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參數(shù)資料
| 型號(hào)： | SC2441EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter |
| 中文描述： | 非常低的輸入電壓2相同步降壓與升壓轉(zhuǎn)換器控制器 |
| 文件頁數(shù)： | 18/35頁 |
| 文件大?。?/td> | 733K |
| 代理商： | SC2441EVB |
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18
2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
Applications Information (Cont.)
SC2441
The losses in power MOSFET’s consist of
a) conduction loss due to the channel resistance
R
,
b) switching loss due to the switch rise time
t
r
and fall time
t
and
c) the gate loss due to the gate resistance
R
G
.
Top Switch:
The
RMS
value of the top switch current is
.
1
D
I
I
12
o
rms
,
Q
2
δ
+
=
Its conduction loss is then
P
tc
= I
Q1,rms
2
R
ds(on)
.
R
varies with temperature and gate-source voltage.
Curves showing R
variations can be found in
manufacturers’ data sheet. From the Si7882DP datasheet,
R
is less than 4.5m
when V
is greater than 5V.
However R
increases by nearly 40% as the junction
temperature increases from 25°C to 125°C.
The switching losses can be estimated using the simple
formula
+
=
where t
is the rise time and t
is the fall time of the switching
process. Different manufactures have different definitions
and test conditions for tr and tf. To clarify these, we sketch
the typical MOSFET switching characteristics under clamped
inductive mode in Figure 11.
.
f
V
I
2
1
)(
t
t
P
s
in
o
f
r
2
1
ts
δ
+
Figure 11. MOSFET switching characteristics
In Figure 11,
Q
is the gate charge needed to bring the gate-to-source
voltage
V
to the threshold
V
,
Q
is the additional gate charge required for the switch
current to reach its full-scale value
I
.
and
Q
is the charge needed to charge gate-to-drain (Miller)
capacitance when
V
is falling.
Switching losses occur during the time interval [
t
1
, t
3
].
Defining
t
r
= t
3
-t
1
. t
r
can be approximated as
Q
(
t
cc
where
R
is the total resistance from the driver supply rail
to the gate of the MOSFET. It includes the gate driver internal
impedance
R
, external resistance
R
ge
and the gate
resistance
R
g
within the MOSFET i.e.
.
V
V
R
)
Q
gsp
gt
gd
2
gs
r
+
=
R
gt
= R
gi
+R
ge
+R
g
.
V
is the Miller plateau voltage shown in Figure 11.
Similarly
an approximate expression for t
f
is
.
V
R
)
Q
Q
(
t
gsp
gt
gd
2
gs
f
+
=
Only a portion of the total losses P
= Q
V
f
is dissipated in
the MOSFET package. Here
Q
is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is
.
f
V
Q
R
R
P
s
cc
g
gt
g
tg
=
The total power loss of the top switch is then
P
t
= P
tc
+P
ts
+P
tg
.
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction loss is inversely proportional to the
input voltage. Switching loss however increases with the
input voltage. The total power loss of MOSFET should be
calculated and compared for high-line and low-line
cases. The worst case is then used for thermal design.
Bottom Switch:
The
RMS
current in bottom switch can be shown to be
.
1
)(
D
1
I
I
12
o
rms
,
Q
2
δ
+
=
charge
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| SC2441ITSTRT | Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter |
| SC2442 | High Performance Wide Input Range Dual Synchronous Buck Controller |
| SC2442H | High Performance Wide Input Range Dual Synchronous Buck Controller |
| SC2442HITSTR | High Performance Wide Input Range Dual Synchronous Buck Controller |
| SC2442HITSTRT | High Performance Wide Input Range Dual Synchronous Buck Controller |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| SC2441ITSTRT | 制造商:Semtech Corporation 功能描述:Controller IC |
| SC2442 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Wide Input Range Dual Synchronous Buck Controller |
| SC2442H | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:High Performance Wide Input Range Dual Synchronous Buck Controller |
| SC2442HITSTR | 制造商:Semtech Corporation 功能描述: |
| SC2442HITSTRT | 制造商:Semtech Corporation 功能描述:Controller IC |
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