R5972D datasheet(23/35 Pages) STMICROELECTRONICS

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R5972D

Application information

The overall losses are:

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 65 °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 5.3: Current protection on page 9). 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 freewheeling diode) while during TON the voltage applied the

inductor is instead maximized as approximately equal to VIN.

PTOT

RDSON IOUT





DVIN IOUT



TSW



FSW VIN IQ=



0.4 1.5



0.3 12 1.5



–



250



12 2.5



–



0.615W



TA RthJA

–

PTOT



70 0.615 65 110

C





IL TON



VIN Vout

–

DCRL RDSON

 I



–

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



IL TOFF



VD Vout DCRL I





–

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



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