Datasheet

Application information B5973D
28/44 DocID14117 Rev 4
Example:
 V
IN
= 12 V
 V
OUT
= 3.3 V
 I
OUT
= 2 A
R
DS(on)
has a typical value of 0.25 at 25 °C and increases up to a maximum value of 0.5
 at 150 °C. We can consider a value of 0.4 .
T
SW
is approximately 70 ns.
I
Q
has a typical value of 2.5 mA at V
IN
= 12 V.
The overall losses are:
Equation 25
The junction temperature of device will be:
Equation 26
Where T
A
is the ambient temperature and Rth
J-A
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 42 °C/W, and an ambient
temperature of about 70 °C:
Equation 27
8.4 Short-circuit protection
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 Section 5.4: Current protection on page 12). 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
Equation 28
 OFF phase
P
TOT
R
DSON
I
OUT

2
DV
IN
I
OUT
 T
SW
 F
SW
V
IN
I
Q
=++=
0.4 2
2
 0.3 12 2 70 10
9–
 250 10
3–
12 2.5 10
3–
++ 0.93W=
T
J
T
A
Rth
JA–
P
TOT
+=
T
J
70 0.93 42 110C+=
I
L TON

V
IN
V
out
– DCR
L
R
DSON
+I–
L
------------------------------------------------------------------------------------ T
ON
=