Intersil is ready for the cold with a full lineup of power and analog components spec’d to operate down to -40 °C.
The snowflake ornament reference design has a series of LEDs that fade on and off in an eye-pleasing pattern and plays the familiar song “Let it Snow, Let it Snow, Let it Snow” via a small buzzer when the button on the back of the board is pressed.
The brightness of the LEDs adjust to the ambient light in the room in order to keep the brightness appropriate to the surroundings. Additionally, there is a low-battery detect feature as well as a 4 hour time-out alert to remind the user to turn off the reference design and avoid depleting the batteries.
The reference design is powered by two AAA batteries and is light-weight enough to prominently hang on a tree via a hole at the top of the reference design with the included ribbon.
Facing the challenge of designing a product that must operate in extreme cold conditions? Check out our full lineup power supply and analog components designed to withstand the industrial temperature range.
|Snowflake Design Files
Gerber and Schematic Files for the Snowflake Reference Design Board
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The two AAA batteries that power the ornament can be Alkaline, Lithium, NiMH or traditional heavy duty batteries. This means that the input voltage can range from 3.4V down to 1.8V. The LEDs require about 2V of forward bias in order to turn on, so the design cannot run directly off of the batteries and meet the design goals. Therefore, a buck-boost converter is necessary. The ISL91127IR is one of Intersil’s latest buck-boost converters and features operation from 1.8V to 5.5V, typical quiescent current of 30μA, up to 96% efficiency and is packaged in a small but easy-to-use 4x4mm QFN. These features are a perfect fit for the design.
As a further benefit, the internal switching frequency of the ISL91127IR is 2.5MHz which allows very small inductors to be utilized. Regarding size, note that there is a chip-scale version of this converter that comes in a miniscule 2.15 x 1.74mm WLCSP package. Since the expected production volume for this design is relatively low, it was decided to use packages that can be hand-soldered for prototype and packages that facilitate manufacturing of the production board.
Via the on/off switch, the batteries are tied directly to the ISL91127IR and a resistor divider sets the output voltage of the converter to 3.3V. Note, there is a version of this device called ISL9117IRN which creates a nominal 3.3V output with no external resistors required. The fixed output version would have been a good fit for the ornament design, but it was decided to go with the adjustable version to allow flexibility in case the output voltage needed to be raised or lowered during testing.
If Lithium AAA batteries are installed, the initial voltage will be as high as 3.4V and, in this case, the ISL91127IR will function in buck mode, will switch to buck-boost mode and then eventually into pure boost mode as the batteries deplete. The ornament will continue to function until the batteries are fully depleted and drop down to 1.8V output.
Low Battery Detect Circuit
Although the ornament functions normally all the way down to 1.8V, the user is alerted at 2V that the batteries will soon need to be changed. In order to detect this low battery condition, two Intersil devices are used.
When the battery voltage drops below 2.048V, the output of the comparator is driven high and the microcontroller is alerted of a low battery condition. In this mode, the LEDs strobe to form a large “X” and indicate to the user that the batteries need to be replaced. The ISL28915 comparator has a 1.8V to 5.5V supply range, a 6 lead SOT-23 package and has an active current of 600nA. The ISL60002-20 voltage reference features an operating voltage of 2.7V to 5.5V, 3 lead SOT-23 and active current of 350nA typical.
In order to adjust the brightness of the LEDs for the operating environment, an ambient light sensor (ALS) is employed. The ISL29102 is a great choice because it has a spectral response very close to the human eye requires no external components and has very low operating current of 650nA to 10μA depending on the ambient light in the room.
The ISL29102 also operates on 1.8- 3.3V voltage rails and comes in a 2x2.1mm 6 lead ODFN package. The output of the ISL29102 is fed to the microcontroller’s A/D converter and the LED light intensity is adjusted by changing the maximum duty cycle of each LED string.
The ambient light sensor is powered by a 3.0V rail rather than the 3.3V rail of the rest of the board. While the ISL29102 can be powered by the 3.3V rail as well, the 3.0V rail produced by the ISL80505 low drop-out regulator provides a low noise supply to the sensor. The intent is primarily to serve as a reminder that Intersil has low linear regulators that are excellent candidates anywhere a low noise voltage source is required. In this case, the ISL80505 is a 500mA regulator with a drop-out as low as 45mV, has excellent accuracy and PSRR, and is packaged in an 8 lead DFN.