Datasheet

A5973D Application information
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R
DS(on)
has a typical value of 0.25 @ 25 °C and increases up to a maximum value of 0.5. @
150 °C. We can consider a value of 0.4 Ω.
T
SW
is approximately 70 ns.
I
Q
has a typical value of 2.5 mA @ 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). 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
Equation 29
where V
D
is the voltage drop across the diode, DCR
L
is the series resistance of the inductor.
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
Δ
V
IN
V
out
– DCR
L
R
DSON
+()I⋅–
L
------------------------------------------------------------------------------------
T
ON
()=
I
L TOFF
Δ
V
D
V
out
DCR
L
I•++()–
L
---------------------------------------------------------------
T
OFF
()=