SNVS616H April 2009 – July 2015 LM3429 , LM3429-Q1
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 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 Current Regulators
- 7.3.2 Predictive Off-Time (PRO) Control
- 7.3.3 Switching Frequency
- 7.3.4 Average LED Current
- 7.3.5 Analog Dimming
- 7.3.6 Current Sense and Current Limit
- 7.3.7 Control Loop Compensation
- 7.3.8 Output Overvoltage Lockout (OVLO)
- 7.3.9 Input Undervoltage Lockout (UVLO)
- 7.3.10 PWM Dimming
- 7.3.11 Startup Regulator (VCC LDO)
- 7.3.12 Thermal Shutdown
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Basic Topology Schematics
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Operating Point
- 8.2.1.2.2 Switching Frequency
- 8.2.1.2.3 Average LED Current
- 8.2.1.2.4 Inductor Ripple Current
- 8.2.1.2.5 LED Ripple Current
- 8.2.1.2.6 Peak Current Limit
- 8.2.1.2.7 Loop Compensation
- 8.2.1.2.8 Input Capacitance
- 8.2.1.2.9 NFET
- 8.2.1.2.10 Diode
- 8.2.1.2.11 Output OVLO
- 8.2.1.2.12 Input UVLO
- 8.2.1.2.13 PWM Dimming Method
- 8.2.1.2.14 Analog Dimming Method
- 8.2.2
Buck-Boost Application - 6 LEDs at 1 A
- 8.2.2.1 Design Requirements
- 8.2.2.2
Detailed Design Procedure
- 8.2.2.2.1 Operating Point
- 8.2.2.2.2 Switching Frequency
- 8.2.2.2.3 Average LED Current
- 8.2.2.2.4 Inductor Ripple Current
- 8.2.2.2.5 Output Capacitance
- 8.2.2.2.6 Peak Current Limit
- 8.2.2.2.7 Loop Compensation
- 8.2.2.2.8 Input Capacitance
- 8.2.2.2.9 NFET
- 8.2.2.2.10 Diode
- 8.2.2.2.11 Input UVLO
- 8.2.2.2.12 Output OVLO
- 8.2.2.3 Application Curve
- 8.2.3 Boost PWM Dimming Application - 9 LEDs at 1 A
- 8.2.4 Buck-Boost Analog Dimming Application - 4 LEDs at 2A
- 8.2.5 Boost Analog Dimming Application - 12 LEDs at 700 mA
- 8.2.6 Buck-Boost PWM Dimming Application - 6 LEDs at 500 mA
- 8.2.7 Buck Application - 3 LEDS at 1.25 A
- 8.2.8 Buck-Boost Thermal Foldback Application - 8 LEDs at 2.5 A
- 8.2.9 SEPIC Application - 5 LEDs at 750 mA
- 8.2.1
Basic Topology Schematics
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
10 Layout
10.1 Layout Guidelines
The performance of any switching regulator depends as much upon the layout of the PCB as the component selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI within the circuit.
Discontinuous currents are the most likely to generate EMI; therefore, take care when routing these paths. The main path for discontinuous current in the LM3429 buck regulator contains the input capacitor (CIN), the recirculating diode (D1), the N-channel MosFET (Q1), and the switch sense resistor (RLIM). In the LM3429 boost and buck-boost regulators, the discontinuous current flows through the output capacitor (CO), D1, Q1, and RLIM. In either case, this loop should be kept as small as possible and the connections between all the components should be short and thick to minimize parasitic inductance. In particular, the switch node (where L1, D1 and Q1 connect) should be just large enough to connect the components. To minimize excessive heating, large copper pours can be placed adjacent to the short current path of the switch node.
The RCT, COMP, CSH, IS, HSP and HSN pins are all high-impedance inputs which couple external noise easily, therefore the loops containing these nodes should be minimized whenever possible.
In some applications the LED or LED array can be far away (several inches or more) from the LM3429, or on a separate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large or separated from the rest of the regulator, the output capacitor should be placed close to the LEDs to reduce the effects of parasitic inductance on the AC impedance of the capacitor.
10.2 Layout Example
Figure 38. LM3429 Layout Guideline