R5973AD datasheet(26/38 Pages) STMICROELECTRONICS

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Application information

R5973AD

26/38

DocID027027 Rev 1

Equation 25

The junction temperature of device will be:

Equation 26

Where TA is the ambient temperature and RthJ-A is the thermal resistance junction to

ambient. Considering that the device is mounted on the board with a good ground plane,

that it has a thermal resistance junction to ambient (RthJ-A) of about 40 °C/W, and an

ambient temperature of about 70 °C:

Equation 27

8.4

Short-circuit protection

In overcurrent protection mode, when the peak current reaches the current limit, the device

reduces the TON down to its minimum value (approximately 250 nsec) and the switching

frequency to approximately one third of its nominal value even when synchronized to an

external signal (see Section 5.4: Current protection on page 11). In these conditions, the

duty cycle is strongly reduced and, in most applications, this is enough to limit the current to

ILIM. In any event, in case of heavy short-circuit at the output (VO = 0 V) and depending on

the application conditions (Vcc value and parasitic effect of external components) the current

peak could reach values higher than ILIM. This can be understood considering the inductor

current ripple during the ON and OFF phases:



ON phase

Equation 28



OFF phase

Equation 29

where VD is the voltage drop across the diode, DCRL is the series resistance of the inductor.

In short-circuit conditions VOUT is negligible so during TOFF the voltage across the inductor

is very small as equal to the voltage drop across parasitic components (typically the DCR of

the inductor and the VFW of the free wheeling diode) while during TON the voltage applied

the inductor is instead maximized as approximately equal to VIN.

So the Equation 28 and the Equation 29 in overcurrent conditions can be simplified to:

Equation 30

PTOT

RDSON IOUT





DVIN IOUT



TSW



FSW VIN IQ=



0.4 1.5



0.3 12 1.5



–



500



12 2.7



–



0.93W



TA RthJA

–

PTOT



70 0.93 42 110

C





IL TON



VIN Vout

–

DCRL RDSON

 I



–

------------------------------------------------------------------------------------ TON



IL TOFF



VD Vout DCRL I





–

--------------------------------------------------------------- TOFF



IL TON



VIN DCRL RDSON

 I



–

---------------------------------------------------------------- TON MIN



VIN

--------- 250ns





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