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參數(shù)資料
| 型號： | ML4895 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | Synchronous Buck Controller For DC/DC conversion(用于DC/DC變換的同步補償控制器（5.9V～15V到2.5V～4V變換）) |
| 中文描述： | 同步降壓控制器，用于直流/直流轉(zhuǎn)換（用于直流/直流變換的同步補償控制器（高于全?15V的到為2.5V?4V變換）） |
| 文件頁數(shù)： | 6/10頁 |
| 文件大?。?/td> | 248K |
| 代理商： | ML4895 |
ML4895
6
Figure 4. One Shot and Synchronous Rectifier Timing
DESIGN CONSIDERATIONS
A typical design can be implemented by using the
following design procedure. Note that this procedure is
not intended to give final values, but to give a good
starting point, and provide the relationships necessary to
make trade-off decisions. Some experimentation will be
necessary to optimize values and to verify that the design
operates over worst case conditions.
DESIGN SPECIFICATIONS
It is important to start with a clear definition of the design
specifications. Make sure the specifications reflect worst
case conditions. Key specifications include the minimum
and maximum input voltage and the output voltage and
load current.
INDUCTOR AND SENSE RESISTOR SELECTION
Figure 5 shows the inductor current of the buck regulator.
The inductor current is made up of two components: the
DC current level set by the transconductance amplifier,
I
SENSE
, and the inductor ripple current,
D
I
L
. The figure
also shows that I
OUT
is the summation of I
SENSE
and
D
I
L
.
I
I
I
V
R
t
V
2
V
L
OUT
SENSE
L
SENSE
SENSE
ON
IN
OUT
=
+
=
+
-
1
2
1
6
(2)
Therefore, the selection of the inductance value
determines how much of the output current is made up of
the ripple current. Higher inductor ripple current allows
smaller inductor values, but results in higher peak
currents, lower efficiency, and higher output voltage
ripple.
Inductor ripple currents in the range of 30% to 70% of the
maximum output current are typical. As a good starting
point, set the inductor ripple current to 50% of the
maximum output current:
D
I
t
V
V
L
F
I
L
ON
IN
OUT
IRC
OUT MAX
(
=
-
=
1
6
)
(3)
where F
IRC
= ratio of inductor ripple current to the
maximum output current, or:
1
05
(
)
L
t
V
I
V
ON
IN
OUT
OUT MAX
=
-
6
(4)
Calculate the inductance using the volt-seconds value
given in Figure 3 at the maximum input voltage. Choose
the nearest standard value, realizing the trade-offs
mentioned before. Then, using the inductance value
chosen, determine the actual inductor ripple current at
the maximum and minimum input voltage using Equation
4 and Figure 3.
The sense resistor value can be determined using the
inductor ripple current value calculated above and
Equation 3 rearranged as follows:
R
V
I
I
SENSE
SENSE MIN
OUT MAX
(
L MIN
(
=
-
(
)
)
)
1
2
D
(5)
Having determined the values for the inductor and sense
resistor, we can now specify the inductor peak current
rating. This value is calculated at current limit and at the
maximum input voltage, and is given by:
I
I
I
L PEAK MAX
(
SENSE MAX
V
SENSE MAX
L MAX
(
(
))
(
)
)
=
+
D
5
(6)
I
R
I
L PEAK MAX
SENSE
V
L MAX
5
7
5
=
+
D
I
R
I
L PEAK MAX
SENSE
L MAX
5
7
5
=
+
01
D
Q (ONE SHOT)
INDUCTOR CURRENT (IL)
VSR
tON
VC
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