DAC output OSCILLATING when combining MULTIPLE CHANNEL

Hi Antonie,

Please share your schematic and load details.

Also share the register configurations for the above plots.

Hi,

You will find attached the schematic. Channels 0 to 3 are linked together and a 22uH inductor is used to flatten out the current. The load used to get the aforementioned result was a power LED cree XML (Vf~2.5V and If ~ 400mA). The above plot was obtained with those C commands :

"LTC2672_set_output_span(LTC2672_1, LTC2672_SPAN_200MA, LTC2672_CHANNEL_0);
LTC2672_set_output_span(LTC2672_1, LTC2672_SPAN_200MA, LTC2672_CHANNEL_1);
LTC2672_set_output_span(LTC2672_1, LTC2672_SPAN_200MA, LTC2672_CHANNEL_2);
LTC2672_set_output_span(LTC2672_1, LTC2672_SPAN_200MA, LTC2672_CHANNEL_3);

LTC2672_set_output_current(LTC2672_1, 100.f, LTC2672_SPAN_200MA, LTC2672_CHANNEL_0);
LTC2672_set_output_current(LTC2672_1, 100.f, LTC2672_SPAN_200MA, LTC2672_CHANNEL_1);
LTC2672_set_output_current(LTC2672_1, 100.f, LTC2672_SPAN_200MA, LTC2672_CHANNEL_2);
LTC2672_set_output_current(LTC2672_1, 100.f, LTC2672_SPAN_200MA, LTC2672_CHANNEL_3);"

Here is the simplified LTC2672_set_output_span function :

"LTC2672_sendCommand(deviceID, (unsigned char)LTC2672_CMD_WRITE_SPAN_CH, (unsigned char)channel, (unsigned short)span);"

Here is the simplified LTC2672_set_output_current function :

"LTC2672_sendCommand(deviceID, LTC2672_CMD_WRITE_PWRUP_CH, channel, code);"

Here is my simplified LTC2672_sendCommand function (the same as on the LTC2672 git) :

"LTC2672[deviceID].CS_level(0); // Select device
// Send command byte
LTC2672[deviceID].spi_sendByte((unsigned char)(command | address & 0x0F));// Send 16-bit data
LTC2672[deviceID].spi_sendByte((unsigned char)((data >> 8) & 0xFF)); // MSB
LTC2672[deviceID].spi_sendByte((unsigned char)(data & 0xFF)); // LSB
LTC2672[deviceID].CS_level(1); // Unselect device"

The #defines are from the datasheet.

The oscillation amplitude and frequency vary with the output current and the LTC2672 input voltage.

Best,

Antoine Richard

Hi Antonie,

Please share the entire schematic - power supply circuit and the load datasheet.

Hi, 

Here is the load datasheet : https://downloads.cree-led.com/files/ds/x/XLamp-XML2.pdf.

I currently use an ultra-stable laboratory supply that supplies the LTC2662, so no on-board power circuit can be the cause of problem (the 5V supply is routed to an alimentation plane supplying the LTC2662). I cannot really share the entire PCB schematic/routing. When observing the 5V supply at the VCC pin or VDDx pins, it is very stable. Only the output have ripple/oscillation.

Best, 

Antoine Richard

Hi,

I send a second message because the first one was flagged as SPAM (I sent a link). You can find the datasheet of the load searching for "XLamp® XM-L2 LEDs". The load is a CREE XML high power LED.

We are using a laboratory ultra-stable power-supply, so no on board DCDC power circuit can be the source of problem. The 5V is directly supplying the LTC2662 via a power plane. Also, when I probe the 5V of the VCC and VDDx pins, it is stable. You'll understand that I cannot share the entire design, but I can share some specific parts.

Best,

Antoine Richard 

Hi Antonie,

I am taking it offline.

Regards

Vikash

Hi Antonie,

What is the status of V-?

Regards

Vikash

Hi,

I answered offline.

Regards,

Antoine

Would it be possible to post the resolution of this issue, without disclosing any confidential information? I have a similar problem.

Thanks
Andrew

Hi,

I was told several advices to try to minimize the issue :

1) Do not use any serial inductor to filter the output current, just use decoupling capacitors.

2) Do not use any serial inductor to filter the MUX output, and use a voltage follower because this output is unbuffered. Drawing too much current on this output can cause the chip to malfunction.

3) Use a stable power rail to power the chip. Excessive ripple can propagate easily.

Hope it helps,

Antoine