ZHCSKV2A February 2020 – July 2020 LM76005-Q1
PRODUCTION DATA
- 1 特性
- 2 应用
- 3 说明
- 4 Revision History
- 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 Control
- 7.3.2 Light Load Operation Modes — PFM and FPWM
- 7.3.3 Adjustable Output Voltage
- 7.3.4 Enable (EN Pin) and UVLO
- 7.3.5 Internal LDO, VCC UVLO, and Bias Input
- 7.3.6 Soft Start and Voltage Tracking (SS/TRK)
- 7.3.7 Adjustable Switching Frequency (RT) and Frequency Synchronization
- 7.3.8 Minimum On-Time, Minimum Off-Time, and Frequency Foldback at Dropout Conditions
- 7.3.9 Bootstrap Voltage and VBOOT UVLO (BOOT Pin)
- 7.3.10 Power Good and Overvoltage Protection (PGOOD)
- 7.3.11 Overcurrent and Short-Circuit Protection
- 7.3.12 Thermal Shutdown
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 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 Capacitors
- 8.2.2.10 BIAS Capacitors
- 8.2.2.11 Soft-Start Capacitors
- 8.2.2.12 Undervoltage Lockout Setpoint
- 8.2.2.13 PGOOD
- 8.2.2.14 Synchronization
- 8.2.3 Application Curves
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
8.2.2.7 Feedforward Capacitor
The LM76005-Q1 is internally compensated. Depending on the VOUT and frequency FS, if the output capacitor COUT is dominated by low ESR (ceramic types) capacitors, it can 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 22, assuming COUT has very small ESR.
Equation 23 for CFF was tested:
If capacitors with high ESR are used, CFF is not required. The CFF capacitor creates a time constant with RFBT that couples the attenuated output voltage ripple to the FB node. Using a value that is too large for CFF can couple too much ripple to FB node and affect output voltage regulation. For capacitors with medium ESR (20 – 200 mΩ), Equation 23 can be used as a quick starting point. For the application in this design example, a 47-pF C0G capacitor is used.