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TPS81256

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SLVSAZ9C – JUNE 2012 – REVISED FEBRUARY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

8.4 Device Functional Modes

8.4.1 Softstart, Enable

The TPS81256 device starts operation when EN is set high and starts up with the soft-start sequence. For proper

operation, the EN pin must be terminated and must not be left floating.

The TPS81256 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.

Pulling the EN pin low forces the device in shutdown, with a shutdown current of typically 1µA. In this mode, true

8.4.2 Load Disconnect and Reverse Current Protection

Regular boost converters do not disconnect the load from the input supply and therefore a connected battery will

be discharge during shutdown. The advantage of TPS81256 is that this converter is disconnecting the output

from the input of the power supply when it is disabled (so called true shutdown mode). In case of a connected

battery it prevents it from being discharge during shutdown of the converter.

8.4.3 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 typically 2.1V.

8.4.4 Thermal Regulation

The TPS81256 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.

8.4.5 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.