Key Input Parameter | Key Output Signal | Recommended Device |
---|---|---|
SPI or I2C communication to program DAC codes 0x000 to 0xFFF, GPI trigger | 0 A to 250 µA, and 0 mA to 20 mA LED current | DAC43204 (8 bit), DAC53204 (10 bit), DAC63204 (12 bit) |
Objective: Bias an LED with a high-side current source using a smart DAC.
This design uses a four-channel buffered voltage or current output smart DAC such as the DAC43204, DAC53204, or DAC63204 (DACx3204) to bias a light-emitting diode (LED). The smart DAC can configured in voltage output mode and be connected in a force-sense configuration with a bipolar junction transistor (BJT) in LED biasing applications needing a few milliamps of current as shown on channel 1 of the DACx3204 in the circuit schematic. The DAC sets the collector current of the PNP-type BJTx, such as the 2N2905, and controls the amount of current through the LED by varying the base voltage. The LED is connected between the collector of the BJT and ground. The DAC can be used in current output mode to drive the LED directly with up to 250 µA for low-current LED biasing applications as shown on channel 0 of the DACx3204 in the circuit schematic. The feedback pin (VFB) of the DACx3204 compensates for the base-emitter voltage (VBE) drop and the drift of the BJT when using the BJT configuration. The DACx3204 have a general-purpose input-output (GPIO) pin that can be used to switch the LEDs between two current values, or on and off. All register settings can be saved using the non-volatile memory (NVM) on the smart DAC, meaning that the device can be used without a processor, even after a power cycle. This circuit can be used in applications such as barcode scanners, barcode readers, currency counters, POS printers, optical modules, and appliance lighting.
The DAC codes for the 10-bit DAC53204 can be calculated by:
VDAC is calculated by:If a 5-V VDD is used as the reference, the high and low DAC codes become:
This is rounded to 1023d and 819d to give a high value of 4.995 V and low value of 3.999 V. This configuration compensates the VBE voltage drop caused by temperature, collector current, and aging of the BJT. A BJT provides a smaller VBE drop as compared to a typical gate-source voltage (VGS) drop of a MOSFET.The DACx3204 can be programmed with the initial register settings described in the Register Settings section using I2C or SPI. The initial register settings can be saved in the NVM by writing a 1 to the NVM-PROG field of the COMMON-TRIGGER register. After programming the NVM, the device loads all registers with the values stored in the NVM after a reset or a power cycle.