ZHCS573I August 2010 – April 2018 LM25119
PRODUCTION DATA.
- 1 特性
- 2 應用
- 3 說明
- 4 修訂歷史記錄
- 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 High Voltage Start-Up Regulator
- 7.3.2 UVLO
- 7.3.3 Enable 2
- 7.3.4 Oscillator and Sync Capability
- 7.3.5 Error Amplifiers and PWM Comparators
- 7.3.6 Ramp Generator
- 7.3.7 Current Limit
- 7.3.8 Hiccup Mode Current Limiting
- 7.3.9 Soft Start
- 7.3.10 HO and LO Output Drivers
- 7.3.11 Maximum Duty Cycle
- 7.3.12 Thermal Protection
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Dual-output Design Example
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Timing Resistor
- 8.2.1.2.2 Output Inductor
- 8.2.1.2.3 Current Sense Resistor
- 8.2.1.2.4 Ramp Resistor and Ramp Capacitor
- 8.2.1.2.5 Output Capacitors
- 8.2.1.2.6 Input Capacitors
- 8.2.1.2.7 VCC Capacitor
- 8.2.1.2.8 Bootstrap Capacitor
- 8.2.1.2.9 Soft Start Capacitor
- 8.2.1.2.10 Restart Capacitor
- 8.2.1.2.11 Output Voltage Divider
- 8.2.1.2.12 UVLO Divider
- 8.2.1.2.13 MOSFET Selection
- 8.2.1.2.14 MOSFET Snubber
- 8.2.1.2.15 Error Amplifier Compensation
- 8.2.1.3 Application Curves
- 8.2.2 Two-Phase Design Example
- 8.2.1
Dual-output Design Example
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文檔支持
- 12機械、封裝和可訂購信息
7.3.4 Oscillator and Sync Capability
The LM25119 switching frequency is set by a single external resistor connected between the RT pin and the AGND pin (RT). The resistor must be located very close to the device and connected directly to the pins of the IC (RT and AGND). To set a desired switching frequency (fSW) of each channel, the resistor can be calculated with Equation 1.
where
- RT is in ohms (Ω)
- fSW is in hertz (Hz)
The frequency fSW is the output switching frequency of each channel. The internal oscillator runs at twice the switching frequency and an internal frequency divider interleaves the two channels with 180° phase shift between PWM pulses at the HO pins.
The RT pin can be used to synchronize the internal oscillator to an external clock. The internal oscillator can be synchronized by AC coupling a positive edge into the RT pin. The voltage at the RT pin is nominally 1.25 V and the voltage at the RT pin must exceed 4 V to trip the internal synchronization pulse detector. A 5-V amplitude signal and 100-pF coupling capacitor are recommended. Synchronizing at greater than twice the free-running frequency may result in abnormal behavior of the pulse width modulator. Also, note that the output switching frequency of each channel is one-half the applied synchronization frequency.