Datasheet

Application information LED2000
28/40 DocID023432 Rev 4
Example 3(see Example 1):
V
IN
= 12 V, V
FW_LED
= 3.5 V, n
LED
= 2, I
LED
= 700 mA
The typical output voltage is:
Equation 36
R
DSON_HS
has a typical value of 95 m and R
DSON_LS
is 69 m at 25 °C.
For the calculation we can estimate R
DSON_HS
= 140 m and R
DSON_LS
= 100 mas
a consequence of T
J
increase during the operation.
T
SW_EQ
is approximately 12 ns.
I
Q
has a typical value of 1.5 mA at V
IN
= 12 V.
The overall losses are:
Equation 37
Equation 38
The junction temperature of the device is:
Equation 39
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 the
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:
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 typical) to keep the inductor current limited.
This is the pulse-by-pulse current limitation to implement the constant current protection
feature.
V
OUT
n
LED
V
FW_LED
V
FB
+ 7,1V==
P
TOT
R
DSON_HS
I
OUT

2
DR
DSON_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+=