Budgeting LED Current Sink Headroom Across Supply and Forward Voltage

A constant current LED sink still needs voltage across its output stage to regulate. If the supply barely covers the LED forward voltage, current can fall below the programmed value. Raising the supply solves that particular problem but increases dissipation in the sink. Review both ends of the voltage budget.

Define the current path

Draw the LED supply, series LEDs, wiring, and current sink output. The TI TLC5926 data sheet specifies current regulation under defined output voltage conditions and provides output current behavior versus voltage. Those conditions are more useful than assuming a current setting guarantees regulation at any output voltage.

For the intended current, collect the LED string’s forward voltage range over relevant temperature and production variation. Include connector and trace drop where it affects the available voltage.

Find the minimum available headroom

Subtract the highest relevant LED string voltage and wiring drop from the lowest supply voltage at the board. Compare the remainder with the selected driver’s regulation requirement at the programmed current.

For a hypothetical 5 V rail, a 3.6 V LED and 0.1 V wiring drop leave 1.3 V across the sink. That arithmetic does not establish compliance unless the chosen sink’s limits support the current at that remaining voltage.

Check the opposite corner for heating

Use the highest supply and lowest LED voltage to estimate maximum sink voltage. Multiply each enabled channel’s sink voltage by its current and include duty cycle when calculating average dissipation. Add device supply consumption where material.

For illustration, a channel carrying 20 mA with 2 V across its sink dissipates 40 mW in that path. Sixteen such channels contribute 640 mW before other losses. Evaluate the actual package and board thermal conditions instead of judging one channel in isolation.

Measure current and temperature together

Test the relevant supply extremes, LED populations, and display patterns. Measure sink voltage and current at the same time so reduced brightness can be distinguished from a command or regulation issue. Include the all-on pattern if the application permits it.

Save the voltage budget, channel configuration, measured regulation boundary, and thermal result. Recheck after changing LED bins, series count, current setting, or supply tolerance. A satisfactory design maintains current where required while keeping sink dissipation acceptable. A brightly illuminated prototype at nominal supply demonstrates neither the low voltage regulation margin nor the high voltage thermal condition that determines reliable operation across the intended range. Include the measured board temperature with each current reading.