Efficient DC/DC converter saves power in portable design

Developing a power supply circuit for portable electronic equipment requires design engineers to extend battery life by maximising the power efficiency and minimising power dissipation throughout the system, which in turn enables the devices themselves to become smaller and contributes to more flexibility in the design of the end product. One of the most important components of such a design is the power management IC or DC/DC converter.

A highly efficient DC/DC converter is essential to any portable design. Many portable electronic applications are designed to operate using a single AA or AAA battery cell, presenting challenges for the power supply designer. Producing a constant 3.3V system output from an input voltage ranging anywhere from 850mV to 1.5V requires a synchronous boost DC/DC converter capable of operating at a fixed switching frequency with on-board compensation circuitry and requiring only tiny, low-profile inductors and ceramic capacitors, preferably in a miniature standard IC package to reduce its overall footprint in the device design.

A proven circuit design, consisting of a thin SOT IC package and a few external components, completes a 90 per cent efficient, single-cell to 3.3V/150mA converter that occupies just 7-by-9mm of board area. Efficiencies of more than 90 per cent may be achieved for load currents between 25mA and 80mA when operating from a single-cell input (1.5V). An external, low-current Schottky diode, although not required, will maximise efficiency at higher output currents.

This circuit design integrates a high-efficiency DC/DC converter with low-gate-charge internal switches rated at 0.35Ω (n) and 0.45Ω (p) typical. Switch current limit is typically 850mA over the full operating temperature range, enabling output power of 0.66W with a fresh single alkaline AA cell input and 2.5W from two cells.

Current mode control delivers excellent input line and output load transient response. Slope compensation (required to prevent subharmonic instability when duty cycle exceeds 50 per cent) can be built in to the converter, along with circuitry to maintain a constant current limit threshold regardless of input voltage.

Key features
Two features of an advanced power management IC design contribute to the efficiency of its operation: the integration of internal feedback mechanisms and the inclusion of a power-save mode to conserve energy during operation. The addition of internal feedback loop compensation eliminates the need for external components, lowering overall cost and simplifying the design process. Power-save operation improves the converter efficiency at light loads (ILOAD <3mA typ) by activating the power converter only when required to keep the output voltage regulated to within 1 per cent. Once the output voltage is in regulation, the converter switches to a sleep state, reducing both gate charge losses and quiescent current. A similar IC without the power-save mode would be forced to remain in constant PWM over the entire operating range, increasing the quiescent current. While a constant frequency PWM may be desirable in some frequency-critical applications, it decreases the overall system efficiency.

Shutdown current is <1?A, and hysteresis on this pin allows for a simple, resistive pull-up to V_in for continuous operation. Note also that in shutdown, V_out remains an unregulated 600mV below V_in. This is particularly useful when a memory or real-time clock must remain active during power down. The output voltage can be easily set by changing the resistor values of the voltage divider.

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

This feature would eliminate the need for a large input bypass capacitor, saving board space and reducing costs. The ability to operate down to 0.65V at the input is an important part of getting more life from a nearly exhausted battery.

As an example, the battery life comparison of two portable devices powered by single-cell batteries shows that the ability of the power management IC to operate in low-voltage mode provides some 6hrs more operating time than a conventional DC/DC converter under identical test conditions. A 40 per cent increase in operating life gives the end product obvious advantages.

Suppressing EMI
EMI issues can be problematic when boost converters operate in discontinuous mode (i.e., when inductor current falls to zero before the start of the flowing power cycle). To help reduce potential reference, an internal 100Ω damping circuit can be connected across the inductor when the inductor current is zero and the part is in shutdown.

EMI and overall performance quality are also affected by the PCB layout. The high-speed operation of a low-voltage input device demands careful attention to board layout, in particular the high-current paths during an operation cycle involving the switching of the n-channel and p-channel internal switches. The current paths between SW-pin, V_in-pin C_in, C_out and ground should be short and wide for lowest intrinsic resistive loss and lowest stray inductance.

- Steven Chen
EE Times