SNVSB95 July 2019 LM3421-Q1 , LM3423-Q1
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Device Comparison
- 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 Current Regulators
- 8.3.2 Predictive Off-Time (PRO) Control
- 8.3.3 Average LED Current
- 8.3.4 Analog Dimming
- 8.3.5 Current Sense and Current Limit
- 8.3.6 Overcurrent Protection
- 8.3.7 Zero Current Shutdown
- 8.3.8 Control Loop Compensation
- 8.3.9 Start-Up Regulator
- 8.3.10 Overvoltage Lockout (OVLO)
- 8.3.11 Input Undervoltage Lockout (UVLO)
- 8.3.12 PWM Dimming
- 8.3.13 LM3423-Q1 Only: DPOL, FLT, TIMR, and LRDY
-
9 Application and Implementation
- 9.1 Application Information
- 9.2
Typical Applications
- 9.2.1
Basic Topology Schematics
- 9.2.1.1 Design Requirements
- 9.2.1.2
Detailed Design Procedure
- 9.2.1.2.1 Operating Point
- 9.2.1.2.2 Switching Frequency
- 9.2.1.2.3 Average LED Current
- 9.2.1.2.4 Inductor Ripple Current
- 9.2.1.2.5 LED Ripple Current
- 9.2.1.2.6 Peak Current Limit
- 9.2.1.2.7 Loop Compensation
- 9.2.1.2.8 Input Capacitance
- 9.2.1.2.9 N-channel FET
- 9.2.1.2.10 Diode
- 9.2.1.2.11 Output OVLO
- 9.2.1.2.12 Input UVLO
- 9.2.1.2.13 PWM Dimming Method
- 9.2.1.2.14 Analog Dimming Method
- 9.2.2
LM3421 Buck-Boost Application
- 9.2.2.1 Design Requirements
- 9.2.2.2
Detailed Design Procedure
- 9.2.2.2.1 Operating Point
- 9.2.2.2.2 Switching Frequency
- 9.2.2.2.3 Average LED Current
- 9.2.2.2.4 Inductor Ripple Current
- 9.2.2.2.5 Output Capacitance
- 9.2.2.2.6 Peak Current Limit
- 9.2.2.2.7 Loop Compensation
- 9.2.2.2.8 Input Capacitance
- 9.2.2.2.9 N-channel FET
- 9.2.2.2.10 Diode
- 9.2.2.2.11 Input UVLO
- 9.2.2.2.12 Output OVLO
- 9.2.2.3 Application Curve
- 9.2.3 LM3421-Q1 BOOST Application
- 9.2.4 LM3421-Q1 Buck-Boost Application
- 9.2.5 LM3423-Q1 Boost Application
- 9.2.6 LM3421 Buck-Boost Application
- 9.2.7 LM3423 Buck Application
- 9.2.8 LM3423 Buck-Boost Application
- 9.2.9 LM3421 SEPIC Application
- 9.2.1
Basic Topology Schematics
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information
8.3.9 Start-Up Regulator
The controller includes a high voltage, low dropout bias regulator. When power is applied, the regulator is enabled and sources current into an external capacitor (CBYP) connected to the VCC pin. The recommended bypass capacitance for the VCC regulator is 2.2 µF to 3.3 µF. The output of the VCC regulator is monitored by an internal UVLO circuit that protects the device from attempting to operate with insufficient supply voltage and the supply is also internally current limited. Figure 20 shows the typical start-up waveforms.
Figure 20. Start-Up Waveforms First, CBYP is charged to be above VCC UVLO threshold (approximately 4.2 V). The CVCC charging time (tVCC) can be estimated using Equation 19.
CCMP is then charged to 0.9 V over the charging time (tCMP), which can be estimated using Equation 20.
Once CCMP = 0.9 V, the part starts switching to charge CO until the LED current is in regulation. The CO charging time (tCO) can be roughly estimated using Equation 21.
The system start-up time (tSU) is defined using Equation 22.
In some configurations, the start-up waveform overshoots the steady state COMP pin voltage. In this case, the LED current and output voltage overshoots also, which can trip the overvoltage or protection, causing a race condition. The easiest way to prevent this is to use a larger compensation capacitor (CCMP), thereby slowing down the control loop.