DLPS052 October 2015 DLPA3000
The current through the LEDs is determined by a digital number stored in the respective IDAC registers 0x03h to 0x08h. These registers determine the LED current which is measured through the sense resistor RLIM. The voltage across RLIM is compared with the current setting from the IDAC registers and the loop regulates the current to its set value.
When current is flowing through an LED, a forward voltage is built up over the LED. The LED also represents a (low) differential resistance, which is part of the load circuit for VLED. Together with the wire resistance (RWIRE) and the RON resistance of the FET switch, a voltage divider is created with RLIM that is a factor in the loop gain of the ILED control. Under normal conditions, the loop is able to produce a well-regulated LED current of up to 6 A.
Since this voltage divider is part of the control loop, care must be taken while designing the system.
When, for instance, two LEDs in series are connected, or when a relatively high wiring resistance is present in the loop, the loop gain will reduce due to the extra attenuation caused by the increased series resistances of rLED + RWIRE +RON. As a result, the loop response time lowers. To compensate for this increased attenuation, the loop gain can be increased by selecting a higher gain for the programmable gain block. The gain increase can be set through register 0x25h [3:0].
Under normal circumstances, the default gain setting (00h) is sufficient. In case of a series, connection of two LEDs setting 01h or 02h might suffice.
As discussed before, wiring resistance also impacts the control-loop performance. It is advisable to prevent unnecessary large-wire length in the loop. Keeping wiring resistance as low as possible is good for efficiency reasons. In case wiring resistance still impacts the response time of the loop, an appropriate setting of the gain block can be selected. The same goes for connector resistance and PCB tracks. Keep in mind that basically every mΩ counts. Following these precautions will help get a proper functioning of the ILED current loop.