NS公司的LM3434是自适应的DC/DC降压恒流电流控制器,提供了用于大功率LED照明的恒定电流.输出配置可使多个LED阳极连接到地以获得散热器最大效率.输入工作电压-9V到-30V,输出电流大于6A,PWM频率大于30kHz,开关频率高达1MHz,主要用在投映系统,固态照明和汽车照明.本文介绍了LM3434主要特性,方框图,10A输出和20A输出应用电路图,以及LM3434 20A输出评估板电路,材料清单和PCB布局图.

The LM3434 is an adaptive constant on-time DC/DC buck (step-down) constant current controller (a true current source). The LM3434 provides a constant current for illuminating high power LEDs. The output configuration allows the anodes of multiple LEDs to be tied directly to the ground referenced chassis for maximum heat sink efficacy. The high frequency capable architecture allows the use of small external passive components and no output capacitor while maintaining low LED ripple current. Two control inputs are used to modulate LED brightness. An analog current control input is provided so the LM3434 can be adjusted to compensate for LED manufacturing variations and/or color temperature correction.

The other input is a logic level PWM control of LED current. The PWM functions by shorting out the LED with a parallel switch allowing high PWM dimming frequencies. High frequency PWM dimming allows digital color temperature control, interference blanking, field sequential illumination, and brightness control. Additional features include thermal shutdown, VCC under-voltage lockout, and logic level shutdown mode. The LM3434 is available in a low profile LLP-24 package.

LM3434主要特性:

■ Operating input voltage range of -9V to -30V w.r.t. LED anode

■ Control inputs are referenced to the LED anode

■ Output current greater than 6A

■ Greater than 30kHz PWM frequency capable

■ Negative output voltage capability allows LED anode to be tied directly to chassis for maximum heat sink efficacy

■ No output capacitor required

■ Up to 1MHz switching frequency

■ Low IQ, 1mA typical

■ Soft start

■ Adaptive programmable ON time allows for constant ripple current

■ LLP-24 package

LM3434应用:

■ Projection systems

■ Solid state lighting

■ Automotive lighting

图1. LM3434方框图

图2. LM3434 10A输出应用电路图

图3. LM3434 20A输出应用电路图t

LM3434 20A输出评估板

The evaluation board is designed to provide a constant current in the range of 4A to 20A. The LM3434 requires two input voltages for operation. A positive voltage with respect to GND is required for the bias and control circuitry and a negative voltage with respect to GND is required for the main power input. This allows for the capability of using common anode LEDs so that the anodes can be tied to the ground referenced chassis. The evaluation board only requires one input voltage of -9V to -30V with respect to GND. The positive voltage is supplied by the LM5002 circuit. The LM5002 circuit also provides a UVLO function to remove the possibility of the LM3434 from drawing high currents at low input voltages during startup.

Initially the output current is set at the minimum of approximately 4A with the POT P1 fully counter-clockwise. To set the desired current level a short may be connected between LED+ and LED-, then use a current probe and turn the POT clockwise until the desired current is reached. The current may be adjusted with P1 up to 18A. 20A output may be acheived either by bypassing P1 and applying an analog voltage directly to ADJ or by adjusting the values of R1 and/or R2 to get higher than 1.5V with P1 fully clockwise. PWM dimming FETs are included on-board for testing when the LED can be connected directly next to the board. A shutdown test post on J2, ENA, is included so that startup and shutdown functions can be tested using an external voltage.

图4.LM3434 20A输出评估板电路图
LM3434 20A输出评估板材料清单:



图5.LM3434 20A输出评估板PCB布局图
详情请见:
http://www.national.com/ds/LM/LM3434.pdf
和
http://www.national.com/an/AN/AN-2041.pdf