ZHCSOE3E August 2021 – September 2023 LMR38020
PRODUCTION DATA
- 1
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 5 Device Comparison Table
- 6 Pin Configuration and Functions
- 7 Specifications
-
8 Detailed Description
- 8.1 Overview
- 8.2 Functional Block Diagram
- 8.3
Feature Description
- 8.3.1 Fixed Frequency Peak Current Mode Control
- 8.3.2 Adjustable Output Voltage
- 8.3.3 Enable
- 8.3.4 Switching Frequency and Synchronization (RT/SYNC)
- 8.3.5 Power-Good Flag Output
- 8.3.6 Minimum On Time, Minimum Off Time, and Frequency Foldback
- 8.3.7 Bootstrap Voltage
- 8.3.8 Overcurrent and Short Circuit Protection
- 8.3.9 Soft Start
- 8.3.10 Thermal Shutdown
- 8.4 Device Functional Modes
- 9 Application and Implementation
- 10Device and Documentation Support
- 11Mechanical, Packaging, and Orderable Information
8.3.1 Fixed Frequency Peak Current Mode Control
The LMR38020 is a step-down synchronous buck converter with integrated high-side (HS) and low-side (LS) switches (synchronous rectifier). The LMR38020 supplies a regulated output voltage by turning on the high-side and low-side NMOS switches with controlled duty cycle. During high-side switch on time, the SW pin voltage swings up to approximately VIN, and the inductor current, iL, increases with a linear slope (VIN – VOUT) / L. When the high-side switch is turned off by the control logic, the low-side switch is turned on after an anti-shoot-through dead time. Inductor current discharges through the low-side switch with a slope of –VOUT / L. The control parameter of a buck converter is defined as Duty Cycle D = tON / TSW, where tON is the high-side switch on time and TSW is the switching period. The converter control loop maintains a constant output voltage by adjusting the duty cycle D. In an ideal buck converter where losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage: D = VOUT / VIN.
Figure 8-1 SW Node and Inductor Current
Waveforms in Continuous Conduction Mode (CCM)The LMR38020 employs fixed-frequency peak-current mode control. A voltage feedback loop is used to get accurate DC voltage regulation by adjusting the peak-current command based on voltage offset. The peak inductor current is sensed from the high-side switch and compared to the peak current threshold to control the on time of the high-side switch. The voltage feedback loop is internally compensated, which allows for fewer external components, making it easy to design and providing stable operation with almost any combination of output capacitors. The converter operates with fixed switching frequency at normal load condition. At light-load condition, the LMR38020 operates in PFM mode to maintain high efficiency (PFM version) or in FPWM mode for low output voltage ripple, tight output voltage regulation, and constant switching frequency (FPWM version).