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Temperature rising using the LTC3440

Hello,

i am using the LTC3440 to convert the voltage from a LiPo Battery into a stable 3.3V. As you can see in my schematic, i am the following the datasheet. My circuit is drawing 300mA in worst case.

I did some tests with 300mA Load and the outputs is stable on 3.3V. The problem is the temperature on the IC. I am going to measure the temperature but its really hot ( cannot touch it for more than 2-3 seconds).

The inductor is rated for 1.2A max DC current and i am using the max frequency on the converter.

Should i increase the inductors value and decrease the frequency?

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  • Even when i draw 60mA from 3.3V, the input current to IC is 250mA.

    Any suggestions?

    I have 2 same PCBs which i test with same LiPo (so same VBAT):

    - In the first PCB i measure 50mA on converter's input when i draw 40mA from 3.3V

    - In the second PCB i measure 250mA-280mA on converter's input when i draw 60mA from 3.3V

    Seems like IC failure? 

  • Even without load (by removing the R34) the LTC3440 is drawing over 200mA and its getting hot.

    I am going to replace the LTC3440 with new one, just in case something went wrong when soldering (maybe exceed the absolute maximum temperature or timing).

    The Mode/Sync pin is floating in my desing, it is not clear in the datasheet if i must ground,left floating or tie it to VCC. Could this floating Pin damage my IC? The output is on 3.3V so i guess the internal oscilator is working.

  • Hi ,

    There could be several reasons for the temperature rise- part might have been damaged, poor layout or any other incorrect component placement on the board. I would recommend you to get a demo board for LTC3440 and compare your design and layout with it.

    The Mode/SYNC pin should not be floating. If you want to run in PWM mode it should be driven low else it should be pulled high for Burst Mode.

    Thanks

  • Could Mode/Sync pin left floating damage the part;

  • Solved.

    Mode/Sync was not the problem. The problem was that Rt resistor on both PCBs was not placed correctly when i first power-up the board in a way that Rt pin was floating.

    When i noticed the solder issue, i soldered the resistor correctly but seems that IC was already damaged.That missing resistor as i spoke with AD support may drive the IC on unexpected mode/function.

    Also i simulated the circuit on LTSpice and when Rt resistor is missing, i can see a currect spike on IC's input 2.7A+.

    Thanks.

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  • Solved.

    Mode/Sync was not the problem. The problem was that Rt resistor on both PCBs was not placed correctly when i first power-up the board in a way that Rt pin was floating.

    When i noticed the solder issue, i soldered the resistor correctly but seems that IC was already damaged.That missing resistor as i spoke with AD support may drive the IC on unexpected mode/function.

    Also i simulated the circuit on LTSpice and when Rt resistor is missing, i can see a currect spike on IC's input 2.7A+.

    Thanks.

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