AN2603 datasheet(9/23 Pages) STMICROELECTRONICS

9 / 23 page

AN2603

ST1S09 description

Doc ID 13778 Rev 3

9/23

1.4

Short circuit protection

In over-current protection mode, when the peak current reaches the current limit, the device

reduces the TON down to its minimum value. In these conditions, the duty cycle is strongly

reduced and, in most applications, this is enough to limit the current to Ilim.

In cases of heavy short circuit at the output (VOUT = 0 V) and depending on the application

conditions (VIN value and parasitic effect of external components) the current peak could

reach values higher than Ilim. This can be understood by considering the inductor current

ripple during the ON and OFF phases:

●

ON phase

Equation 2

●

OFF phase

Equation 3

Where VD is the voltage drop across the internal NDMOS 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 is possible that the current ripple in this phase will

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 VIN = VINmax. If the

application is required to sustain the short-circuit condition for an extended period, the

external components (mainly the inductor) must be selected based on this value.

ΔI

OUT

–

DCR

⋅

–

---------------------------------------------------------- T

⋅

ΔI

OUT

DCR

⋅

--------------------------------------------------------- T

OFF

⋅

AN2603 Folha de dados(PDF) 9 Page - STMicroelectronics