L5988D datasheet(39/52 Pages) STMICROELECTRONICS

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L5988D

Application information

If we suppose, as an example, TA = 40 C, 140 C is the maximum operating temperature

before triggering the thermal shutdown and RTH = 40 C/W so the maximum power loss

achievable with the thermal performance of the system will be:

Equation 47

The switching, conduction and quiescent losses in case of VIN = 12 V, VOUT = 1.2 V,

fSW = 400 kHz are plotted in Figure 21. The calculations are performed considering the

typical RDS(on) of the power element for a junction temperature of 125 C

(RDS_ON HS = 120 m, RDS_ON LS = 83 m; see Section 3: Maximum ratings on page 8 for

details).

Figure 21. Estimation of the internal power losses (VIN = 12 V, VOUT = 1.2 V,

fSW = 400 kHz)

The red trace represents the maximum power which can be taken away as calculated

above, whilst the purple trace is the total internal losses.

As a consequence, given these operating conditions, the system can manage a continuous

output current up to 4.2 A. The device could deliver a continuous output current up to 5 A to

the load (see Section 6.5), however the maximum power loss of 2.5 W is reached with an

output current of 4.2 A, so the maximum output power is derated.

The calculation of the internal power losses must be done for each specific operating

condition given by the final application.

For example, the result showed in Figure 21. is not valid in case the VIN is equal to 5 V

instead of 12 V: the lower contribution of the switching losses, which are proportional to the

input voltage, increases the maximum output current from 4.2 A to 4.5 A (see Figure 22.).

PMAX DC

T

RTH

-----------

TJ MAX TAMB

–

RTH

--------------------------------------

100

----------

2.5W

L5988D 데이터시트(PDF) 39 Page - STMicroelectronics