SNVS085X July 2000 – December 2017 LM3478
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Typical High Efficiency Step-Up (Boost) Converter
- 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 Power Inductor Selection
- 8.2.1.2.3 Programming the Output Voltage
- 8.2.1.2.4 Setting the Current Limit
- 8.2.1.2.5 Current Limit with External Slope Compensation
- 8.2.1.2.6 Power Diode Selection
- 8.2.1.2.7 Power MOSFET Selection
- 8.2.1.2.8 Input Capacitor Selection
- 8.2.1.2.9 Output Capacitor Selection
- 8.2.1.2.10 Compensation
- 8.2.1.3 Application Curves
- 8.2.2 Typical SEPIC Converter
- 8.2.1
Typical High Efficiency Step-Up (Boost) Converter
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
8.2.1.2.5 Current Limit with External Slope Compensation
RSL is used to add additional slope compensation when required. It is not necessary in most designs and RSL should be no larger than necessary. Select RSL according to Equation 22.
Where RSEN is the selected value based on current limit. With RSL installed, the control signal includes additional external slope to stabilize the loop, which will also have an effect on the current limit threshold. Therefore, the current limit threshold must be re-verified, as illustrated in Equation 23, Equation 24, and Equation 25 below.
Where ΔVSL is the additional slope compensation generated as discussed in the slope compensation ramp section and calculated using Equation 24.
This changes the equation for current limit (or RSEN) as shown in Equation 25.
The RSEN and RSL values may have to be calculated iteratively in order to achieve both the desired current limit and stable operation. In some designs RSL can also help to filter noise on the ISEN pin.
If the inductor is selected such that ripple current is the recommended 30% value, and the current limit threshold is 120% of the maximum peak, a simpler method can be used to determine RSEN. Equation 26 below will provide optimum stability without RSL, provided that the above 2 conditions are met.
