Turbo-boost charger to support CPU turbo mode

Introduction
In order to continuously improve the dynamic performance of the CPU and enable laptops to handle complex multitasking at high speed, we must first increase the CPU clock frequency for a short period of time and make full use of its heat dissipation capacity. However, doing so will cause the total power consumption required by the system to exceed the power provided by the power supply (such as: AC adapter, etc.), causing the adapter to crash. One possible solution is to increase the rated power of the adapter, but the cost will also increase. The turbo boost charger introduced in this article allows the adapter and battery to power the system at the same time to meet the sudden and ultra-high power requirements when the laptop is working in the CPU core acceleration mode.

In a traditional notebook system, an AC adapter is used for power, and the battery is charged with power not required by the system. When the AC adapter is not available, the battery is used to power the system by turning on the S1 switch (see Figure 1). The adapter can power the system and charge the battery at the same time, so it is required to have a higher power rating, which makes it difficult to effectively control the size and cost. Dynamic power management (DPM) is generally used to accurately monitor the total power of the adapter to prioritize powering the system.

Figure 1 Adapter and battery charger system

Once the adapter's power limit is reached, DPM regulates the input current (power) by reducing the charging current and directly powering the system from the adapter without optimal efficiency power conversion. When the system is at maximum load, all adapter power is used to power the system and the battery is not charged. Therefore, the main design criteria is to ensure that the adapter's power rating is sufficient to support peak CPU power and other system power.

There is an increasing demand for high system performance using multiple CPU cores and enhanced graphics processor units (GPUs) to process complex tasks at high speed. To meet this demand, Intel developed turbo-boost technology for its Sandy Bridge processors. This technology allows the processor to have a burst of power demand that exceeds the thermal design power (TDP) for a short period of time (tens of milliseconds to tens of seconds). However, the design of the AC adapter can only meet the high power requirements of the processor and platform at a certain TDP level, taking into account the design tolerance. When the charger system finds that the adapter reaches its input rated power after the charging current is reduced to zero by the dynamic power management unit, one of the simplest ways to avoid the AC adapter crashing is to implement CPU throttling by reducing the CPU frequency, but this will reduce system performance. How can the CPU be allowed to run at high speed above the TDP level for a short period of time without crashing the adapter or increasing its rated power?

Turbo-boost battery charger
When the total power required by the system load and the battery charger reaches the adapter power limit, dynamic power management starts to reduce the battery charge current. The battery charger stops charging and its charge current drops to zero when the system load reaches the AC adapter power limit. As the system continues to increase its load in CPU core turbo mode, the battery charger (usually a synchronous buck converter) sits idle because there is no remaining power available to charge the battery. This synchronous buck converter is actually a bidirectional DC/DC converter that can operate in buck mode or boost mode depending on the operating state. If the battery charge is sufficient, the battery charger works in boost mode and powers the system together with the AC adapter. Figure 2 shows the block diagram of a turbo-boost battery charger.

Figure 2. Turbo-boost battery charger operating in CPU core acceleration mode

So when and how does the battery charger switch from buck mode to boost discharge mode? The system can enter CPU core acceleration mode at any time, so it is often not possible to notify the charger through the SMBus in time to start this mode transition. The charger should be able to automatically detect which operating mode the system requires. In addition, it is very important that the system design can achieve fast transitions between buck and boost modes. DC/DC converters require soft-start times of hundreds of microseconds to several milliseconds to minimize inrush current. The adapter should have strong overload capability to support the total system peak power demand before the charger switches to boost discharge mode. Most current AC adapters can maintain their output voltage for several milliseconds.

Figure 3 shows an application circuit for a turbo-boost battery charger supporting CPU core acceleration mode. The RAC current sense resistor is used to sense the AC adapter current to implement dynamic power management functions and determine whether the battery charger operates in buck charging mode or boost discharge mode. The current sense resistor R7 detects the host programming battery battery charging current through the SMBus based on the battery status. If necessary, the total power provided by the charger and the system can be monitored through the IOUT output, which is 20 times the voltage drop across the sense resistor RAC (to achieve CPU downclocking). The battery boost discharge mode can be turned on or off based on the battery charge status and temperature conditions through the SMBus control register. In boost discharge mode, the circuit provides additional cycle-by-cycle current limit protection by monitoring the voltage drop across the low-side MOSFET Q4. To implement ultra-thin notebook computers such as Intel UltrabooksTM, the switching frequency can be set to 615, 750 or 885 kHz. This minimizes the inductor size and the number of output capacitors. The charger control chip fully integrates the charging current loop compensator, charging voltage and input current regulation loops to further reduce the number of external components. The power selector MOSFET controller is also integrated into the charger. In addition, the charger system uses all n-channel MOSFETs instead of p-channel power MOSFETs used in traditional charging solutions to reduce costs. Another benefit of using this turbo-boost charger system is that it can be used for any of the above functions without changing the bill of materials. System designers can perform rapid system performance evaluation without increasing the hardware design workload.

Figure 3 Turbo-boost battery charger application circuit

Figure 4 shows the switching waveforms that occur during the transition from buck charging mode to boost discharging mode. When the input current reaches the maximum power limit of the adapter due to increased system load, the battery charger stops charging and the battery switches to boost mode to provide additional power to the system.

Figure 4 Waveforms between buck charging mode and boost discharging mode

Figure 5 shows the efficiency of the turbo-boost charger. We can see that more than 94% efficiency can be achieved when charging and discharging a 3- or 4-cell battery pack. If the battery is removed or the remaining battery power is too low, the CPU must be throttled to avoid adapter crashes.

Figure 5 Turbo-boost charger efficiency

Now, the battery can be discharged even when the adapter is connected. However, one potential issue is battery life. Since boost discharge mode only lasts for tens of milliseconds to seconds, its impact on battery life is minimized. Battery aging is proportional to the cell voltage; therefore, the higher this voltage, the faster the battery ages, and the faster the battery ages, the shorter its life. Discharging the battery in boost discharge mode results in a lower cell voltage, which reduces battery aging and ultimately extends its life.

Conclusion
The turbo-boost charger is a simple, cost-effective method. It allows the battery to make up for the lack of AC adapter power for a short period of time when both the AC adapter and the battery are powering the system. This topology supports CPU core acceleration mode, ensuring the lowest system cost, and does not require the AC adapter power rating to be increased to meet peak system power requirements. Test results show that the turbo-boost charger is a practical solution for real-world notebook computer designs.