LTC3453 fails and short circuit

Should I change the EN1 to EN2 to increase the current limiting rating? I thought the small current rating would help to protect the IC, maybe that is the wrong interpreation. 

Hello, 

1. I'm glad an output cap was added. This should be close to the VOUT and GND pins on the IC.

2. The current gain for ISET1 and ISET 2 and formulas are the same. They will provide the same current with the same resistance.

3. How is the output current being measured?

4. The input current will be different between the RED LED and the Yellow LED because they have different Vf voltages. The output and input power will be different for each LED. This can also cause the regulator to operate in different modes, buck with one, buck-boost or boost for the other. The input current will be much different than the output current. The LED current is much different than the output current.

6. ILED is 384*0.8V/R but should be programed not to exceed more than 500mA output current. The resistor values being used should be OK.

It is good that the schematic was changed to control the EN. Is the EN pin pulled low when disconnected?

I see the LEDs are connected with a header. Does VOUT ever go open circuit?

I see the LED is on a cable. I am concerned about the output voltage spiking after turn off. 

When the problem occurs. Does the IC ever recover or is it permanently damaged?

It was observed on a similar device that turning on-off-on quickly before the VOUT cap discharged caused an issue. What helped in this case was to load the output cap with enough resistance to discharge the cap between on-off-on transitions. Maybe 1k or 10k might be enough.

Hello, 

1. I'm glad an output cap was added. This should be close to the VOUT and GND pins on the IC.

2. The current gain for ISET1 and ISET 2 and formulas are the same. They will provide the same current with the same resistance.

3. How is the output current being measured?

4. The input current will be different between the RED LED and the Yellow LED because they have different Vf voltages. The output and input power will be different for each LED. This can also cause the regulator to operate in different modes, buck with one, buck-boost or boost for the other. The input current will be much different than the output current. The LED current is much different than the output current.

6. ILED is 384*0.8V/R but should be programed not to exceed more than 500mA output current. The resistor values being used should be OK.

It is good that the schematic was changed to control the EN. Is the EN pin pulled low when disconnected?

I see the LEDs are connected with a header. Does VOUT ever go open circuit?

I see the LED is on a cable. I am concerned about the output voltage spiking after turn off. 

When the problem occurs. Does the IC ever recover or is it permanently damaged?

It was observed on a similar device that turning on-off-on quickly before the VOUT cap discharged caused an issue. What helped in this case was to load the output cap with enough resistance to discharge the cap between on-off-on transitions. Maybe 1k or 10k might be enough.

Thanks Marty. 

4. Do you think that would be the cause of IC failure?

6.  I don't think the current would exceed more than 500mA. Based on the current configuration, a fixed resistor is always connected and the current should be limited to <150mA. A fuse is connected to the powerline, it should break before the current excess its maxmum current. 

The EN is pull down to GND by a 10k resistor. 

The VOUT is always connected to LED terminal (either yellow or red).  the header is used for testing only. 

The IC is permanently damaged, short straight away after being powered on. 

I am also concern about the spiking. So you suggest connecting a 10K in parallel with the output cap? I can try it when the new PCB arrives. 

Hello,

4. Was basically stating that the input current will not be the same as the LED current and it will differ depending on the VIN/VOUT conditions. If the LTC3453 is boosting up the output voltage the input current will be higher than the LED current. If it is bucking down the vout voltage, the input current can be lower than the LED current.

6. If the LED current is 150mA or less I expect the input current to be less than 500mA. Below is from the datasheet. This was taken using the typical app in on the front page with a white LED, LUMILEDS LXCL-PWF1, I think with a Vf of close to 3V. (I would need to look at the spec to be sure.) This should provide an idea what is expected for the input current for this setup. It sounded like you app with the yellow LED should be similar.

If the LED can be turned on-off-on quickly I do suggest adding a resistor in parallel with the output. I would start with 1k first. It that works then try something with more resistance. Also make sure the input and output caps are close the the pins of the LTC3453 with a good GND path. The cables from and to the battery and LED should be as short if possible or as short as in the application if possible. If possible twist the LED and return wires together and also twist the battery and GND wires together. If it appears that the input or output voltage is spiking above abs max or below GND then some added TVS diodes maybe needed to help prevent this from spiking up or down and breaking the part. These spikes are typically vary fast and difficult to capture on the oscilloscope. 

It is difficult to say what is breaking the part without good scope captures showing what is happening when the part breaks, not easy to do.