ZHCSPF2B December 2021 – December 2024 LM5168 , LM5169
PRODUCTION DATA
- 1
- 1 特性
- 2 應(yīng)用
- 3 說明
- 4 Device Comparison Table
- 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 Control Architecture
- 7.3.2 Internal VCC Regulator and Bootstrap Capacitor
- 7.3.3 Internal Soft Start
- 7.3.4 On-Time Generator
- 7.3.5 Current Limit
- 7.3.6 N-Channel Buck Switch and Driver
- 7.3.7 Synchronous Rectifier
- 7.3.8 Enable, Undervoltage Lockout (EN/UVLO)
- 7.3.9 Power Good (PGOOD)
- 7.3.10 Thermal Protection
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Fly-Buck? Converter Application
- 8.2.1 Design Requirements
- 8.2.2
Detailed Design Procedure
- 8.2.2.1 Switching Frequency (RT)
- 8.2.2.2 Transformer Selection
- 8.2.2.3 Output Capacitor Selection
- 8.2.2.4 Secondary Output Diode
- 8.2.2.5 Setting Output Voltage
- 8.2.2.6 Input Capacitor
- 8.2.2.7 Type-3 Ripple Network
- 8.2.2.8 CBST Selection
- 8.2.2.9 Minimum Secondary Output Load
- 8.2.2.10 Example Design Summary
- 8.2.3 Application Curves
- 8.3 Typical Buck Application
- 8.4 Power Supply Recommendations
- 8.5 Layout
- 9 Device and Documentation Support
- 10Revision History
- 11Mechanical, Packaging, and Orderable Information
封裝選項(xiàng)
機(jī)械數(shù)據(jù) (封裝 | 引腳)
散熱焊盤機(jī)械數(shù)據(jù) (封裝 | 引腳)
- DDA|8
訂購信息
8.2.2.7 Type-3 Ripple Network
A Type 3 ripple generation network uses an RC filter consisting of RA and CA across SW and VOUT1 to generate a triangular ramp that is in phase with the inductor current. This triangular ramp is then AC-coupled into the feedback node through capacitor CB. Type 3 ripple injection is an excellent choice for applications where low output voltage ripple is crucial, and is chosen for this example.
Equation 30 is used to calculate CA. With the values used in this example, CA > 245 pF. A value of 3300 pF is selected to keep RA within practical limits. In general, the user needs 20 mV of ripple at the feedback pin for reliable operation, calculated at nominal input voltage. The minimum value of ripple must not be less than 12 mV at minimum input voltage. Using Equation 31 with nominal input voltage, a value of RA > 117 kΩ was found and a value of 118 kΩ is selected.
While the magnitude of the generated ripple does not affect the output voltage ripple, the magnitude produces a DC error of approximately half the amplitude of the generated ripple, scaled by the feedback divider ratio. Therefore, the amount of DC offset, tolerable in the output voltage, imposes an upper bound on the feedback ripple.
Finally, Equation 32 is used to calculate the coupling capacitance CB. In the equation, TR is the approximate settling time of the control loop to a load transient disturbance. This was taken as 50 μs.
where
- TR = 50 μs (typical)
In this example, a value of > 37 pF was calculated for CB and a value of 56 pF is selected. This value avoids excessive coupling capacitor discharge by the feedback resistors during sleep intervals when operating at light loads. Note that the minimum value of CB is 47 pF.