It was Würth the wait!
I placed an order with Element14 for some Würth Elektronik 8.2uH inductors and some other parts.
What are they for? They're to enable the EFM32 microcontroller to drive the 24 heart LEDs from a single voltage.
I've arranged the 24 LEDs in a four chains of six LEDs like so.
This was done as a compromise between part count, reliability and serviceability.
This array arrangement has a Vf of about 12V (6 x 2.0V) and a current requirement of 80mA (4 x 20mA)
A single string of 24 LEDS is susceptable to total display outage with one failed open circuit LED.
It requires a supply voltage of 48 volts with a current of 20mA.
For the array the effects of single open circuit LED would cause only 5 other LEDs in the same string to not light up but cause the current destined for broken string to be channelled across the remaining working strings.
To the other extreme, 24 separate LED circuits would require 24 separate current limiting devices like resistors but has the advantage of only the broken LED not working with no other collateral damage.
This configuration would require a supply voltage of about 2 volts with a current of 480mA.
The EFM32 microcontroller itself is not capable of driving 24 LEDs simultaneously so additional circuitry has been added.
Instead of directly driving a single LED using an EFM32 PWM output this output will be used to drive a LED driver IC that accepts a PWM output.
There are many of these devices around and all have different capabilities.
For my purpose I'd like to drive all the LEDs and microcontroller using a single voltage source with the minimum number of components and device pins.
To accomplish this the LED driver must have;
- Programmable Current Limiting
- Voltage up or down conversion
- PWM input
Linear Technologies has a huge range of suitable devices but I needed something simple and in a hurry for this design challenge so I chose the Texas Instruments LM3410 because it was in stock at Element14 and apart from bypass capacitors and a logic input pullup resistor the only external components are;
- inductor
- schottky diode
- current setting resistor
The LM3410 is a essentially 5 pin device available in 3 different package types. SOT-23, 6 pin WSON and 8 pin MSOP-PowerPad.
The WSON and MSOP packages have a thermal pad to disspate power more efficiently and have separate AGND and PGND pins rather than a combined GND pin.
Feeling a little legless?
Due to its availability and thermal capabilities I ordered the legless WSON packing.
Being legless makes it considerably harder to mount. I used localised IR (InfraRed Radiation) for isolation soldering.
This is essentially an IR oven confined to a single component to minimise collateral damage.
It is great for prototype construction because it allows each component to be added when needed.
What is Unit Testing?
Unit testing is the functional testing of a component in isolation to determine whether it is working correctly.
I performed unit testing on the LED Driver Circuit to ensure that it worked correctly.
This video shows the LED driver firstly driving one string of LEDs and then three strings of LEDS.
What is System Integration Testing?
System integration testing is the functional testing of all components and controls that constitute a system.
Ideally this is a comprehensive and formal set of testing procedures. The formality is ensure test repeatability - a must for quality control.
In this case it is an informal test where the STK3700 kit driving the LED driver that is driving the all 24 LEDs of the heart and checking the slider control against the LED output.
This is a video of my first test of the untuned system. The PWM rate hasn't been tuned yet. It works with human vision persistence but not with video cameras.
It could be as simple as a case of ISO tuning on the camera.
