Efficient DC/DC converters are the foundation of all portable designs. Many portable electronic applications are designed to operate with a single AA or AAA battery, which presents challenges for power supply design engineers. Producing a constant 3.3V system output from an input voltage of 850mV to 1.5V requires a synchronous step-up DC/DC converter to operate at a fixed switching frequency, with on-chip compensation circuitry, and requires miniature low-inductance and ceramic capacitors. It is best to use a miniature IC package to reduce its total footprint in the design of the device.
A proven circuit design consisting of a thin SOT IC package and a small number of external components enables a single cell to 3.3V/150mA converter with an efficiency of only 90% of the 7&TImes;9mm2 board area. When operating at a single cell input (1.5V), load currents between 25mA and 80mA may achieve efficiencies greater than 90%. 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 electrode voltage internal switch with a nominal resistance of 0.35 Ω(N) and a typical resistance of 0.45 Ω(P). The switch current limit is typically 850 mA over the entire operating temperature range, enabling 0.66 W and 2.5 W output power for the 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 (which is necessary to prevent cross-over harmonic instability when the duty cycle exceeds 50%) can be integrated into the converter to maintain a constant current limit threshold with the circuit, regardless of the input voltage.
Main characteristics
Two features of advanced power management IC design can affect its efficiency: the integration of internal feedback mechanisms and the addition of a power-saving mode that saves energy during operation. The increased internal feedback loop compensation eliminates the need for external components, reducing overall cost and simplifying the design process. The power-saving mode of operation increases converter efficiency at light loads (ILOAD "3mA, typ") by activating the power converter only when needed to keep the output voltage modulation within 1%. Once the output voltage is being modulated, the converter switches to sleep, reducing gate charge loss and quiescent current. A similar IC without a power save mode will be forced to maintain a constant PWM over the entire operating range, thereby increasing the quiescent current. While constant frequency PWM may be popular in some frequency sensitive applications, it will reduce overall system efficiency.
The shutdown current is less than 1mA, and the hysteresis on this pin allows for a simple resistive pull-up of VIN for continuous operation. Also note that during shutdown, VOUT remains below the unmodulated 600mV of VIN. This feature is especially useful when the memory or real-time clock must remain active during a power outage. The output voltage can be easily set by changing the resistor value of the voltage divider.
In order to get the most power from the battery power supply, the DC/DC converter must be able to operate at input voltages below 1V and provide an adjustable output voltage ranging from 2.5V to 5V. Ideally, the device will also be able to operate at input voltages as low as 0.65V. The only limitation is the ability of the input supply to provide sufficient power.
This feature eliminates the need for large input bypass capacitors, saving board space and reducing cost. The ability to operate at input voltages as low as 0.65V is an important feature for achieving longer life from batteries that are nearly exhausted.
Taking two portable devices powered by a single battery as an example, the comparison of battery life shows that under ideal test conditions, the ability of the power management IC to operate in low voltage mode allows it to provide six more than conventional DC/DC converters. More than an hour of battery life. A 40% increase in working life provides a distinct advantage for end products. The comparison is shown in the figure.
EMI suppression method
When the boost converter is operating in discontinuous mode (ie, when the inductor current drops to zero before the power-drive cycle begins), there may be EMI issues. To help reduce the potential reference point, a 100Ω internal damper circuit can be connected across the inductor when the inductor current is zero and the device is off.
EMI and overall performance quality are also affected by PCB layout. High-speed operation of low-voltage input devices requires special attention to board layout, especially during high-current paths during duty cycles involving N-channel and P-channel switching. The current path between the SW pin, VIN pin CIN, COUT, and ground should be short and wide to create the lowest inherent resistive losses and the lowest leakage inductance.
Figure: A power management IC (purple) that operates in low-voltage mode provides more than six hours of battery life than a traditional DC/DC converter (red).
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