Datasheet

TPS61253, TPS61254, TPS61256
TPS61258, TPS61259
www.ti.com SLVSAG8C –SEPTEMBER 2011–REVISED AUGUST 2012
DETAILED DESCRIPTION
OPERATION
The TPS6125x 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 TPS6125x 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 TPS6125x 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,
requiring minimal output filtering. Internal soft-start and loop compensation simplifies the design process while
minimizing the number of external components.
POWER-SAVE MODE
The TPS6125X 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.
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Product Folder Links: TPS61253 TPS61254 TPS61256 TPS61258 TPS61259