10.6 Thermal Considerations

Implementation of integrated circuits in low-profile and fine-pitch surface-mount packages typically requires special attention to power dissipation. Many system-dependent issues such as thermal coupling, airflow, added heat sinks and convection surfaces, and the presence of other heat-generating components affect the power dissipation limits of a given component. In general three basic approaches for enhancing thermal performance can be used; these are listed below:

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

The DLPA3000 is a device with efficient power converters. Nevertheless, since the power delivered to the LEDs can be quite large (more than 30 W in some cases), the power dissipated in the DLPA3000 device can still be considerable. In order to have proper operation of the DLPA3000, guidance is given below on the thermal dimensioning of the DLPA3000 application.

The target of the dimensioning is to keep the junction temperature below the maximum recommendation of 120°C during operation. In order to determine the junction temperature of the DLPA3000, a summation of all power dissipation terms, P_diss, needs to be made. The junction temperature, T_junction, is then given by:

in which T_ambient is the ambient temperature and R_θJA is the thermal resistance from junction to ambient.

Depending on the application of the DLPA3000, the total power dissipation can vary. The main contributors in the DLPA3000 will typically be the:

• Buck converters
• RGB strobe decoder switches
• LDOs

The calculation of the dissipation for these blocks is shown below.

For a 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 buck converter PWR1,2,5,6,7, the efficiency can be determined using the curves in Figure 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, however, an extra term needs to be added to the dissipation, i.e. the dissipation of the LED switch. So, the dissipation for the illumination block, P_{diss_illum}, can be described by:

where P_OUT represents the total power supplied to the LEDs, I_{LED_avg} is the average LED current, and R_{sw_P,Q,R} 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 is not added to this dissipation term. Since the R_LIM is external to the DLPA3000, it does not contribute to the heating of the DLPA3000, at least not directly, although potentially it does through increasing the ambient temperature. For total system dissipation, R_LIM should of course be included.

These discussed buck converters potentially handle the highest power levels, which is why they need to be power efficient. In contrast, linear regulators, such as LDOs, handle less power. However, since the efficiency of an LDO can be relative low, the related power dissipation can be significant. To calculate the power dissipation of an LDO, P_{diss_LDO}, the following equation can be used:

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. Since the voltage drop over the LDO (V_in–V_out) can be relative large, a relatively small load current can yield significant DLPA3000 dissipation. If this situation occurs, one might consider using one of the general purpose bucks to have a more power-efficient (less dissipation) solution.

One LDO, the LDO DMD, needs special attention, since it is used as the power supply of a boost power converter. The boost converter is used to supply the high voltages for the DMD (such as V_BIAS, V_OFS, and V_RST). The loading on these lines can be up to I_load,max=10 mA simultaneously. Thus, the maximum related power level is moderate. Assuming an efficiency on the order of 80% for the boost converter, η_boost, this implies a maximum boost converter dissipation, P_{diss_DMD_boost,max} of:

In perspective of the dissipation of the illumination buck converter, this is likely negligible. The term that might count to the total power dissipation is P_{diss_LDO_DMD}. The input current of the DMD boost converter is supplied by this LDO. In case of a high-supply voltage, a non-negligible dissipation term is obtained. 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.

Thus, the worst-case dissipation of the LDO, can be on the order of 1.5 W for an input supply voltage of 19.5 V. However, this is a worst-case scenario. In most cases, the load current of the LDO DMD is significantly less. It is advised to check this LDO current level for the specific application.

Finally, the DLPA3000 will draw a quiescent current. This quiescent current is relatively independent of the power supply voltage. For the buck converters, the quiescent current is comprised in the efficiency numbers. For the LDOs, a quiescent current on the order of 0.5 mA can be used. For the rest of the DLPA3000 circuitry, not included in the buck converters or LDOs, a quiescent current on the order of 3 mA applies. So, overall, when the power dissipation of the buck converters, illumination block (illumination buck + P,Q,R switches) and the LDOs are summed, a good estimate of the DLPA3000 dissipation, P_{diss_DLPA3000}, is obtained. Given as an equation:

Once this total power dissipation is know, the thermal design can be done. A few examples are given. Assume the total P_{diss_DLPA3000}= 7.5 W and the heatsink and airflow is as given in Thermal Information. What is the maximum ambient temperature that can be allowed?

Know parameters: T_junction,max= 120 °C, R_θJA= 7 °C/W, Pdiss_DLPA3000 = 7.5 W.

Using Equation 11 the maximum ambient temperature can be calculated as:

In the same way, the junction temperature of the DLPA3000 can be calculated once the dissipated power and the ambient temperature is known. For instance:

T_ambient= 50 °C, R_θJA= 7 °C/W, P_{diss_DLPA3000}= 8.5 W.

For the heat sink configuration and airflow as indicated in Thermal Information, the junction temperature can be calculated to be:

In case the combination of ambient temperature and DLPA3000 power dissipation does not yield an acceptable junction temperature (such as <120°C), two approaches can be used:

1. Using larger heatsink or more airflow to reduce R_θJA
2. Reduce power dissipation in DLPA3000 by for instance not using an internal general purpose buck converter, but an external one. Or lowering maximum LED current.

As a final example, it is shown below how to determine a de-rating of the maximum I_LED in case the junction temperature at I_LED= 6 A exceeds the maximum allowed temperature. Assume the following parameters:

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.

In order to find the maximum acceptable LED current, a few steps are required. First, the total maximum allowed dissipation for the DLPA3000 needs to be determined

Since the buck converters and LDOs do dissipate in total 2.5 W, for the illumination block the dissipation budget is 4.9 W. The dissipation of the illumination block comprises two terms: the illumination buck converter dissipation and the P,Q,R-switches. Note that the dissipation of R_LIM is not included here since this calculation is about the junction temperature. For overall system dissipation, of course R_LIM should be included.

Information needed to calculate I_LED are the illumination buck converter efficiency and the on-resistance of the P,Q,R-switches.

The efficiency of the converter can be derived from Figure 14. For V_LED= 3.5 V and I_LED is between 4 A and 6 A, the efficiency is on average 80%. The on resistance of switch P,Q,R is given in the tables and is typically 30 mOhm. Assuming V_LED to be independent of I_LED, the dissipation of the illumination block is given by:

Rewriting this expression for I_LED yields:

Thus, to meet the maximum junction temperature requirement, the LED current should stay below 4.8 A. Once the maximum current selected, it is advised to redo the thermal calculations based on the LED current. It might be that the assumed efficiency is too high for the first calculated LED current. That would require the calculations to be redone, but now with a better estimate for the efficiency. The same goes for the LED voltage. At lower current, a lower LED voltage is to be expected. That implies a lower power delivered to the LED and less power dissipated in the buck converter.

Once the system is dimensioned and built, the actual junction temperature can be derived from measuring the internal VOTS using the AFE. This is described in Measurement System.