EN/SYNC

EN/SYNC

EN

CLK

Figure 7-1. Synchronization with Separate Enable Signal (Optional)

7.3.4 Spread Spectrum Modulation

Using the S-CONF pin enables or disables spread spectrum modulation. DC/DC converters generate an output

voltage ripple at the switching frequency. When powering ADCs or an analog front end (AFE), the switching

frequency generates high frequency mixing spurs as well as a low frequency spur in the output frequency

spectrum. Using the optional second stage L-C filter reduces the ripple of the converter and spurs by up to 30

dB.

The device has integrated two different spread spectrum modulation (SSM) schemes that are selected by

the resistor connected to the S-CONF pin according to Table 7-1. It is possible to select random or triangle

modulation to spread the switching frequency over a larger frequency range. The triangular SSM is modulated

based on the switching frequency, and results in 1.9-kHz for 1-MHz switching frequency and 4.3-kHz for

2.2-MHz switching frequency. The modulation spread is ±10% of the device switching frequency. This SSM

provides high attenuation when the receiver bandwidth is less than the modulation frequency, typically the

case for systems using Fast Fourier Transforms (FFT) post processing as in high speed ADC applications.

For applications sensitive to noise at the modulation frequency, random SSM is used. Using a random spread

spectrum modulation also reduces the spurs in the output spectrum as shown in Figure 6-2. The random

SSM operates with the same frequency spread and modulation period as the triangular SSM. The randomized

modulation uses a Fibonacci Linear-Feedback Shift Register (LFSR) so that every tone is generated once during

the pseudo-random generation period. The frequency spreading is shown in Figure 7-2.The attenuation using

random or triangle SSM is shown in Figure 6-26.

fMOD

Triangular

(single tone)

fSW

Unmodulated

(single tone)

Spectral envelope of

triangular modulation

(+/- 10% fSW)

Spectral envelope of

random modulation

(+/- 10% fSW)

Increased noise floor

with random modulation

Figure 7-2. Spread Spectrum Modulation

7.3.5 Output Discharge

Output discharge is enabled or disabled, depending on the S-CONF setting. With output discharge enabled,

the output voltage is pulled low by a discharge resistor RDIS of typically 7 Ω. The output discharge function is

enabled during thermal shutdown, UVLO, or when EN/SYNC is pulled low.

7.3.6 Undervoltage Lockout (UVLO)

To avoid misoperation of the device at low input voltages, the device is enabled after the input voltage is above

the undervoltage lockout threshold. The device is disabled after the input voltage falls below the undervoltage

threshold.

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TPSM82913

SLVSGJ4A – OCTOBER 2022 – REVISED DECEMBER 2022

Copyright © 2022 Texas Instruments Incorporated

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