SNVSCH7 February 2024 LMR38025
PRODUCTION DATA
- 1
- 1 Features
- 2 Applications
- 3 Description
- 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 Fixed Frequency Peak Current Mode Control
- 7.3.2 Adjustable Output Voltage
- 7.3.3 Enable
- 7.3.4 Switching Frequency and Synchronization (RT/SYNC)
- 7.3.5 Power-Good Flag Output
- 7.3.6 Minimum On Time, Minimum Off Time, and Frequency Foldback
- 7.3.7 Bootstrap Voltage
- 7.3.8 Overcurrent and Short-Circuit Protection
- 7.3.9 Soft Start
- 7.3.10 Thermal Shutdown
- 7.4 Device Functional Modes
- 8 Application and Implementation
- 9 Device and Documentation Support
- 10Revision History
- 11Mechanical, Packaging, and Orderable Information
7.3.8 Overcurrent and Short-Circuit Protection
The LMR38025 is protected from overcurrent conditions by cycle-by-cycle current limit on both the peak and valley of the inductor current. Hiccup mode is activated if a fault condition persists to prevent overheating.
The High-side MOSFET overcurrent protection is implemented by the nature of the peak current mode control. The high-side switch current is sensed when the high-side is turned on after a set blanking time. The high-side switch current is compared to the output of the error amplifier (EA) minus slope compensation every switching cycle. The peak current of high-side switch is limited by a clamped maximum peak current threshold, ISC_LIMIT, which is constant.
The current going through the low-side MOSFET is also sensed and monitored. When the low-side switch turns on, the inductor current begins to ramp down. The low-side switch is turned OFF at the end of a switching cycle if the current is above the low-side current limit, ILS_LIMIT. The low-side switch is kept on so that inductor current keeps ramping down until the inductor current ramps below the ILS_LIMIT. Then the low-side switch is turned OFF and the high-side switch is turned on. The figure below describes how the device operates under an overcurrent condition. Use Equation 7 for calculating the maximum load current.
Figure 7-9 Overcurrent Response to a Load
StepIf the feedback voltage drops below 40% of VREF, a counter is activated. If the current through the low-side switch triggers ILS_LIMIT for 256 consecutive cycles, the device enters hiccup mode. In hiccup mode, the converter shuts down and stays off for a hiccup period, THICCUP (76ms typical), before the LMR38025 tries to soft start again. If the overcurrent or short-circuit fault condition still exists, hiccup repeats until the fault condition is removed. Hiccup mode reduces power dissipation under severe overcurrent conditions and prevents overheating and potential electrical overstress to the device.
For FPWM version, the inductor current is allowed to go negative. When this current exceeds the low-side negative current limit, ILS_NEG, the low-side switch is turned off and high-side switch is turned on immediately. This action is used to protect the low-side switch from excessive negative current.