8.1 Overview

The TPS61256A synchronous step-up converter typically operates at a quasi-constant 3.5-MHz frequency pulse width modulation (PWM) at moderate to heavy load currents. At light load currents, the TPS61256A converter operates in power-save mode with pulse frequency modulation (PFM).

During PWM operation, the converter uses a novel quasi-constant on-time valley current mode control scheme to achieve excellent line/load regulation and allows the use of a small ceramic inductor and capacitors. Based on the V_IN/V_OUT ratio, a simple circuit predicts the required on-time.

At the beginning of the switching cycle, the low-side N-MOS switch is turned-on and the inductor current ramps up to a peak current that is defined by the on-time and the inductance. In the second phase, once the on-timer has expired, the rectifier is turned-on and the inductor current decays to a preset valley current threshold. Finally, the switching cycle repeats by setting the on timer again and activating the low-side N-MOS switch.

In general, a dc/dc step-up converter can only operate in "true" boost mode, i.e. the output “boosted” by a certain amount above the input voltage. The TPS61256A device operates differently as it can smoothly transition in and out of zero duty cycle operation. Therefore the output can be kept as close as possible to its regulation limits even though the converter is subject to an input voltage that tends to be excessive. Refer to the typical characteristics section (DC Output Voltage vs. Input Voltage) for further details.

The current mode architecture with adaptive slope compensation provides excellent transient load response, requiring minimal output filtering. Internal soft-start and loop compensation simplifies the design process while minimizing the number of external components.