Datasheet

Application information ST1CC40
26/37 DocID18279 Rev 5
The overall losses are:
Equation 32
Equation 33
The junction temperature of the device is:
Equation 34
where T
A
is the ambient temperature and Rth
J-A
is the thermal resistance junction-to-
ambient. The junction-to-ambient (Rth
J-A
) thermal resistance of the device assembled in
HSO8 package and mounted on the board is about 40 °C/W.
Assuming the ambient temperature is around 40 °C, the estimated junction temperature is:
Equation 35
7.4 Short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit threshold,
the device disables the power element and it is able to reduce the conduction time down to
the minimum value (approximately 100 nsec typ.) to keep the inductor current limited. This
is the pulse-by-pulse current limitation to implement the constant current protection feature.
In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is
enough to limit the switch current to the current threshold.
The inductor current ripple during ON and OFF phases can be written as:
 ON phase
Equation 36
 OFF phase
Equation 37
where DCR
L
is the series resistance of the inductor.
P
TOT
R
DS(on)_HS
I
OUT

2
DR
DS(on)_LS
I
OUT

2
1D– V
IN
I
OUT
f
SW
T
SW
 V
IN
I
Q
+++=
P
TOT
0,14 0,7
2
0,6 0,1 0,7
2
0,4 12+ 0,7 12 10
9–
850 10
3
12 1,5 10
3–
+  + 205mW=
T
J
T
A
Rth
JA–
P
TOT
+=
T
J
60 0,205 40 68C+=
I
L TON

V
IN
V
OUT
– DCR
L
R
DS(on) HS
+I–
L
------------------------------------------------------------------------------------------------ - T
ON
=
I
L TON

V
OUT
DCR
L
R
DS(on) LS
+I+–
L
----------------------------------------------------------------------------------------- T
OFF
=