SLAA998 May 2021 DAC43701 , DAC43701-Q1 , DAC53701 , DAC53701-Q1
Key Input Parameter |
Key Output Signal |
Recommended Device |
---|---|---|
GPI trigger, Values programmed in MARGIN_HIGH and MARGIN_LOW registers | DAC output transitioning from MARGIN_LOW to MARGIN_HIGH value at a programmed slew rate, current output to adjust the brightness of LED | DAC53701, DAC43701 |
Objective: Programmable LED biasing.
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 VSET 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 VGS drop of the MOSFET. The LED current is given by VSET/RSET 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.
The design procedure is laid out with the following requirements and components in mind
Choose a small VSET so that the power dissipation across RSET is minimum. While there is an option to use the DACx3701 with either an external or an internal reference, it is important to note that the DACx3701 has a single supply voltage which will also serve as a reference in the external mode. In a practical scenario, it is unlikely that a user would have a precision reference available for VDD. With the standard power supplies, the noise and accuracy of an external reference will not be at par with the internal reference of the DACx3701. It is therefore recommended to use the internal reference of the DACx3701 to have an accurate ISET.
A VSET of 1V is chosen for the bright condition. The internal reference of the device is 1.212V with optional gain settings of 1.5×, 2×, 3×, or 4×. Using the 1.5× gain setting, the DAC53701 will have an output span of (the internal reference of the DAC is trimmed to 1.212V (typical) at room temperature). This results in RSETof
The output buffer of the DAC is connected in a force-sense configuration to the MOSFET as previously shown. This configuration compensates the gate-source voltage drop caused by temperature, drain current, and aging of the MOSFET. Considering a typical gate-source voltage of 1.2V and a power supply headroom of 200mV, the VDD for the DAC must be a minimum of (1V + 1.2V + 200mV) = 2.4V. Use a standard 3.3-V or 5-V power supply for the DAC. A bipolar junction transistor (BJT) provides a much smaller base-emitter voltage drop, but a MOSFET has better matching between the drain and source currents. It is recommended to choose BJT over the MOSFET when less than 2.4-V supply voltage is available for the DAC. The fact that the power supply of the DAC should be kept at or below 5.5V imposes a constraint on the VGS across the MOSFET for higher values of VSET. The VGS of the MOSFET will change with temperature and with ISET as well. A higher ISET requires a higher VGS but a high VSET may clip the VGS and subsequently the MOSFET current due to the supply limitations of the DAC. This presents a stronger case to use a lower VSET.
Configure the MARGIN-HIGH register value to the code equivalent of 1V; that is or 0x233. The MARGIN-LOW value should be the equivalent of the dim LED current that is 10mA, which corresponds to a DAC voltage of . The code for MARGIN-LOW is or 0x11A.