1. Technical Field
This disclosure relates to power converters. In particular, this disclosure relates to a step down (buck) power converter with inductor based switching, suitable for use with a memory device (e.g., a flash memory device) or other device.
2. Related Art
Continual development and rapid improvement in semiconductor manufacturing techniques have led to extremely high density memory devices. The memory devices are available in a wide range of types, speeds, and functionality. Memory devices often take the forms, as examples, of flash memory cards and flash memory drives. Today, capacities for memory devices have reached 64 gigabytes or more for portable memory devices such as Universal Serial Bus (USB) flash drives, and one terabyte or more for solid state disk drives. Memory devices form a critical part of the data storage subsystem for digital cameras, digital media players, home computers, and an entire range of other host devices.
One important characteristic of a memory device is its power consumption. In an age when many host devices are powered by limited capacity batteries, every fraction of a watt in power saving translates into extended battery life and extended functionality between recharges for the host device. While the memory device is in operation, a power converter provides the power supply to the memory device. A buck power converter typically has much higher efficiency than other types of power converters. This is one reason that buck converters are frequently preferred over linear regulators and charge pump regulators.
However, memory devices present significant technical challenges to the use of a buck regulator. As one example, the form factor of the circuit board in a memory device is often very small. As a result, it is difficult to find space for large off-chip components like the inductor or capacitor used in a buck regulator. Furthermore, the components add extra cost to the memory device, and cost margins for memory devices are already very small.
The sizes of the inductor and capacitor are inversely proportional to the switching frequency of the control loop in the buck regulator. According, in the past, very high switching frequencies on the order of tens or hundreds of MHz or higher were used. Unfortunately, high switching frequencies increase design complexity and cost, while reducing the overall power efficiency. Moreover, the bandwidth of the components of a buck regulator is generally preferred to be significantly higher (e.g., 10 times or higher) than the switching frequency. This is often a difficult condition to meet, and commonly imposes significant restrictions on the maximum possible switching frequency.
One technique for addressing the technical challenges associated with buck converters is to use a multiphase approach with multiple control loops. Each phase requires its own distinct inductor and switching power transistor pairs. The control loops are driven 180 degrees out of phase. Separate pulse width modulated (PWM) signals drive the distinct power transistor pairs. In other words, the conventional multiphase approach requires multiple inductors equal in number to the number of phases. The convention approach also requires multiple power transistor pairs. As noted above, it is difficult and financially prohibitive to provide these extra components, particularly in a small, inexpensive memory device.