AN1330 datasheet(21/31 Pages) STMICROELECTRONICS

21 / 31 page

AN1330

Application information

21/31

Considering the device in an SO-8 package mounted on the board with a good groundplane,

that it has a thermal resistance-junction to-ambient (RthJ-A) of about 115 °C/W and an

ambient temperature of about 70 °C.

Equation 26

5.4

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 ns) and the switching

frequency to approximately one third of its nominal value (see Section 2.4: Current

protection). In these conditions, the duty cycle is strongly reduced and, in most of

applications, this is enough to limit the current to ILIM. In any event, in case of heavy short-

circuit at the output (VOUT=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 27

●

OFF phase

Equation 28

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

inductor.

In short-circuit conditions, VOUT is negligible. So, during the TOFF, the voltage applied to the

inductor is very small and it may be that the current ripple in this phase does not

compensate for the current ripple during the TON.

The maximum current peak can be easily measured through the inductor with VOUT=0 V

(short-circuit) and VCC=VINmax. In cases where the application must sustain the short-

circuit condition for an extended period, the external components (mainly the inductor and

diode) must be selected based on this value.

0.44

115

121

°C

≅

•

∆

out

DCR

•

–

()

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

•

∆

out

DCR

•

()

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

OFF

•

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