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TPS61087-Q1

SLVSB50B – DECEMBER 2011 – REVISED JUNE 2016

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Copyright © 2011–2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 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 SS pin and charged with a constant current,

is used to slowly ramp up the internal current limit of the boost converter. When the EN pin is pulled high, the

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is completed. As the size of the capacitor increases the ramp of the current limit slows and the soft-start time

increases. 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.

7.3.2 Frequency Select Pin (FREQ)

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

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

efficiency. Another benefit of higher switching frequency is a lower output ripple voltage. Unless light load

efficiency is a major concern, TI recommends using a 1.2-MHz switching frequency.

7.3.3 Undervoltage Lockout (UVLO)

To avoid misoperation of the device at low input voltages, an undervoltage lockout is included, which disables the

device if the input voltage falls below 2.4 V.

7.3.4 Thermal Shutdown

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

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

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

again.

7.3.5 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.

7.4 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, the converter switches into discontinuous conduction

mode (DCM). If the load is further reduced, the part starts to skip pulses to maintain the output voltage.