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
7.3.8 Configurable Soft Start (RSS)
The soft-start feature of the LM25137 allows the regulator to gradually reach the steady-state operating point, thus reducing start-up stresses and inrush current. Program the soft-start time with a resistor from RSS to AGND. If the RSS pin is shorted to AGND, the soft-start time is 1.5ms. If the RSS resistance is greater than 500kΩ (or the pin is left open circuit), the LM25137 defaults to a soft-start time of 6.5ms.
Use Equation 3 to calculate the RSS resistance for a given soft-start time.
Equation 3. 