LM5145 datasheet(18/70 Pages) TI

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When the LM5148 is configured as a primary controller, the PG/SYNC pin becomes a synchronization clock

output for the secondary controller. The synchronization signal is 180° out-of-phase with the primary HO driver

output.

8.3.5 Switching Frequency (RT)

Program the LM5148 oscillator with a resistor from RT to AGND to set an oscillator frequency from 100 kHz and

2.2 MHz. Calculate the RT resistance for a given switching frequency using Equation 4.

(kHz)

R (k )

(4)

Under low VIN conditions when the on time of the high-side MOSFET exceeds the programmed oscillator period,

the LM5148 extends the switching period until the PWM latch is reset by the current sense ramp exceeding the

controller compensation voltage.

The approximate input voltage level at which this occurs is given by Equation 5.

IN(min)

OUT

OFF(min)

(5)

where

• tSW is the switching period.

• tOFF(min) is the minimum off time of 90 ns.

8.3.6 Dual Random Spread Spectrum (DRSS)

The LM5148 provides a digital spread spectrum, which reduces the EMI of the power supply over a wide

frequency range. DRSS combines a low-frequency triangular modulation profile with a high frequency cycle-by-

cycle random modulation profile. The low-frequency triangular modulation improves performance in lower radio-

frequency bands, while the high-frequency random modulation improves performance in higher radio frequency

bands.

Spread spectrum works by converting a narrowband signal into a wideband signal that spreads the energy over

multiple frequencies. Because industry standards require different EMI receiver resolution bandwidth (RBW)

settings for different frequency bands, the RBW has an impact on the spread spectrum performance. DRSS

is able to simultaneously improve the EMI performance in the low and high RBWs using its low-frequency

triangular modulation profile and at high frequency cycle-by-cycle random modulation, respectively. DRSS can

reduce conducted emissions up to 15 dBμV in the low-frequency band (150 kHz to 30 MHz) and 5 dBμV in the

high-frequency band (30 MHz to 108 MHz).

To enable DRSS, connect either a 41.2-kΩ or 71.5-kΩ resistor from CNFG to AGND. DRSS is disabled when an

external clock is applied to the PFM/SYNC pin.

LM5148

SNVSC01 – FEBRUARY 2023

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