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Axelite Confidential Materials, do not copy or distribute without written consent.

Rev.1.2 Sep.23, 2019

AX1255

Thermal Considerations

The AX1255 series can deliver a current of up to 2A over the full operating junction

temperature range. However, the maximum output current must be dated at higher

ambient temperature to ensure the junction temperature does not exceed 125°C. With all

possible conditions, the junction temperature must be within the range specified under

operating conditions. Power dissipation can be calculated based on the output current and

the voltage drop across regulator.

PD = (VIN - VOUT) × IOUT+ VIN × IQ

The final operating junction temperature for any set of conditions can be estimated

by the following thermal equation:

PD (MAX) = (TJ (MAX) - TA) / θJA

Where TJ (MAX) is the maximum junction temperature of the die (125°C) and TA is the

maximum ambient temperature. The junction to ambient thermal resistance (θJA) for

SOP-8L-EP (Exposed pad) package at recommended minimum footprint is 75°C/W on 1.5

in2 and Multi-layer PCB layout. The maximum power dissipation at TA = 25°C can be

calculated by following formula:

PD (MAX) = (125°C - 25°C) / 75°C/W = 1.33W

The thermal resistance θJA of SOP-8L-EP (Exposed Pad) is determined by the

package design and the PCB design. However, the package design has been decided. If

possible, it's useful to increase thermal performance by the PCB design. The thermal

resistance can be decreased by adding copper under the expose pad of SOP-8L-EP

package. We have to consider the copper couldn't stretch infinitely and avoid the tin

overflow.