AMC7135 PWM LED control – works?

The AMC7135 is a constant current driver (sink) meant for 1W power LEDs. It sinks 350 mA into its OUT terminal, thud driving the load (power LED) with 350 milliamps of current. Assuming that the supply voltage is under 6V, the setup can drive a single 1W white power LED easily with minimal component count and no current sense resistor. But what happens when you use PWM to drive the AMC7135? The AMC7135 PWM circuit must operate in a particular frequency and duty limit to work smoothly!

Can AMC7135 handle PWM?

Yes, it can.
Is it recommended to do this? Probably not! There is nothing in the datasheet to hint that the current sink can switch in kHz range (usually required in lighting control). However, when I tested the AMC7135 out, it seemed as if it can handle fairly high PWM frequency and thus control the brightness of the connected LED flexibly.

However, assuming that the AMC7135 is a feedback based current control system, you must consider the time that the device takes to settle to a stable constant current output. Not caring about the settling time of the current sink will cause loss of total effective power delivered to the LED.

How fast can you run the AMC7135 PWM?

The Vdd pin is used to enable/disable the output of the LED driver. The input is very high impedance and so any microcontroller that outputs a 3.3V or 5V logic level should be able to directly switch the AMC7135 current sink. How fast can you run the PWM? Here is the test circuit I used to check how long it takes for the circuit to draw a stable 350mA through the LED:

amc7135-pwm-led-control

Apologies for my terrible drawing skills! But this should be sufficient to get the point across. In the scope screenshots shown below, the nice rectangular wave is the signal generator output (3.3V CMOS level) and the other waveform that is struggling to follow the PWM input is the AMC7135 current output.

AMC7135 PWM drive response speed

amc7135-pwm-frequency-1

At about 2kHz PWM frequency, things seem okay! The time required by the current sink to become stable is about 8us.
That is,
Turn-on time: 8 us
Turn-off time: 4 us

This should tell you that you do not want the frequency anywhere close to these time periods. If you make the frequency so high that the current sink cannot turn on properly, you will lose on your full potential power that can be delivered to the LED, i.e. PWM control will not be effective over all duty cycles.
This can be seen here:

amc7135-pwm-duty-restricted-high-frequency

As in this screenshot, the duty cycle is at its minimum possible here. If you go to a lower duty cycle, things will not stabilize enough before the PWM turns the LED off again. That is not a good way to drive the AMC7135 as the brightness will suddenly decrease in the lower ranges of duty cycle.

So if you want control over the lower ranges of duty cycle, you need to reduce the PWM frequency. The best approach to drive the AMC7135 PWM based LED systems is:

Frequency of PWM: 6 kHz
Minimum duty: 5%
Maximum duty: 100%

This will ensure smooth brightness control over all duty ranges, as well as good enough precision of PWM control even on slow microcontrollers. Note that reducing the PWM frequency to under 5 kHz may cause visible flicker in the LED!

2 Responses

  1. Would the response be faster if the pwm voltage is 3.3v instead of 5v?

    • It did not seem to make any huge difference. The time to get stable actually depends on the load capacitance and how fast the source can ramp up the current (ESR of source, etc matter too). But keeping all the things modest… I think this is the right frequency. You can of course go higher, but it will reduce smoothness in brightness control.

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