本文介紹了LM3492主要特性, 簡化功能方框圖, 典型應用電路圖以及LM3492評估板參考設計電路圖和評估板材料清單(BOM),PCB元件布局圖.NS公司的LM3492是兩路單獨調光LED驅動器,集成了升壓轉換器和快速電流穩壓器.可編程開關頻率從200 kHz到1 MHz,輸入電壓4.5V-65V,可編程LED電流從50 mA 到 200 mA,調光電壓達65V,總功率可達15W,主要應用在超高對比度6.5”-10”難關LCD顯示屏背光以及汽車或航海GPS顯示器.

LM3492主要特性:

Boost Converter:

■ Very wide input voltage ranged from 4.5V-65V

■ Programmable soft-start

■ No loop compensation required

■ Stable with ceramic and other low ESR capacitors with no audible noise

■ Nearly constant switching frequency programmable from 200 kHz to 1 MHz

Current Regulators:

■ Programmable LED current from 50 mA to 200 mA

■ 1000:1 contrast ratio at a dimming frequency of more than 3 kHz, minimum LED current pulse width is 300 ns

■ Two individual dimmable LED strings up to 65V, total 15W (typically 28 LEDs @ 150 mA)

■ Dynamic Headroom Control maximizes efficiency

■ Over-Power protection

■ ±3% current accuracy

Supervisory Functions:

■ Precision enable

■ COMM I/O pin for diagnostic and commands

■ Thermal shutdown protection

■ Thermally enhanced eTSSOP-20 package

LM3492應用:

■ Ultra-high contrast ratio 6.5”-10” LCD display backlight up to 28 LEDs

■ Automotive or marine GPS display

圖1.LM3492簡化功能方框圖

圖2.LM3492典型應用電路圖

LM3492評估板參考設計

LM3492 Evaluation Board Reference Design

The LM3492 integrates a boost converter and a two-channel current regulator to implement a high efficient and cost effective LED driver for driving two individually dimmable LED strings with a maximum power of 15W and an output voltage of up to 65V. The boost converter employs a proprietary Projected- On-Time control method to give a fast transient response with no compensation required, and a nearly constant switching frequency programmable from 200 kHz to 1 MHz.

The application circuit is stable with ceramic capacitors and produces no audible noise on dimming. The programmable peak current limit and soft-start features reduce current surges at startup, and an integrated 190 mΩ, 3.9A N-Channel MOSFET switch minimizes the solution size. The fast slew rate current regulator allows high frequency and narrow pulse width dimming signals to achieve a very high contrast ratio of 1000:1 at a dimming frequency of more than 3 kHz. The LED current is programmable from 50 mA to 200 mA by a single resistor.

To maximize the efficiency, Dynamic Headroom Control (DHC) automatically adjusts the output voltage to a minimum. DHC also facilitates a single BOM for different number of LED in a string, which is required for backlight panels of different size, thereby reducing overall development time and cost. The LM3492 comes with a versatile COMM pin which serves as a bi-directional I/O pin interfacing with an external MCU for the following functions: power-good, over-temperature, IOUT over- and under-voltage indications, switching frequency tuning, and channel 1 disabling. Other supervisory functions of the LM3492 include precise enable, VCC under-voltage lockout, current regulator Over-Power protection, and thermal shutdown protection. The LM3492 is available in the thermally enhanced eTSSOP-20 package.

This application note details the design of a LM3492 evaluation board which drives 2 LED strings, each of which consists of 10 LEDs running at 150 mA and the forward voltage of each LED is typically 3.8V. The input voltage is from 9V to 16V. The evaluation board schematic, PCB layout, Bill of Materials, and circuit design descriptions are shown. Typical performance and operating waveforms are also provided for reference.

圖3.LM3492評估板電路圖
評估板材料清單(BOM):


圖4.評估板布局圖(頂層)