The role of on-chip voltage regulators and on-chip voltage regulators

What are on-chip voltage regulators and on-chip voltage regulators? What is their role? As our lives increasingly require mobile computing and handheld devices, system design is shifting from discrete devices to highly integrated system-on-chip (SoC) , at the same time, the back-end server requires faster computing processing power to meet the growing data processing needs. One such trend is the development of batteries that are environmentally friendly and last longer. This requires a more complex power management scheme, in which voltage regulators play a key role, so how they are placed is critical to improving performance.

A typical voltage regulator. "Power Supply Unit" is the power supply for the voltage regulator. The power supply for servers, desktops and laptops is usually 12V, and the power supply for mobile devices is 3.3V. 1.0V regulators are usually used to power core digital circuits, while IO is usually 1.8V. The duty cycle of the "gate driver" determines the output voltage of the switching regulator. A low-pass LC filter is used to eliminate ripple on the output voltage rail. To power sensitive analog circuits, a separate LDO is used.

Placing the voltage regulator on the die has several benefits:

• Finer, faster power management: On-chip voltage regulators provide greater control over turning off unused circuits to save power. For off-chip regulators, the time required to wake up from sleep is on the order of microseconds (μs), while for on-chip regulators it is on the order of nanoseconds (ns). This helps provide tighter power control, extending battery life for handheld devices and reducing cooling costs for servers.

•Reduced I2R power consumption: A typical server processor draws 65W at full load. If the voltage is 1V, 65A current needs to be delivered. Such a large current requires wider traces to ensure low I2R loss. If the same power is provided at 2V voltage, the current is reduced to half (32.5A), and the trace width becomes smaller with the same I2R loss. The reduction in I2R losses can also reduce board size.

•Save PCB area: The available area of ​​PCB is very precious, and reducing the PCB area can reduce the overall size. By placing the regulator on the die, its associated components can be removed, saving PCB area and reducing BOM.

Given these benefits of on-chip regulators, why do some designs still prefer off-chip regulators? To answer this question, let’s first discuss the building blocks of a voltage regulator and the benefits of using an on-chip regulator or an off-chip regulator. Weigh the pros and cons.

•Power train: The power train provides two conduction paths for the voltage regulator, one is from VIN to the filter, and the other is from the filter to ground. In all regulators, the path from VIN to the filter is a PFET or NFET switch. On the other hand, the path from the filter to ground is either a diode or an NFET. Using FETs on both paths results in better voltage regulation performance. The first trade-off is the voltage rating on the FET. There are several types of off-chip FETs and can support voltages in excess of 400V. In contrast, on-chip FETs can support up to 3.3V due to the typical CMOS process, while for processors, they can only support up to 2V.

•Another trade-off is the power consumption of the power delivery circuitry. Off-chip FETs have extra space to dissipate heat, but on-chip FETs do not, which increases the overall die temperature and requires additional thermal management design. Placing the power delivery circuitry on the die can provide switching frequencies up to several hundred megahertz, thereby reducing filter size. Also having the power delivery circuit on the die will double the number of power pins because additional pins are needed to connect the output of the power delivery circuit to the filter.

•Inductor: The inductor and the capacitor together form a low-pass filter to suppress the ripple voltage at the output. The inductance required for a voltage regulator is usually large, so the inductor cannot be placed on the die. The off-chip regulator uses off-the-shelf SMD inductors. On-chip regulators help achieve the same efficiency by reducing the size of the required inductor by allowing higher switching frequencies, and can be implemented through PCB traces. Doing so can reduce the number of components on the PCB, but at the expense of increased DC resistance and thus increased losses.

•Capacitor: Capacitor is used to reduce the ripple at the output of the voltage regulator. For off-chip regulators, larger capacitors are needed to suppress ripple due to the lower switching frequency. Using an on-chip voltage regulator with high-frequency switching allows the on-board capacitors to be made smaller or even eliminated. However, this still occupies a considerable area on the die and typically has a higher ESR (equivalent series resistance) than an off-chip capacitor.

•Controller: The voltage regulation loop controller has little effect on the placement of the regulator. For off-chip regulators, the controller uses a separate chip; while for on-chip regulators, the controller is part of the control loop on the die. The advantage of an on-chip controller is that it allows better control of the voltage regulation loop, and the controller can be implemented in the digital domain and therefore reusable as technology changes.

Finally, whether the voltage regulator is placed on-chip or off-chip still depends on the actual application. When the load current is small, the input voltage is high, slower wake-up and sleep are allowed, or power management granularity is not important, an off-chip voltage regulator is a better choice. For these reasons, LED lights are often powered from an off-chip voltage regulator. Conversely, if the load current is high, the input voltage is low, and efficient operation requires fast wake-up or finer granularity, an on-chip voltage regulator is a better choice. A typical desktop/server processor is an example of using an on-chip voltage regulator to help improve power management. The above is the relevant analysis of on-chip voltage regulators and on-chip voltage regulators, and the roles they play on the chip.