ZHCSG23A March 2017 – February 2018 LM25141-Q1
PRODUCTION DATA.
- 1 特性
- 2 应用
- 3 说明
- 4 修订历史记录
- 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 VCC Regulator
- 7.3.3 Oscillator
- 7.3.4 Synchronization
- 7.3.5 Frequency Dithering (Spread Spectrum)
- 7.3.6 Enable
- 7.3.7 Power Good
- 7.3.8 Output Voltage
- 7.3.9 Current Sense
- 7.3.10 DCR Current Sensing
- 7.3.11 Error Amplifier and PWM Comparator
- 7.3.12 Slope Compensation
- 7.3.13 Hiccup Mode Current Limiting
- 7.3.14 Standby Mode
- 7.3.15 Soft Start
- 7.3.16 Diode Emulation
- 7.3.17 High- and Low-Side Drivers
-
8 Application and Implementation
- 8.1 Application Information
- 8.2 Typical Application
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文档支持
- 12机械、封装和可订购信息
7.3.8.1 Minimum Output Voltage Adjustment
There are two limitations to the minimum output voltage adjustment range: the LM25141-Q1 voltage reference of 1.2 V and the minimum switch node pulse width, tSW.
The minimum controllable on-time at the switch node (tSW) limits the voltage conversion ratio (VOUT/VIN). For fixed-frequency PWM operation, the voltage conversion ratio should meet the condition in Equation 8:
where
- tSW is 70 ns (typical)
- Fsw is the switching frequency
If the desired voltage conversion ratio does not meet the above condition, the controller transitions from fixed frequency operation into a pulse skipping mode to maintain regulation of the output voltage.
For example, if the desired output voltage is 3.3 V with a VIN of 20 V and operating at 2.2 MHz, the voltage conversion ratio test is satisfied:
For wide VIN applications and lower output voltages, an alternative is to use the LM25141-Q1 with a 440-kHz oscillator frequency. Operating at 440 kHz, the limitation of the minimum tSW time is less significant. For example, if a 1.8-V output is required with a VIN of 42 V:
