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AN2728 датащи(PDF) 11 Page - STMicroelectronics

номер детали AN2728
подробное описание детали  ST1S12 small synchronous buck converter
PDF  20 Pages
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производитель  STMICROELECTRONICS [STMicroelectronics]
домашняя страница  http://www.st.com
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AN2728 датащи(HTML) 11 Page - STMicroelectronics

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AN2728
Thermal considerations
11/20
3
Thermal considerations
The dissipated power of the device is determined by three different factors:
●
Switch losses due to the non-negligible RDS(on). These are equal to:
Equation 8
and
Equation 9
where D is the duty cycle of the application.
Note:
The duty cycle is theoretically given by the ratio between VOUT and VIN, but in practice it is
significantly higher than this value in order to compensate for the losses of the overall
application. Due to this reason, the switching losses related to the RDS(on) increase
compared to an ideal case.
●
On and off switching losses. These are given by the following relation:
Equation 10
where TON and TOFF are the overlap times of the voltage across the power switch and the
current flowing into it during the turn-on and turnoff phases. TSW is the equivalent switching
time.
●
Quiescent current losses:
Equation 11
where IQ is the quiescent current.
The overall losses are:
Equation 12
When the device operates at VOUT close to VIN (high duty cycle) the power dissipated can
be approximated by:
Equation 13
D
I
R
P
2
OUT
DS(on)_P
ON_P
×
×
=
D
I
R
P
2
OUT
DS(on)_P
ON_P
×
×
=
D)
1
(
I
R
P
2
OUT
DS(on)_N
ON_N
-
×
×
=
D)
1
(
I
R
P
2
OUT
DS(on)_N
ON_N
-
×
×
=
SW
SW
OUT
IN
SW
OFF
ON
OUT
IN
SW
F
T
I
V
F
2
)
T
(T
I
V
P
×
×
×
=
×
+
×
×
=
SW
SW
OUT
IN
SW
OFF
ON
OUT
IN
SW
F
T
I
V
F
2
)
T
(T
I
V
P
×
×
×
=
×
+
×
×
=
PQ = VIN x IQ
Q
IN
SW
SW
OUT
IN
2
OUT
DS(on)_N
2
OUT
DS(on)_P
TOT
I
V
F
T
I
V
D)
1
(
I
R
D
I
R
P
×
+
×
×
×
+
-
×
×
+
×
×
=
Q
IN
SW
SW
OUT
IN
2
OUT
DS(on)_N
2
OUT
DS(on)_P
TOT
I
V
F
T
I
V
D)
1
(
I
R
D
I
R
P
×
+
×
×
×
+
-
×
×
+
×
×
=
Q
IN
2
OUT
DS(ON)_P
TOT
I
V
I
R
P
x
+
x
=
Q
IN
2
OUT
DS(ON)_P
TOT
I
V
I
R
P
x
+
x
=



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