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LP2985-N: Stepping down 9V battery to 3.3V

Ashish Mistry
Prodigy 50 points
Part Number: LP2985-N

We are trying to use the 3.3 V LDO regulator to step down a 9 V battery. It is working correctly and it says on the data sheet that it can handle voltages up to 16 V, but I am concerned about it being able to handle the power dissipation (@9V and below)Will it be able to handle it? Also is there any data about the power dissipation that you would be able to share? 

  • 0
    TI__Guru 55940 points

    Hi Ashish,

    The amount of power that can be dissipated depends on application specific conditions including heatsinking, ambient temperature, air flow, etc.  Often before a board is laid out the best way to estimate if a device is adequately able to dissipated the intended power, we use the junction to ambient thermal resistance and the rated junction temperature of the device.  LP2985-N is rated for the conditions listed in the Recommended Operating Conditions.

    The power you intend to be dissipated in your application can be calculated as Pd = (Vin - Vout) * Iout.  You already have stated that you will have a headroom of 5.7 V (Vin- Vout).  If you plug in your load current you will have your intended power dissipation.

    We can look up the junction to ambient thermal resistance in the Thermal Information table:

    Now we have all the information needed to estimate what the junction temperature would be in your application using the following equation:

    Tj = Pd * RθJA + Ta

    From the Recommended Operating Conditions we know that the maximum rated junction temperature is 125 C.  If your estimated junction temperature is below 125 C, then LP2985-N is adequate for your intended power dissipation.  If not, you will want to either consider another device or adjust your application by reducing the headroom (Vin - Vout).  You could do this by using a DCDC before the LDO (most power efficient), using another LDO (highest system PSRR), or even a power resistor in series before the current LDO.  All of these will reduce the power dissipation in the LDO.

    Very Respectfully,

    Ryan