8.3.1 Soft Start

The TPS8268x has an internal soft start circuit that controls the ramp up of the output voltage. Once the converter is enabled and the input voltage is above the undervoltage lockout threshold V_UVLO, the output voltage ramps up to 95% of its nominal value within t_Ramp of typ. 150μs. This ensures a controlled ramp up of the output voltage and limits the input voltage drop when a battery or a high-impedance power source is connected to the input of the DC/DC converter.

The inrush current during start-up is directly related to the effective capacitance and load present at the output of the converter.

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

As soon as the junction temperature, T_J, exceeds typically 140°C, the device goes into thermal shutdown. In this 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 0.5μA. 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 12Ω. 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.

8.4.1 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.

Switching regulators can be particularly troublesome in applications where electromagnetic interference (EMI) is a concern. Switching regulators operate on a cycle-by-cycle basis to transfer power to their output. In most cases, the frequency of operation is either fixed or regulated, based on the output load. This method of conversion creates large components of noise at the frequency of operation (fundamental) and multiples of the operating frequency (harmonics).

The spread spectrum architecture varies the switching frequency by around ±10% of the nominal switching frequency, thereby significantly reducing the peak radiated and conducted noise on both the input and output supplies. The frequency dithering scheme is modulated with a triangle profile and a modulation frequency f_m.

Figure 21. Spectrum Of A Frequency Modulated Sin. Wave With Sinusoidal Variation In Time

Figure 22. Spread Bands Of Harmonics In Modulated Square Signals

(1) Spectrum illustrations and formulae (Figure 21 and Figure 22) copyright IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 47, NO.3, AUGUST 2005. See References Section for full citation.

The above figures show that after modulation the side-band harmonic is attenuated compared to the non-modulated harmonic, and the harmonic energy is spread into a certain frequency band. The higher the modulation index (m_f), the larger the attenuation.

Equation 1.

where:

f_c is the carrier frequency (5.5 MHz)

f_m is the modulating frequency (approx. 0.008*f_c)

δ is the modulation ratio (approx 0.1)

Equation 2.

The maximum switching frequency f_c is limited by the device and finally the parameter modulation ratio (δ), together with f_m, which is the side-band harmonic´s bandwidth around the carrier frequency f_c. The bandwidth of a frequency modulated waveform is approximately given by Carson’s rule and is summarized as:

Equation 3.

f_m < RBW (resolution bandwidth): The receiver is not able to distinguish individual side-band harmonics, so, several harmonics are added in the input filter and the measured value is higher than expected in theoretical calculations.

f_m > RBW: The receiver is able to properly measure each individual side-band harmonic separately, so the measurements match with the theoretical calculations.