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:
- easy to solder : SOT23 is ok, QFN isn't
- over voltage protection (when LED is not connected or broken). Proper solutions directly sense the output voltage using an extra pin. This brings the minimum number of pins for the IC to 6 : gnd, vin, vout, sw, fb, en.
- VIN min < 5V
- VIN max > 5V
- VOUT min > 12V
- IOUT > 1A (we need 500mA). Don't confuse this with the switch current. The input current of a boost converter is essentially the inductor current. When the boost converter switch is conducting, the input current is equal to the switch current. If we need 6W at the output, we need at least 8W at the input. The switch current should be at least 2A to be safe.
- low power. Remark that the output LEDs continue to draw current when the LED module is in shutdown. External circuitry is needed to fully shut down the LED module.
- dimming capability (analog & digital dimming are explained in the MP3202 datasheet)
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.
Details about this circuit can be found on EasyEDA.
The circuitry above has some issues:
- Thick wires must be used for VCC & GND because they have to carry up 1.1A. These wires will probably too thick to fit in the JST-XH. For debug, I soldered thicker wires on bottom side.
- The current is set for 500mA, but only about 400mA is being drawn. The feedback voltage never reached 200mV. The 0.4ohm sense resistor combination had to be replaced by a 0.5ohm (2x 1ohm) resistor combination.
- Value of the resistors for analog dimming are too big. As the PCB is quite small, they pick up some noise. Making them a factor of 10 smaller improves it a lot.
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:
- The maximum input voltage for the RT4533 is 5.5V
- 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.
- So maximum input power is 5.5 * 1.2 = 6.6W
- Let's be optimistic and assume the output power is 80% of the input power : 5.3W, which is far from the advertized 10W.
The output current drops linearly with the control voltage, as expected.
Efficiency is around 80%, as shown on the RT4533 datasheet.
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.