ACME AS123 LED Driver with Dimmer Control - Application Note

The AS123 LED Driver from ACME Semiconductor (fictional) provides fully integrated PWM dimming and a programmable current set-point. This IC is specifically for automotive rear combination (tail/stop) lighting applications. It tightly regulates the desired string current under conditions of widely varying applied DC voltage.

The PWM dimming function, which switches the LED on and off at just over 300 Hz, is active when the line input pin is high and the "PWM_disable" pin is low (< 6V). This PWM_disable pin should be connected to the stop/brake signal input, as shown in the application schematic. Then the PWM switching will stop when braking, providing full LED brightness.

The AS123 supports current programming using a single external resistor (see r_iset in the schematic). The resistor value can be selected to give the desired LED string current, using the following formula:

r_iset = 1.85/i_desired

The AS123 is capable of regulating up to 100mA per string continuously. Therefore the value of r_iset should be no less than 18.5 Ohms.

In the simulation results, note that the brake/stop signal is active (12.6V) for the first 100ms, and then low (0 V) for the next 100ms. The tail light input is active the entire time, pulsing between 11.6V and 17.6V. Note that the LED current is approximately constant, at just under 30mA during the brake/stop on-time. When the brake/stop signal goes to 0V, then the LED current begins to PWM (55% duty-cycle, 325Hz).

The user can move the waveform probes around, to look at other signals both on the schematic nets and within components. For example, move a probe onto one of the LED models and select to view the "power_dissipated" or the "lumens_output".

If the user creates a free account on systemvision, he can make a copy of this design. Then he can make changes to it, such as adjusting the r_iset resistor value. He can then simply re-run the simulation to see the effect of that change.

See a "block diagram" schematic model of the ACME AS123 here: https://www.systemvision.com/design/acme-as123-led-driver-schematic-model

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Mike Donnelly

Joined November 8, 2013
Design added Monday, February 29, 2016 | 1:03 pm PST