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TPS61085 Datasheet(HTML) 8 Page - Texas Instruments
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SLVS859B – JUNE 2008 – REVISED DECEMBER 2014
8.3 Feature Description
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
constant current. When the EN pin is pulled high, the soft-start capacitor C
is immediately charged to 0.3 V.
The capacitor is then charged at a constant current of 10
μA typically until the output of the boost converter V
has reached its Power Good threshold (roughly 98% of V
nominal value). During this time, the SS voltage
directly controls the peak inductor current, starting with 0 A at V
= 0.3 V up to the full current limit at V
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
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.
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