ZHCSCW6B August 2014 – January 2018 LM43601
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 Fixed-Frequency, Peak-Current-Mode Controlled, Step-Down Regulator
- 7.3.2 Light Load Operation
- 7.3.3 Adjustable Output Voltage
- 7.3.4 Enable (ENABLE)
- 7.3.5 VCC, UVLO and BIAS
- 7.3.6 Soft Start and Voltage Tracking (SS/TRK)
- 7.3.7 Switching Frequency (RT) and Synchronization (SYNC)
- 7.3.8 Minimum ON-Time, Minimum OFF-Time, and Frequency Foldback at Dropout Conditions
- 7.3.9 Internal Compensation and CFF
- 7.3.10 Bootstrap Voltage (BOOT)
- 7.3.11 Power Good (PGOOD)
- 7.3.12 Overcurrent and Short-Circuit Protection
- 7.3.13 Thermal Shutdown
- 7.4 Device Functional Modes
-
8 Applications and Implementation
- 8.1 Application Information
- 8.2
Typical Application
- 8.2.1 Design Requirements
- 8.2.2
Detailed Design Procedure
- 8.2.2.1 Custom Design With WEBENCH? Tools
- 8.2.2.2 Output Voltage Setpoint
- 8.2.2.3 Switching Frequency
- 8.2.2.4 Input Capacitors
- 8.2.2.5 Inductor Selection
- 8.2.2.6 Output Capacitor Selection
- 8.2.2.7 Feedforward Capacitor
- 8.2.2.8 Bootstrap Capacitors
- 8.2.2.9 VCC Capacitor
- 8.2.2.10 BIAS Capacitors
- 8.2.2.11 Soft-Start Capacitors
- 8.2.2.12 Undervoltage Lockout Set-Point
- 8.2.2.13 PGOOD
- 8.2.3 Application Curves
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文檔支持
- 12機械、封裝和可訂購信息
8.2.2.7 Feedforward Capacitor
The LM43601 is internally compensated and the internal R-C values are 400 kΩ and 50 pF respectively. Depending on the VOUT and frequency FS, if the output capacitor COUT is dominated by low ESR (ceramic types) capacitors, it could result in low phase margin. To improve the phase boost an external feedforward capacitor CFF can be added in parallel with RFBT. CFF is chosen such that phase margin is boosted at the crossover frequency without CFF. A simple estimation for the crossover frequency without CFF (fx) is shown in Equation 21, assuming COUT has very small ESR.
Equation 22 for CFF was tested:
Equation 22 indicates that the crossover frequency is geometrically centered on the zero and pole frequencies caused by the CFF capacitor.
For designs with higher ESR, CFF is not needed when COUT has very high ESR, and CFF calculated from Equation 22 should be reduced with medium ESR. Table 2 can be used as a quick starting point.
For the application in this design example, a 33-pF COG capacitor is selected.