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LTC3118 - Burst Mode output current

florian.sommer
on Jul 22, 2019

Hi,

I am using the ltc3118 in a battery-powered device. Because of that, I want to use it in burst mode. I used the typical application note from the datasheet. IN1: 18V wall socket, IN2: Battery pack 12V, OUT: 5V. With almost no load the voltage is quite stable and I can see the burst characteristics on the scope. As soon as I want to get higher output currents the output voltage drops to 0-1V.

Is there any problem with the specifications or the circuit from the datasheet?

Thank you

Florian

  • 0
    •  Analog Employees 
    on Jul 23, 2019 1:36 PM

    Hi,

    Can you share us your schematics with your operating input voltages, output current and other test conditions if any. Ideally the part an have max output current of 2A in buck mode however there could be issues in the schematic or layout or instability in the voltage loop that might lead to Vout dropping out of regulation. Application circuit from datasheet usually are verified designed so should work normally. If you share your schematics and design details we would try to look into the issue. You can send me the schematics through private message if  you do not want to share it here.

    Thanks

  • 0 on Jul 24, 2019 6:35 AM in reply to Anisha

    Hi, we use the following schematics/pcp layout (C48 is not placed). We will use multiple sensors which will pull about 200mA each, therefor we wanted to pull 100mA at 5 Volts with a power resistance (47Ohm). Our Wallsocket VIN1 has 18V (at the moment it is a lab supply), our VIN2 is a batterypack with 3 x 3.7 to 4.1V and 10.4Ah each. For the first tests we used the wall socket only with 18V and no active crrent control. 
    Thanks

  • 0
    •  Analog Employees 
    on Jul 29, 2019 2:26 PM in reply to florian.sommer

     Hi,

    Which application circuit from datasheet you have used for your design? The components value doesn’t seem to match. For loop stability, component selection is very crucial.  Also please note that the output capacitance values mentioned in the application circuits of the datasheet are the measured value after considering the DC biasing effect. So pls check you component values and loop compensation.

    I would recommend you to refer the demo board  DC2045A for component selection and loop compensation as it is similar to your design specification.

    Also the layout seems to have issues and can also be a reason for the instability. To name a few the layout has common ground for power signals and control signals, the copper area for input/output voltage is very small etc. 

    I would recommend to refer the design files of the demo board available in the website for you layout https://www.analog.com/en/products/ltc3118.html#product-evaluationkit.

    You can also read through good PCB layout guidelines for switching power supplies here https://www.analog.com/media/en/technical-documentation/application-notes/an136f.pdf

     Thanks