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TPS61256A

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SLVSB07A – JULY 2011 – REVISED MARCH 2018

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8 Detailed Description

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

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.

8.2 Softstart

The TPS61256A device has an internal softstart circuit that limits the inrush current during start-up. The first step

in the start-up cycle is the pre-charge phase. During pre-charge, the rectifying switch is turned on until the output

capacitor is charged to a value close to the input voltage. The rectifying switch is current limited (approx. 200mA)

during this phase. This mechanism is used to limit the output current under short-circuit condition.

Once the output capacitor has been biased to the input voltage, the converter starts switching. The soft-start

system progressively increases the on-time as a function of the input-to-output voltage ratio. As soon as the

output voltage is reached, the regulation loop takes control and full current operation is permitted.

8.3 Undervoltage Lockout

The under voltage lockout circuit prevents the device from malfunctioning at low input voltages and the battery

threshold plus its hysteresis of 100 mV at typ. 2.1V.

8.4 Thermal Regulation

The TPS61256A device contains a thermal regulation loop that monitors the die temperature during the pre-

charge phase. If the die temperature rises to high values of about 110 °C, the device automatically reduces the

current to prevent the die temperature from increasing further. Once the die temperature drops about 10 °C

below the threshold, the device will automatically increase the current to the target value. This function also

reduces the current during a short-circuit condition.

8.5 Thermal Shutdown

mode, the high-side and low-side MOSFETs are turned-off. When the junction temperature falls below the

thermal shutdown minus its hysteresis, the device continuous the operation.