Efficient DC/DC Converters: The Basis of Portable Design

High-efficiency DC/DC converters are the foundation of all portable designs. Many portable electronic applications are designed to operate from a single AA or AAA battery, which presents a challenge to power supply design engineers. Generating a constant 3.3V system output from an input voltage of 850mV to 1.5V requires a synchronous step-up DC/DC converter that can operate at a fixed switching frequency with on-chip compensation circuitry, and requires a miniature low-profile inductor and ceramic capacitor, preferably in a miniature IC package to reduce its total footprint in the device design.

　　A proven circuit design consisting of a thin SOT IC package and a few external components achieves a 90% efficient single-cell to 3.3V/150mA converter occupying only 7×9mm2 board area. When operating from a single-cell input (1.5V), load currents between 25mA and 80mA are possible with over 90% efficiency. An external low-current Schottky diode (although not required) will maximize efficiency at higher output currents.

　　This circuit design integrates a high efficiency DC/DC converter with a low gate voltage internal switch with a nominal resistance of 0.35Ω(N) and a typical resistance of 0.45Ω(P). The switch current limit is typically 850mA over the entire operating temperature range, resulting in an output power of 0.66W and 2.5W at a new alkaline AA single cell input and two cell inputs, respectively.

　　Current mode control provides excellent input line and output load transient response. Slope compensation (necessary to prevent crossover harmonic instability when duty cycle exceeds 50%) can be incorporated into the converter, along with the circuit to maintain a constant current limit threshold regardless of input voltage.

　　Key Features

　　Two features of advanced power management IC designs affect their operating efficiency: the integration of internal feedback mechanisms and the inclusion of power-save modes that save energy during operation. The addition of internal feedback loop compensation eliminates the need for external components, reducing overall cost and simplifying the design process. Power-save operating modes improve converter efficiency at light loads (ILOAD < 3mA, typical) by activating the power converter only when needed to keep the output voltage modulation within 1%. Once the output voltage is modulating, the converter switches to a sleep state, reducing gate charge losses and quiescent current. A similar IC without a power-save mode would be forced to maintain constant PWM over the entire operating range, increasing quiescent current. While constant-frequency PWM may be desirable in some frequency-sensitive applications, it reduces overall system efficiency.

　　The shutdown current is less than 1mA, and hysteresis on this pin allows a simple resistive pull-up to VIN for continuous operation. Note also that during shutdown, VOUT remains an unregulated 600mV below VIN. This feature is particularly useful when memory or real-time clocks must remain active during power-off. The output voltage can be easily set by changing the resistor value of the voltage divider.

　　To achieve the highest efficiency from a battery source, a DC/DC converter must be able to operate from an input voltage below 1 V and provide an adjustable output voltage in the range of 2.5 V to 5 V. Ideally, such a device would also be able to continue to operate from an input voltage as low as 0.65 V, with the only limitation being the input source's ability to provide sufficient power.

　　This feature will eliminate the need for large input bypass capacitors, saving board space and reducing cost. The ability to operate with input voltages as low as 0.65V is an important feature for getting longer life from nearly depleted batteries.

　　Figure 1: A power management IC capable of operating in low-voltage mode

　　For example, a comparison of the battery life of two portable devices powered by a single battery shows that under ideal test conditions, the ability of the power management IC to operate in low-voltage mode enables it to provide more than six hours of battery life over traditional DC/DC converters. A 40% increase in operating life provides a clear advantage to the end product. The comparison is shown in the figure.

　　EMI Suppression Methods

　　EMI issues may exist when the boost converter is operated in discontinuous mode (i.e., before the power transmission cycle begins, when the inductor current drops to zero). To help lower the potential reference point, an internal 100Ω damping circuit can be placed across the inductor when the inductor current is zero and the device is in the off state.

　　EMI and overall performance quality can also be affected by PCB layout. Low voltage input devices operating at high speeds require extra attention to board layout, especially the high current paths during the duty cycle involving N-channel and P-channel switch switching. The current paths between the SW pin, VIN pin, CIN, COUT, and ground should be short and wide to form the lowest inherent resistance losses and the lowest leakage inductance.