TPS81256

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SLVSAZ9 – JUNE 2012

DETAILED DESCRIPTION

OPERATION

The TPS8125x is a stand-alone, synchronous, step-up converter. The converter operates at a quasi-constant 4-

MHz frequency pulse width modulation (PWM) at moderate to heavy load currents. At light load currents, the

TPS8125x 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

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 TPS8125x 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. In this operation mode, the

output current capability of the regulator is limited to ca. 150mA. 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

while requiring only one external tiny capacitor for output filtering and loop stability purposes. Internal soft-start

and loop compensation simplifies the application design process.

POWER-SAVE MODE

The TPS8125X integrates a power-save mode to improve efficiency at light load. In power save mode the

converter only operates when the output voltage trips below a set threshold voltage.

It ramps up the output voltage with several pulses and goes into power save mode once the output voltage

exceeds the set threshold voltage.

The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM

mode.

Copyright © 2012, Texas Instruments Incorporated

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Product Folder Link(s): TPS81256