L5988D datasheet(38/52 Pages) STMICROELECTRONICS

38 / 52 page

Application information

L5988D

38/52

DocID15324 Rev 4

Figure 20. Switching losses

Quiescent current losses, calculated as:

Equation 45

where IQ is the quiescent current.

The junction temperature TJ can be calculated as:

Equation 46

Where TA is the ambient temperature and PTOT is the sum of the power losses just seen.

RthJA is the equivalent static thermal resistance junction to ambient of the device; it can be

calculated as the parallel of many paths of heat conduction from the junction to the ambient.

For this device the path through the exposed pad is the one conducting the largest amount

of heat. The static RthJA measured on the application is about 40 °/W.

The thermal impedance of the system, considered as the device in HTSSOP16 package

soldered on the application board, takes on an important rule when the maximum output

power is limited by the static thermal performance and not by the electrical performance of

the device. Therefore the embedded power elements could manage an higher current but

the system is already taking away the maximum power generated by the internal losses.

In case the output power increases the thermal shutdown will be triggered because the

junction temperature triggers the designed thermal shutdown threshold.

The RTH is a static parameter of the package: it sets the maximum power loss which can be

generated from the system given the operation conditions.

VIN IQ



TA RthJA PTOT



L5988D Datasheet(PDF) 38 Page - STMicroelectronics