LM3554 IC temperature is too high issue

if you are leaving one of the channels floating then this could be the heating issue.  The boost switching power supply in the device is designed to control the voltage drop across the high side current source(s) (from OUT to LED1 and LED2).  This is regulated to typically 300mV.  The boost will switch until both LED1 and LED2 has at least 300mV drop.  if a channel is left floating then that channel will be pulled up to VOUT and cause the drop from OUT to the floating current source output to go to 0.  This will force the boost to switch continuously at max duty cycle and drive VOUT all the way to the max output voltage of 5.6V.  The other current source which is regulating the LED current will then get a large drop across it. For example, if VLED is 3V at 0.6A, then you get a (5.6V - 3V) x 0.6A power loss across that current sink.  This  1.56W will quickly heat up the device because it is pretty small and will have a relatively high thermal resistance.  There is no good way to only use one current sink.  if you would always have one LED then you can connect LED1 and LED2 together externally and just program each current source to 1/2 the total current.

Thanks

Travis

Jerry,

Yes, the I2C bus can operate on 1.8V, Although if you are using the EVM with the included I2C interface adaptor, it will need a bit higher logic levels due to the micro controller on the adaptor board.  The logic high on SDA/SCL requires a min of 1.22V, and the logic low is a max of 0.4V, so is compatible with 1.8V logic levels.

Thanks

Travis