Техническая Спецификация для STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD

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DocID14661 Rev 5

The junction temperature of device will be:

Where T

 is the ambient temperature and Rth

 is the thermal resistance junction to 

ambient. Considering that the device is mounted on board with a good ground plane, that it 
has a thermal resistance junction to ambient (Rth

J-A

) of about 120°C/W, and an ambient 

temperature of about 50 °C:

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

ON

 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 

). 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 (V

O 

= 0 V) and depending on 

the application conditions (V

cc

 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



OFF phase

where V

 is the voltage drop across the diode, DCR

 is the series resistance of the inductor. 

In short-circuit conditions V

OUT

 is negligible so during T

OFF

 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 V

FW

 of the free wheeling diode) while during T

ON

 the voltage applied 

the inductor is instead maximized as approximately equal to V

IN

. 

So the 

 in overcurrent conditions can be simplified to:

considering T

 that has been already reduced to its minimum.

T

J

T

A

Rth

J A

–

P

TOT



+

=

T

J

50 0.55 120 116

C





+

=

I

L TON



V

IN

V

out

–

DCR

L

R

DSON

+



 I



–

L

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

ON





=

I

L TOFF



V

D

V

out

DCR

L

I



+

+





–

L

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

OFF





=

I

L TON



V

IN

DCR

L

R

DSON

+



 I



–

L

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

ON MIN





V

IN

L

--------- 250ns







=