SLUSE50 November 2023 TPS92642-Q1
PRODUCTION DATA
- 1
- 1 Features
- 2 Applications
- 3 Description
- 4 Pin Configuration and Functions
- 5 Specifications
-
6 Detailed Description
- 6.1 Overview
- 6.2 Functional Block Diagram
- 6.3
Feature Description
- 6.3.1 Internal Regulator
- 6.3.2 Buck Converter Switching Operation
- 6.3.3 Bootstrap Supply
- 6.3.4 Switching Frequency and Adaptive On-Time Control
- 6.3.5 Minimum On-Time, Off-Time, and Inductor Ripple
- 6.3.6 LED Current Regulation and Error Amplifier
- 6.3.7 Start-Up Sequence
- 6.3.8 Analog Dimming and Forced Continuous Conduction Mode
- 6.3.9 External PWM Dimming and Input Undervoltage Lockout (UVLO)
- 6.3.10 Pulse Duty Cycle Limit Circuit
- 6.3.11 Output Short and Open-Circuit Faults
- 6.3.12 Overcurrent Protection
- 6.3.13 Thermal Shutdown
- 6.3.14 Fault Indicator and Diagnostics Summary
- 6.4 Device Functional Modes
-
7 Application and Implementation
- 7.1
Application Information
- 7.1.1 Duty Cycle Considerations
- 7.1.2 Switching Frequency Selection
- 7.1.3 LED Current Programming
- 7.1.4 Inductor Selection
- 7.1.5 Output Capacitor Selection
- 7.1.6 Input Capacitor Selection
- 7.1.7 Bootstrap Capacitor Selection
- 7.1.8 Compensation Capacitor Selection
- 7.1.9 Input Dropout and Undervoltage Protection
- 7.1.10 Pulse Duty Cycle Limit Circuit
- 7.1.11 Protection Diodes
- 7.2
Typical Application
- 7.2.1 Design Requirements
- 7.2.2
Detailed Design Procedure
- 7.2.2.1 Calculating Duty Cycle
- 7.2.2.2 Calculating Minimum On-Time and Off-Time
- 7.2.2.3 Minimum Switching Frequency
- 7.2.2.4 LED Current Set Point
- 7.2.2.5 Inductor Selection
- 7.2.2.6 Output Capacitor Selection
- 7.2.2.7 Bootstrap Capacitor Selection
- 7.2.2.8 Compensation Capacitor Selection
- 7.2.2.9 VIN Dropout Protection and PWM Dimming
- 7.2.3 Application Curves
- 7.3 Power Supply Recommendations
- 7.4 Layout
- 7.1
Application Information
- 8 Device and Documentation Support
- 9 Revision History
- 10Mechanical, Packaging, and Orderable Information
6.3.9 External PWM Dimming and Input Undervoltage Lockout (UVLO)
The UDIM pin is a multifunction input that features an accurate input voltage detection based on band-gap thresholds with programmable hysteresis as shown in Figure 6-7. This pin functions as the external PWM dimming input for the LEDs and monitors VIN to detect dropout and undervoltage conditions. When the rising pin voltage exceeds the 1.22-V threshold, 10 µA (typical) of current is driven out of the UDIM pin into the resistor divider providing programmable hysteresis. TI recommends a bypass capacitor value of 1 nF between the UDIM pin and GND to improve noise immunity.
Figure 6-6 External PWM DimmingThe brightness of LEDs can be varied by modulating the duty cycle of the signal directly connected to the UDIM input. In addition, either an n-channel MOSFET or a Schottky diode can be used to couple an external PWM signal when using UDIM input in conjunction with UVLO functionality. With an n-channel MOSFET, the brightness is proportional to the negative duty cycle of the external PWM signal. With a Schottky diode, the brightness is proportional to the positive duty cycle of the external PWM signal.
Dropout and input undervoltage protection is achieved by connecting the resistor divider network from VIN to UDIM pin and UDIM pin to GND. Dropout protection is activated when UDIM pin voltage drops below VUDIM(EN, FALL) threshold. The minimum input voltage, below which drop protection is activated is programmed using Equation 8.
Additional hysteresis to internal 100 mV is programmed by connecting an external resistor, RUVH in series with UDIM pin. This connection allows the standard resistor divider to have smaller values, minimizing PWM delays.
Input undervoltage protection is triggered when UDIM pin voltage drops below VUDIM(EN) thresholds. The device responds to very low VIN voltage or to the external PWM input signal by disabling the error amplifier, disconnecting the COMP pin and tri-stating the switch node. With switch disabled, inductor current and the LED current drops to zero and the charge on the compensation network is maintained. On rising edge of PWM or when VIN exceeds the internal hysteresis of 100 mV, the converter resumes switching operation. The inductor current quickly ramps to the previous steady-state value.