TPS8268180, TPS8268150, TPS8268120, TPS8268105, TPS8268090

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SLVSBR0C – OCTOBER 2014 – REVISED JUNE 2015

Feature Description (continued)

During soft start, the current limit is reduced to 2/3 of its nominal value. The maximum load current during soft

start should be less than 1A. Once the internal reference voltage has reached 90% of its target value, the current

limit is set to its nominal target value.

8.3.2 Undervoltage Lockout

The undervoltage lockout circuit prevents the device from misoperation at low input voltages. It prevents the

converter from turning on either MOSFET under undefined conditions. The TPS8268x has a rising UVLO

threshold of 2.1V (typical).

8.3.3 Short-Circuit Protection

The TPS8268x integrates current limit circuitry to protect the device against heavy load or short circuits. When

the average current in the high-side MOSFET reaches its current limit, the high-side MOSFET is turned off and

the low-side MOSFET is turned on ramping down the inductor current.

As soon as the converter detects a short circuit condition, it shuts down. After a delay of approximately 20 µs, the

converter restarts. In case the short circuit condition remains, the converter shuts down again after hitting the

current limit threshold. In case the short circuit condition remains present on the converters output, the converter

periodically re-starts with a small duty cycle and shuts down again, thereby limiting the current drawn from the

input.

8.3.4 Thermal Shutdown

mode, the power stage is turned off. The device continues its operation when the junction temperature falls

below typically 130°C.

8.3.5 Enable

The TPS8268x device starts operation when EN is set high. For proper operation, the EN pin must be terminated

and must not be left floating.

Pulling the EN pin low forces the device into shutdown, with a shutdown current of typically 0.5

μA. In this mode,

the internal high-side and low-side MOSFETs are turned off, the internal resistor feedback divider is

disconnected, and the entire internal control circuitry is switched off. The TPS8268x device actively discharges

the output capacitor when it turns off. The integrated discharge resistor has a typical resistance of 12

Ω. This

internal discharge transistor is only turned on after the device had been enabled at least once. The required time

to discharge the output capacitor at the output node depends on load current and the effective output

capacitance.

The TPS8268x is designed such that it can start into a pre-biased output, in case the output discharge circuit

was active for too short a time to fully discharge the output capacitor. In this case, the converter starts switching

as soon as the internal reference has approximately reached the equivalent voltage to the output voltage

present. It then ramps the output from that voltage level to its target value.

8.3.6 MODE Pin

This pin must be tied to the input voltage VIN and must not be left floating.

8.4 Device Functional Modes

8.4.1 Spread Spectrum, PWM Frequency Dithering

The goal is to spread out the emitted RF energy over a larger frequency range, so that the resulting EMI is

similar to white noise. The end result is a spectrum that is continuous and lower in peak amplitude, making it

easier to comply with electromagnetic interference (EMI) standards and with power supply ripple requirements in

cellular and non-cellular wireless applications. Radio receivers are typically susceptible to narrowband noise that

is focused on specific frequencies.

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