LM5145 datasheet(22/73 Pages) TI2

22 / 73 page

(2)

7.3.5 Pulse Frequency Modulation and Synchronization (PFM/SYNC)

A synchronous buck regulator implemented with a low-side MOSFET rather than a diode has the capability to

sink negative current from the output during light load, output overvoltage, and prebias start-up conditions. The

LM25137 provides a diode emulation feature that can be enabled to prevent reverse (drain-to-source) current

flow in the low-side MOSFET. When configured for diode emulation mode, the low-side MOSFET is switched off

when reverse current flow is detected by sensing the SW voltage using a zero-cross comparator. The benefit of

this configuration is lower power loss during light-load conditions. The disadvantage of diode emulation mode is

slower light-load transient response.

Use the PFM/SYNC pin to configure diode emulation. To enable diode emulation and thus achieve high

efficiency at light loads, connect PFM/SYNC to VDDA. If FPWM with continuous conduction mode (CCM)

operation is preferred, tie PFM/SYNC to AGND. Note that diode emulation is automatically engaged to prevent

reverse current flow during a prebiased start-up. A gradual change from DCM to CCM operation provides

monotonic start-up performance.

To synchronize the LM25137 to an external clock source, apply a logic-level signal to PFM/SYNC. The LM25137

is synchronizable to ±20% of the programmed free-running frequency up to a maximum of 2.5MHz. If there is

an RT resistor and a synchronization clock signal, the LM25137 ignores the RT resistor and synchronizes to

the external clock. However, when the minimum off-time is reached at high duty cycle, the synchronization is

ignored, allowing reduction of the switching frequency to maintain output voltage regulation.

7.3.6 Synchronization Out (SYNCOUT)

The SYNCOUT voltage is a logic-level signal with a rising edge approximately 90° lagging HO2 (or 90° leading

HO1). When the SYNCOUT signal is used to synchronize a second LM25137 controller, all four phases are 90°

out of phase, thus optimizing ripple current cancellation.

7.3.7 Dual Random Spread Spectrum (DRSS)

The LM25137 provides a digital spread spectrum, which reduces the EMI signature of the switching regulator

over a wide frequency range. As shown in Figure 7-3, DRSS combines a low-frequency triangular modulation

profile with a high frequency cycle-by-cycle random modulation profile. The low-frequency modulation

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

performance in higher radio-frequency bands.

Spread spectrum functions 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 with low and high RBWs with the low-frequency

triangular and high-frequency cycle-by-cycle random modulation profiles, respectively. DRSS can reduce

conducted emissions by 10dBμV in the CISPR 25 low-frequency band (150kHz to 30MHz) and 5dBμV in the

high-frequency band (30MHz to 108MHz). Applying an external clock signal to PFM/SYNC disables DRSS. See

Table 7-1 to configure the LM25137 using a resistor from CNFG to AGND.

LM25137

SNVSCU4 – OCTOBER 2025

www.ti.com

Submit Document Feedback

Product Folder Links: LM25137

LM5145 Datasheet(PDF) 22 Page - Texas Instruments

LM5145 Datasheet(HTML) 22 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

Datasheet Download

Link URL