SNVSCU4 October 2025 LM25137
PRODMIX
- 1
- 1 Features
- 2 Applications
- 3 Description
- 4 Related Products
- 5 Pin Configuration and Functions
- 6 Specifications
-
7 Detailed Description
- 7.1 Overview
- 7.2 Functional Block Diagram
- 7.3
Feature Description
- 7.3.1 Input Voltage Range (VIN)
- 7.3.2 Bias Supply Regulator (VCC, BIAS1/VOUT1, VDDA)
- 7.3.3 Precision Enable (EN1, EN2)
- 7.3.4 Switching Frequency (RT)
- 7.3.5 Pulse Frequency Modulation and Synchronization (PFM/SYNC)
- 7.3.6 Synchronization Out (SYNCOUT)
- 7.3.7 Dual Random Spread Spectrum (DRSS)
- 7.3.8 Configurable Soft Start (RSS)
- 7.3.9 Output Voltage Setpoints (FB1, FB2)
- 7.3.10 Error Amplifier and PWM Comparator (FB1, FB2, COMP1, COMP2)
- 7.3.11 Inductor Current Sense (ISNS1+, BIAS1/VOUT1, ISNS2+, VOUT2)
- 7.3.12 Minimum Controllable On-Time
- 7.3.13 100% Duty Cycle Capability
- 7.3.14 MOSFET Gate Drivers (HO1, HO2, LO1, LO2)
- 7.3.15 Output Configurations (CNFG)
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Design 1 – Dual 5V and 3.3V, 20A Buck Regulator for 12V
Input Applications
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Custom Design With WEBENCH? Tools
- 8.2.1.2.2 Custom Design With Excel Quickstart Tool
- 8.2.1.2.3 Inductor Calculations
- 8.2.1.2.4 Shunt Resistors
- 8.2.1.2.5 Ceramic Output Capacitors
- 8.2.1.2.6 Ceramic Input Capacitors
- 8.2.1.2.7 Feedback Resistors
- 8.2.1.2.8 Input Voltage UVLO Resistors
- 8.2.1.2.9 Compensation Components
- 8.2.1.3 Application Curves
- 8.2.2 Design 2 – Two-Phase, Single-Output Synchronous Buck Regulator
- 8.2.3 Design 3 – 12V, 25A, 400kHz, Two-Phase Buck Regulator for 24V Input Applications
- 8.2.1
Design 1 – Dual 5V and 3.3V, 20A Buck Regulator for 12V
Input Applications
- 8.3 Power Supply Recommendations
- 8.4 Layout
- 9 Device and Documentation Support
- 10Revision History
- 11Mechanical, Packaging, and Orderable Information
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.
Figure 7-3 DRSS Implementation| RCNFG | PRIMARY / SECONDARY | DRSS | INDEPENDENT OR INTERLEAVED |
|---|---|---|---|
| 10kΩ | Primary | Disabled | Independent |
| 19.1kΩ | Primary | ±5% | Independent |
| 29.4kΩ | Primary | ±10% | Independent |
| 41.2kΩ | Primary | Disabled | Interleaved |
| 54.9kΩ | Primary | ±5% | Interleaved |
| 71.5kΩ | Primary | ±10% | Interleaved |
| 90.9kΩ | Secondary | N/A | Interleaved |