Don’t Be a Sunday (LED) Driver HDDG #0xF Jason Cerundolo

LEDs!

Throwie

Problem

Shunt Resistor I led = 2 mA V led = 1.8 V V R = V batt – 1.8 V I R = 2 mA R = (V batt – 1.8 V)/(2 mA) Vbatt = 3.0 V R = 1.2 V / 2 mA = 600 Ohm

Microcontroller Control LED power sourced by GPIO pin R = (5.0 V – 1.8 V)/(2 mA) R = 1.6 kOhm

Microcontroller Control LED power sunk by GPIO pin

Microcontroller Control Pros Different supplies for LED and MCU Sink more current than source Cons Watch your max rating on GPIO voltage

PWM Control Pros Cheap Easy Cons Possible strobing effect Max f PWM = f CLK / 2^n Ideally f PWM > 500 Hz

Current Control vs Voltage Control Maxim Application Note 3256

Why Current Control? Avoid max current Control brightness Control white balance

Linear Current Control I led = 200 mA V R2 = V BE = 0.6 V R = 3 Ohm V cc > V led V P waste = (V cc – V led ) * 200 mA PWM Control available

Switching Current Control More efficient But this is a voltage supply…

Switching Current Control V FB = 0.5 V I LED = V FB /R P waste = 0.5 V * I LED No output cap Be careful about using EN pin for PWM control

Buck Converter Details Methods for Dimming Control PWM on EN pin Override V REF Add to V FB

PWM Dimming P waste = 0.1 V * I LED Minimum on time 6 cycles 1.5 MHz switching frequency 8 bit control => f PWM = 1.5 MHz / (6 * 2^8) = 977 Hz Datasheet recommends 500 Hz

Analog Dimming Vctrl [V] V R1 [V]

Better PWM Control Pro FET provides a sharper turn-off Con Make sure you can switch the FET V TH_EFF = V TH + V FB

Multiple LEDs Maxim Application Note 3256

Multiple LEDs Only turn on one LED at a time LEDs can be different types (eg, red, green, and blue)

Multiple LEDs Turn on any number of LEDs, as long as D1 is active All LEDs must have similar I-V curve

Strings of LEDs Multiple LEDs in series All LEDs have same current Vcc > n*V f + V FB

Boost Converter Driver Multiple LEDs in series All LEDs have same current Output cap is back Vcc < n*V f + V FB V max > n*V f + V FB

Putting it all together

Thank You Website: Github:

More Information efficiently-drive-an-led efficiently-drive-an-led 03/smart_led_driver.html 03/smart_led_driver.html

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