A6318
REGULATOR CMOS LOW DROPOUT REGULATOR (LDO)
300mA ULTRA-LOW NOISE, ULTRA-FAST
AiT Semiconductor Inc.
www.ait-ic.com
Thermal Considerations
Thermal protection limits power dissipation in A6318. When the operation junction temperature exceeds
165°C, the OTP circuit starts the thermal shutdown function turn the pass element off. The pass element turns
on again after the junction temperature cools by 30°C. For continue operation, do not exceed absolute
maximum operation junction temperature 125°C. The power dissipation definition in device is:
PD = (VIN−VOUT) ×IOUT + VIN×IQ
The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference between junction to ambient. The maximum power
dissipation can be calculated by following formula:
PD(MAX) = ( TJ(MAX) − TA ) /θJA
Where TJ(MAX) is the maximum operation junction temperature 125°C, TA is the ambient temperature and the
θJA is the junction to ambient thermal resistance. For recommended operating conditions specification of
A6318, 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 is layout dependent) for SOT-23-5
package is 250°C/W, on standard JEDEC 51-3 thermal test board. The maximum power dissipation at TA=
25°C can be calculated by following formula:
PD(MAX) = (125°C−25°C)/250 = 400mW
The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. It is also useful to calculate the junction of temperature of the A6318 under a set of specific
conditions. In this example let the Input voltage VIN=3.3V, the output current Io=300mA and the case
temperature TA=40°C measured by a thermal couple during operation. The power dissipation for the
VOUT=2.8V version of the A6318 can be calculated as:
PD = (3.3V−2.8V) ×300mA+3.6V×100uA =150mW
And the junction temperature, TJ, can be calculated as follows:
TJ=TA+PD×θJA=40°C+0.15W×250°C/W=40°C+37.5°C=77.5°C<TJ(MAX) =125°C
REV2.1
- JAN 2010 RELEASED, FEB 2014 UPDATED -
- 10 -
AITSEMI [ AiT Semiconductor ]
