NS公司的LM3421/LM3423是多用途高压LED驱动，可以配置成降压，升压，降压-升压(反激)或SEPIC拓扑，输入电压从4.5V到高达75V，可调开关频率高达2MHz，具有快速(50kHz) PWM调光，关断电流小于1uA，可用在LED驱动器和多种恒流模式稳压器以及热电冷却驱动器.本文介绍了LM3421/LM3423的住要特性，方框图以及多种应用电路图，升压评估板主要特性和电路图和材料清单(BOM).

LM3421/ LM3423:N-Channel Controllers for Constant Current LED Drivers
The LM3421/LM3423 devices are versatile high voltage LED driver controllers. With the capability to be configured in a Buck， Boost， Buck-Boost (Flyback)， or SEPIC topology， and an input operating voltage rating of 75V， these controllers are ideal for illuminating LEDs in a very diverse， large family of applications.

Adjustable high-side current sense with a typical sense voltage of 100mV allows for tight regulation of the LED current with the highest efficiency possible. Output LED current regulation is based on peak current-mode control with predictive Off-Time Control. This method of control eases the design of loop compensation while providing inherent input voltage feed-forward compensation.

The LM3421/LM3423 include a high-voltage startup regulator that operates over a wide input range of 4.5V to 75V. The internal PWM controller is designed for adjustable switching frequencies of up to 2.0MHz， thus enabling compact solutions.

Additional features include: “zero” current shutdown， precision reference， logic compatible DIM input suitable for fast PWM dimming， cycle-by-cycle current limit， and thermal
shutdown.

The LM3423 also includes an LED output status flag， a fault flag， a programmable fault timer， and a logic input to select the polarity of the dimming output driver.

LM3421-3主要特性:
■VIN range from 4.5V to 75V
■2% Internal reference voltage (1.235V)
■Current sense voltage adjustable from 20mV
■High-side current sensing
■2Ω MOSFET gate driver
■Dimming MOSFET gate driver
■Input under-voltage protection
■Over-voltage protection
■Low shutdown current， IQ < 1μA
■Fast (50kHz) PWM dimming
■Cycle-by-cycle current limit
■Programmable switching frequency
■LED output status flag (LM3423 only)
■Fault timer pin (LM3423 only)
■TSSOP EP 16-lead package (LM3421)
■TSSOP EP 20-lead package (LM3423)

LM3421-3应用:
■LED Drivers
■Constant-Current Buck-Boost Regulator
■Constant-Current Boost Regulator
■Constant-Current Flyback Regulator
■Constant-Current SEPIC Regulator
■Thermo-Electric Cooler (Peltier) Driver

图1.LM3421-3方框图

图2.LM3421-3升压拓扑图

图3.LM3421升压LED驱动电路图

图4.带高速调光的LM3421升压拓扑图

图5.带高速调光的LM3421升压/降压(反激)拓扑图

图6.带高速调光的LM3421 SEPIC拓扑图

图7.带高速PWM分流调光的LM3421降压拓扑图

图8.LM3423全特性应用电路图

图9.带高边调光的LM3423升压拓扑图

LM3423升压评估板
LM3423 Boost Evaluation Board
This evaluation board has been designed to demonstrate the LM3423 low-side controller as a step-up (boost) regulator to deliver constant current to high power LEDs. A complete circuit schematic and bill of materials for the evaluation board are included at the end of this document. The printed circuit board consists of two layers of two ounce copper on FR4 material.

Standard Evaluation Board Operating Configuration
fSW = 600 kHz
Over-voltage protection set at 42V
VIN range 4.5V to 35V
Low side PWM fast dimming
2 to 8 series connected LEDs (VO < 35V)
UVLO set at 8.4V
ILED = 1A

Available features that can be configured on the standard evaluation board by the user for are listed below:
Fixed or programmable LED current
High-speed PWM high-side or low-side dimming
User programmable over-voltage protection (OVP)
Under-voltage lock-out (UVLO) protection
Fault protection
Soft-start
Hysteretic current-mode control
Higher Input and / or Output Voltage Modifications
Although the standard LM3423 evaluation board is designed to operate at input and output voltages up to 35V， the device is capable of operating with input and output voltages up to 75V. Operation up to 75V can be achieved by changing the voltage ratings of the input capacitors (C1， C8， C17)， output capacitors (C4， C7， C11， C16)， and transistors Q1， Q7， Q9. For output voltages greater than 35V the OVP resistors R11 and R12 will need to be adjusted.

图10.LM3423评估板电路图

LM3423 Evaluation Board: High Side Current Sense with Low Side High Speed Dimming

LM3423评估板材料清单(BOM):