Power dissipation must be considered when implementing integrated circuits in low-profile and fine-pitch surface-mount packages. Many system related issues may affect power dissipation: thermal coupling, airflow, adding heat sinks and convection surfaces, and the presence of other heat-generating components. In general, there are three basic methods that can be used to improve thermal performance:

• Improving the heat sinking capability of the PCB
• Reducing thermal resistance to the environment of the chip by adding or increasing heat sink capability on top of the package
• Adding or increasing airflow in the system

Power delivered to the LEDs can be greater than 30 W and the power dissipated by the DLPA3000 can be considerable. For proper DLPA3000 operation, the details below outline thermal considerations for a DLPA3000 application.

The recommended junction temperature for the DLPA3000 is below 120°C during operation. The equation that relates junction temperature, T_junction, is given by:

where T_ambient is the ambient temperature, P_diss is the total power dissipation, and R_θJA is the thermal resistance from junction to ambient.

The total power dissipation may vary depending on the application of the DLPA3000. The main contributors in the DLPA3000 are typically:

• Buck converters
• RGB strobe decoder switches
• LDOs

For the buck converter, the dissipated power is given by:

where η_buck is the efficiency of the buck converter, P_in is the power delivered at the input of the buck converter, and P_out is the power delivered to the load of the buck converter. For the buck converter PWR1,2,6, the efficiency can be determined using the curves in ^{Figure 7-22}.

Similarly, for the buck converter in the illumination block, the dissipated power P_{diss_illum_buck} can be calculated using the expression for P_{diss_buck}. For the illumination block, an extra term needs to be added to the dissipation; that is, the dissipation of the LED switch. So, the dissipation for the illumination block, P_{diss_illum}, can be described by:

where P_out-LEDs represents the total power supplied to the LEDs, I_{LED_avg} is the average LED current, and R_{sw_PQR} is the on-resistance of the RGB strobe controller switches. It should be noted here that the sense resistor, R_LIM, also carries the average LED current, but it is not added to the equation of the dissipation for the illumination block. The R_LIM is external to the DLPA3000 and it does not contribute the heat directly to the DLPA3000. For the total system dissipation, R_LIM should be included.

For the LDO. the power dissipation is given by:

where V_in is the input supply voltage, V_out is the output voltage of the LDO, and I_load is the load current of the LDO.

The voltage drops over the LDO (V_in-V_out) can be relatively large; a small load current can result in significant power dissipation. For this situation, a general purpose buck converter can be a more efficient solution.

The LDO DMD provides power to the boost converter, and the boost converter provides high voltages for the DMD; that is, V_BIAS, V_OFS, V_RST. The current load on these lines can increase up to I_load,max=10 mA. Assuming the efficiency of the boost converter, η_boost, is 80%, the maximum boost converter power dissipation, P_{diss_DMD_boost,max}, can be calculated as:

Compared to the power dissipation of the illumination buck converter, the power dissipation of the boost converter is negligible. However, the power dissipation of the LDO DMD, P_{diss_LDO_DMD} should be given consideration in the case of a high supply voltage. The worst-case load current for the LDO is given by:

where the output voltage of the LDO is V_{DRST_5P5V}= 5.5 V.

The worst-case power dissipation of the LDO DMD is approximately 1.5 W when the input supply voltage is 19.5 V. For your specific application, it is recommended to check the LDO current level. Therefore, the total power dissipation of the DLPA3000 can be described as:

The following examples calculate of the maximum ambient temperature and the junction temperature based on known information.

If it is assumed that the total dissipation Pdiss_DLPA3000 = 7.5 W, T_junction,max= 120 °C, R_θJA= 7 °C/W(refer to Thermal Information), then the maximum ambient temperature can be calculated using Equation 11:

If the total power dissipation and the ambient temperature are known as:

T_ambient= 50 °C, R_θJA= 7 °C/W, P_{diss_DLPA3000}= 7.5 W.

the junction temperature can be calculated:

If the combination of ambient temperature and the total power dissipation of the DLPA3000 does not produce an acceptable junction temperature, that is, <120°C, there are two approaches:

1. Using larger heat sink or more airflow to reduce R_θJA
2. Reduce power dissipation in DLPA3000:
   • Use an external buck converter instead of an internal general purpose buck converter.
   • Reduce load current for the buck converter.

The following example shows how to calculate the maximum I_LED at 6A when the junction temperature exceeds the maximum allowed temperature.. If it is assumed that P_{buck_converters}= 1 W, P_LDOs = 0.5 W, T_ambient= 75°C, R_θJA= 7°C/W, V_LED= 3.5 V and T_junction,max= 120°C, then the total maximum allowed dissipation for the DLPA3000 can be calculated:

The total dissipation for the buck converters and LDOs is 1.5W, so the maximum dissipation for the illumination block is 4.9W.

The efficiency of the converter can be determined from Figure 8-13. For V_LED= 3.5 V and I_LED is between 4 A and 6 A, the average efficiency is about 80%. The typically value of the on resistance of switch P,Q,R is 30 mOhm. Assuming V_LED is independent of I_LED, the I_LED can be calculated using Equation 13: