Datasheet

LED5000 Application notes - buck conversion
Doc ID 023951 Rev 1 29/51
For the calculation we can estimate R
DSON_HS
= 300 mΩ as a consequence of Tj increase
during the operation.
T
SW_EQ
is approximately 12 ns.
I
Q
has a typical value of 2.4 mA at V
IN
= 48 V.
The overall internal losses are:
Equation 41
Equation 42
The junction temperature of device will be:
Equation 43
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 evaluation is about 40 °C/W.
Assuming the ambient temperature around 40 °C, the estimated junction temperature is:
5.12 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 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 44
● OFF phase
P
TOT
R
DSON_HS
I
OUT
()⋅
2
DV
IN
I
OUT
f
SW
T
SW
⋅⋅⋅ V
IN
I
Q
⋅++⋅=
P
TOT
0.3 1.5
2
0.7 42 1.5 12 10
9–
850 10
3
42 2.4 10
3–
⋅⋅+⋅⋅⋅ ⋅ ⋅+⋅⋅ 1.2W≅=
T
J
T
A
Rth
JA–
P
TOT
⋅+=
T
J
60 1.2W 40
° C
W
-------
⋅ 110° C≅⋅+=
I
L TON
Δ
V
IN
V
OUT
– DCR
L
R
DSON HS
+()I⋅–
L
--------------------------------------------------------------------------------------------------
T
ON
()=