Design Description

This circuit design describes a key application of the DACx3701 – programmable LED biasing for appliance fade-in and fade-out applications. Appliances such as toaster ovens, microwave ovens, refrigerators and clothes dryers implement door lights for monitoring the status of the function of the appliance. These door lights dim and brighten for a certain time frame when the door closes and opens, respectively. Appliance manufacturers prefer to provide a smooth-dimming transition for a better user experience. However, a microcontroller is required for such an operation, and implementing a separate microcontroller and associated software is an overhead for a heavily-commoditized appliance market. For this reason, only the high-end appliances have such features. The DACx3701 provides an easy-to-implement, low-cost way to control the slew of such lighting without the need for software.

The following images show a simplified circuit diagram of light fade-in fade-out using MOSFET based control and an external LED driver. For high-power LEDs, external LED drivers with headroom control are preferred over MOSFET-based LED control, see AN-1656 Design Challenges of Switching LED Drivers. As the following figures show, a sample use case can involve a mechanical switch coupled to the appliance door. When the door is closed, the switch is ON by default thus keeping the GPI, the general purpose input pin of the DACx3701 low. As soon as the door is opened, the switch is turned OFF, thus pulling the GPI high.

The GPI functionality is configured to margin a high-low function. Thus, a rising edge on the GPI pin takes the DAC output to the value programmed in the MARGIN-HIGH register at a slew-rate defined by the values programmed in the SLEW_RATE and CODE_STEP bits of the GENERAL_CONFIG register. The feedback loop, closed by the MOSFET ensures that V_SET is equal to the DAC output (Here, DAC output means the output of the DAC ladder, not the output pin). This configuration provides a benefit of accounting for the V_GS drop of the MOSFET. The LED current is given by V_SET/R_SET and is thus regulated by the DAC. Similarly when the appliance door is closed, the switch is now turned ON, thus pulling the GPI pin low. This high-to-low trigger on the GPI pin drives the DAC output to the value programmed in MARGIN-LOW register. This way, the LED brightening and dimming can be regulated at a specified rate using the DACx3701.

This design explains how to program the respective DAC registers to set the required LED current and to control the rate at which the LEDs become brighter or dimmer. Also included with this article is pseudocode to get started with the application. The error in LED current is also estimated based on various factors such as load resistor tolerance, DAC feedback impedance and drift in value of a particular DAC code. The power dissipation through MOSFET has been calculated to help the user choose an appropriate part based on their application.