TPS61085

SLVS859B – JUNE 2008 – REVISED DECEMBER 2014

www.ti.com

8.3 Feature Description

8.3.1 Soft-Start

The boost converter has an adjustable soft-start to prevent high inrush current during start-up. To minimize the

inrush current during start-up an external capacitor connected to the soft-start pin SS and charged with a

constant current, is used to slowly ramp up the internal current limit of the boost converter when charged with a

The capacitor is then charged at a constant current of 10

V. The maximum load current is available after the soft-start is completed. The larger the capacitor the slower the

ramp of the current limit and the longer the soft-start time. A 100 nF capacitor is usually sufficient for most of the

applications. When the EN pin is pulled low, the soft-start capacitor is discharged to ground.

8.3.2 Frequency Select Pin (FREQ)

The frequency select pin FREQ allows to set the switching frequency of the device to 650 kHz (FREQ = low) or

1.2 MHz (FREQ = high). Higher switching frequency improves load transient response but reduces slightly the

efficiency. The other benefits of higher switching frequency are a lower output ripple voltage. The use of the 1.2

MHz switching frequency is recommended unless light load efficiency is a major concern.

8.3.3 Undervoltage Lockout (UVLO)

To avoid mis-operation of the device at low input voltages an undervoltage lockout is included that disables the

device, if the input voltage falls below 2.2 V.

8.3.4 Thermal Shutdown

A thermal shutdown is implemented to prevent damages due to excessive heat and power dissipation. Typically

the thermal shutdown threshold happens at a junction temperature of 150°C. When the thermal shutdown is

triggered the device stops switching until the temperature falls below typically 136°C. Then the device starts

switching again.

8.3.5 Overvoltage Prevention

If overvoltage is detected on the FB pin (typically 3 % above the nominal value of 1.238 V) the part stops

switching immediately until the voltage on this pin drops to its nominal value. This prevents overvoltage on the

output and secures the circuits connected to the output from excessive overvoltage.

8.4 Device Functional Modes

The converter operates in continuous conduction mode (CCM) as soon as the input current increases above half

the ripple current in the inductor, for lower load currents it switches into discontinuous conduction mode (DCM). If

the load is further reduced, the part starts to skip pulses to maintain the output voltage.

8

Submit Documentation Feedback

Copyright © 2008–2014, Texas Instruments Incorporated

Product Folder Links: TPS61085