core voltage and a 1.8V I/O supply voltage, both with a tolerance of 5%. If the over-temperature output accuracy of the power supply is 2%, then the designer only has 3% margin to overcome other obstacles. Fortunately, the input voltage of the power supply is relatively stable, and with good decoupling capacitor layout, the designer does not have to worry about line regulation specifications. However, the designer must pay attention to load regulation specifications because DSP processors are subject to multiple loads and need to enter and exit low power modes. Typical load regulation specifications may be between 0.2% and 0.5%, which is an important part of the total tolerance of the power supply.

Finally, load changes will not only affect voltage regulation, but will also cause large and fast load transients to the power supply due to its fast-changing dynamic nature. To maintain the output voltage during these dynamic transients, the power supply must react quickly and aggressively. Large output capacitors can help mitigate voltage drops, but much of this power will come from the loop bandwidth and gain of the power supply. The loop bandwidth of the power supply determines how quickly the power supply reacts to load changes, while the gain determines how aggressive the response is. Figure 1 shows that with a 5% tolerance, 2.2% has been used up by load regulation and power supply accuracy, leaving only 33mV for the power supply to handle any transients the processor may experience. Designers need to pay close attention to these specifications and the load transient behavior of the power supply when selecting a power supply for a DSP.

Figure 1: Voltage tolerance of DSP processors.

[page]People often underestimate the importance of good power layout, but in fact, it can play a large role in meeting the overall power supply tolerance requirements. Proper placement of decoupling capacitors can help reduce noise and crosstalk, which is especially important for switching regulators. Placing the input capacitors of a switching regulator close to the input pins can significantly reduce the deviation in the input supply. This in turn reduces the effects of line transients and can reduce output deviation by 0.2% to 0.5%. Considering that most DSPs have a 5% voltage tolerance, this is a significant amount. Decoupling capacitors and inductors should be placed close to the device to reduce current loops.

In a switching regulator, the switch node is a high-frequency node where the voltage switches between approximately ground and the input voltage. Improper layout can cause the switch node to interfere with other signals in the system. Figure 2 shows a proper switching regulator layout with small current loops and close to the regulator. The red wires are high-power and switch connections and must be physically close to the device to minimize noise and crosstalk. The blue wires are noise-sensitive connections and should be routed away from the switch node. Among the external components, CIN and COUT should be placed closest to the device.

Figure 2: Layout considerations for a switching regulator.

Micrel's MIC22950 provides an ideal solution for powering the DSP processor core. Most DSP manufacturers believe that the DSP power supply should provide a current twice the maximum current consumption (calculated value) of the core. The MIC22950 has a current output capability of 10A to avoid insufficient current supply. Figure 3 shows the block diagram of the MIC22950. An important feature of the MIC22950 is the slope control (RC) function used to solve the inrush current problem. The calculation formula for the current on the capacitor is:

Where C is the capacitor, ΔV is the voltage across the capacitor, and ΔT is time. The inrush current can be controlled by controlling the time slope of the input voltage. The MIC22950 can solve the sequencing problem in DSP processors by using the RC pin in conjunction with the power-on reset (POR) pin. The DSP data sheet contains information on sequencing power on and off. By using the RC pin in conjunction with the POR pin, designers can implement windowed sequencing, delayed sequencing, and proportional sequencing.

Figure 3: Block diagram of the MIC22950

[page]The MIC22950 has an output voltage accuracy of 2%, which is capable of meeting the demanding tolerance range requirements of DSP processors. Its load regulation range is 0.2%, leaving more than 2.8% to overcome any load transients caused by the fast switching of the processor. Figure 4 shows the load transient when the output current changes from 1A to 10A on the MIC22950. Typically, DSP processors will not have such large load transients, but even in this case, the output voltage of the MIC22950 changes by less than 50mV when the output voltage is 1.8V, which is lower than the 2.8% deviation requirement.

Figure 4: Line transients of the MIC22950.

The MIC22950 is also part of a new family of products that use SuperThermal(tm) FET technology. The company has released the MIC22400, MIC22600, and MIC22700, which are capable of delivering 4A, 6A, and 7A output currents, respectively. By using this technology, Micrel has made the MIC22950 one of the highest power density products available in the industry. The ratio of output power to package size is a measure of power density. Since base station board space is limited, designers cannot increase the size of the power supply indefinitely . Therefore, designers will choose the device with the highest power density to ensure that the circuit gets enough power while saving as much valuable board space as possible. The power density of the MIC22950 is 0.4A/mm2, while most other products in the industry are still less than 0.23A/mm2.

The MIC23153 is an ideal solution for powering the I/O voltage of a DSP. With 2A current capability and a new HyperLight Load(tm) (HLL) architecture, the MIC23153 delivers power very efficiently at both light and heavy loads. For light load applications, the HLL architecture uses the charge stored in the output capacitor to maintain the output voltage. Due to the low load current, the output voltage will take longer to fall. In the shutdown state, the MIC23153 turns off all circuits on one side of the current loop except the error comparator and the bandgap comparator, thereby saving more energy in shutdown mode. Once the output voltage drops below the bandgap voltage, the HLL architecture sends a signal to enable the high-side transistor. By turning on the output only when necessary, this patented Micrel architecture uses pulse frequency modulation (PFM) to provide high efficiency at light loads. At heavy loads, the MIC23153 operates in fixed-frequency PWM mode, combining the advantages of both PFM and PWM regulators.

Another benefit of the MIC23153 is the Power Good (PG) feature. When the chip is connected to an output voltage, the PG pin will output a logic high if the output voltage is above 92% of its set voltage. This pin can be used in conjunction with a voltage monitor and the MIC22950 to help with the sequencing required by the DSP processor.

As wireless standards and technologies evolve, the power supply industry must also change to keep up with the changing market. The increasing integration and processing speed of DSP processors put greater pressure on processor power supplies. By understanding the importance of relevant indicators and careful layout, designers can develop higher power but still robust power supplies.