Christoph TackChristoph Tack wrote 11/12/2019 at 20:26 • 4 min read • Like

LED driver modules

The plan is to power the clock from a 5V USB power adapter.  The LEDs require about 12V/500mA.  So we need a boosting LED module, not a buck module.

If we decide to make the module ourselves, we need to select an IC:

Digikey selection (for 1A switch current): RT4533GJ6 and MP3202 (pin compatible), TP61165 (not pin compatible)

In the hindsight, the LM3410 would have been a better option, because it allows for larger drive currents.

Revision 1.0

Details about this circuit can be found on EasyEDA.

The circuitry above has some issues:

The RT4533 announces itself as boost converter for 10W LEDs.  It may well be possible to drive LEDs, but not at 10W.  Let me explain:

  1. The maximum input voltage for the RT4533 is 5.5V
  2. The maximum switch current is 1.2A.  For a boost regulator, this is also the maximum current through the inductor.  The current through the inductor is essentially the input current to the circuit.
  3. So maximum input power is 5.5 * 1.2 = 6.6W
  4. Let's be optimistic and assume the output power is 80% of the input power : 5.3W, which is far from the advertized 10W.
Measurement setup: dual power supply (set to max. 1.96A. Voltage output was disabled when picture was taken), R&S Current probe, R&S differential voltage probe.

Measurement results

The output current drops linearly with the control voltage, as expected.

Output current versus dimming voltage

Efficiency is around 80%, as shown on the RT4533 datasheet.

Efficiency versus output current

The module already saw a second use as a replacement driver for lighting the christmas tree. The original driver was specified for 32V/100mA. Setting the two sense resistors to 3.9ohm and you're done.  Power draw on the 5V power supply was about 800mA.

Dimming the light using PWM

Earlier experiments with dimming LEDs showed me that 8bits is not enough to generate smooth transitions.  The step from 1/256 duty cycle to 2/256 is very noticeable.  Once you're at duty cycle 200/256, increasing the duty cycle doesn't make much difference.

A simple way to increase the PWM resolution is to use two PWM channels, a dual RC-filter and an opamp.  Hackaday already published an article about the subject.