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Simple LED Circuit

Geo
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ACME AS123 LED Driver with Dimmer Control - Application Note

MihaiLuca
Designer93191

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Description

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 (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-------------- End of Example Application Note ------------------If you are a component supplier, this type of "Executable Application Note" can help your potential customers better understand the features and benefits of your components.

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Simple LED Circuit

LuigiBosca
Designer91651

LuigiBosca

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8 years 1 month

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Slayer circuit Tesla coil

ZanderWelsh
Designer91006

ZanderWelsh

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8 years 1 month

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Description

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LED Dimmer Circuit

CaesarB
Designer89066

CaesarB

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8 years 1 month

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Description

This LED Dimmer Circuit uses a 555 Timer to control the PWM duty cycle of the current drive. Rather than apply proportional but continuous current to the LED for dimming, which can cause color shifts, modulating the duty cycle allows the LED to operate at its nominal current during the “ON” portion of the cycle. Because the frequency response of human vision is limited, using a PWM frequency of 250 Hz avoids the perception of flicker for the observer. The LED model has an internal monitor for the "perceived" light output (light-blue waveform), which is a low-pass filtered version of the instantaneous light output. The filter pole frequency is set to 15 Hz to represent the bandwidth of the human eye. The value of the dimmer setting (green waveform) is increased from 10% to 90% at time 100msec. The LED current pulses (purple waveform) are shown just before and after the duty-cycle transition.Part of this design is based on a dimmer schematic found on-line: http://www.555-timer-circuits.com/led-dimmer.html That original circuit is actually incorrect, one of the diodes needs to be reversed. There was a comment from a reader who said he built the circuit as shown and it didn’t work. This is a good example of the value of simulating circuits before building hardware!

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