A5974AD datasheet(31/49 Pages) STMICROELECTRONICS

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A5974AD

Application information

Doc ID 018762 Rev 1

31/49

Figure 17.

Measurement of the thermal impedance of the demonstration board

As can be seen, for example, for load pulses with a duration of 1 second, the actual thermal

impedance is lower than 20

°C/W. This means that, for short pulses, the device can deliver a

higher output current value.

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). 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 29

●

Off phase

Equation 30

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

L TON

out

–

DCR

DS(on)

() I

⋅

–

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

()

L TOFF

out

DCR

⋅

()

–

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

OFF

()

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