Driven by portable and wearable device applications, many designs are moving toward supply voltages of 3.6 volts or less. However, many portable devices require higher voltages for certain functions, which requires designers to convert up to the desired voltage as efficiently as possible through optimally implemented DC-DC boost converters.
The role of DC-DC boost converter
A typical wearable or portable device uses a lithium-ion battery with a nominal output of 3.6 VOLTS direct current. Most battery-powered applications rely on one or more lithium-ion batteries in series to provide the main supply voltage. While that's enough for most applications, certain functions that laptops, tablets and other mobile devices have require higher voltages.
Examples include white light emitting diode (LED) backlight drives, rf transceivers, precision analog circuits, and bias circuits for avalanche photodiodes (APD) in optical receivers. Boost DC-DC regulators can convert low input voltage to high output voltage to meet these application requirements.
Typical boost converter topology
The key components of the boost regulator include: inductor; Semiconductor switches, usually power MOSFET; Rectifier diode; Integrated circuit (IC) control block; Input and output capacitors
Basic boost regulator configuration that displays current direction when switches are on and off
When a VIN voltage is applied and the power switch is turned off, current flows through the inductor to ground along the blue path. An inductor stores electrical energy in its magnetic field. The diode is reverse biased, and as its stored electrical energy is supplied to the load, the voltage across the output capacitor decreases.
Instead, when the power switch is on, the current flows along the red path because the collapsing magnetic field generates a positive voltage and transmits the inductor energy through the forward-biased diode, charging the output capacitor and supplying it to the load.
By changing the duty cycle of the power switch, the control block maintains a constant output voltage in response to input voltage changes and load changes. A resistive voltage divider at the output provides voltage feedback to the control block to adjust duty ratio and maintain the desired output voltage value.
In addition to these basic functions, the integrated design includes optional protection against overtemperature, output short circuit, open load conditions, and input overcurrent.
A common improvement to the base circuit is to replace the diode with a second MOSFET. The second MOSFET acts as a synchronous rectifier that turns on when the power switch is off. Its lower voltage drop reduces power dissipation and improves the efficiency of the regulator.
In battery-powered devices, more efficiency equals longer battery life, so synchronous design is a big advantage. In addition, portable and wearable devices are often spatially limited, so boost converters for these applications have a high degree of integration. The inclusion of power components in the package will limit their flow capacity, but this disadvantage is acceptable in the design of battery-powered devices. Many of these applications are in off mode for long periods of time, making ultra-low static current consumption critical.