PAM2800 Datasheet, PDF (4/7 Pages) Power Analog Micoelectronics – High Power White LED Driver

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PAM2800
Application Information
In the typical application (see Figure 1), the LED current will come to constant current level little by little after the device is powered. A 62ΚΩ
resistor is recommended for R1, the value for R2 should be adjusted around 33ΚΩ due to LED forward voltage from lot-to-lot or brand-to-brand.
Power Dissipation and Thermal Consideration
Thermal protection limits power dissipation in the PAM2800. When the operation junction temperature exceeds +150°C, the OTP (Over
Temperature Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after the junction
temperature drops below +120°C.
For continuous operation, the junction temperature should be maintained below +125°C. The power dissipation is defined as:
( ) _PD = _VIN − _VOUT * _IO + _VIN * _IGND
The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surrounding airflow and temperature
difference between junction and ambient. The maximum power dissipation can be calculated by the following formula:
( ) _PD(MAX) = _TJ(MAX) − _TA / _θJA
Where T_J(MAX) is the maximum operation junction temperature +125°C. T_A is the ambient temperature, and is the thermal resistance from the
junction to the ambient.
For example, as θ_JA is +250°C/W for SOT-23, based on the standard JEDEC 51-3 for a single layer thermal test board, the maximum power
dissipation for SOT-23 package at T_A = +25°C can be calculated by following formula:
PD(MAX) ^{= (125°C − 25°C)/ 250 = 0.4W}
To calculate the junction temperature of the PAM2800 SOT-23 package, if we use input voltage V_IN = 4V at an output current I_O = 300mA and
the case temperature T_A = 40°C measured by the thermal couple while operating, the power dissipation is defined as:
PD ^{= (4V − 2.8V) * 300mA + 4V * 70μA ≈ 360mW}
PAM2800
Document number: DSxxxxx Rev. 1 - 3
4 of 7
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October 2012
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