Led driving device, led light source device, and vehicle onboard display device

An LED driving method using a master LED driving device configured to drive LEDs in a plurality of channels and two or more slave LED driving devices configured to drive LEDs in a plurality of channels, the method comprising:

• a step in which the slave LED driving device in a last stage outputs information on a lowest voltage among cathode voltages of those LEDs in the plurality of channels which said slave LED driving device drives, to the slave LED driving device in a preceding stage;
• a step in which the slave LED driving devices in stages except the last stage output information on a lowest voltage among the cathode voltages of those LEDs in the plurality of channels which said slave LED driving devices drive and the lowest voltage input from the preceding stage, to the slave LED driving device in the preceding stage;
• a step in which the master LED driving device selects a lowest voltage among the cathode voltages of those LEDs in the plurality of channels which the master LED driving device drives and the lowest voltage input from the slave LED driving device in the preceding stage;
• and
• a step in which, using the selected lowest voltage, the master LED driving device controls one DC/DC converter configured to feed an output voltage to anodes of the LEDs in all the channels driven by the master LED driving device and the slave LED driving devices.