參數(shù)資料
型號(hào)： 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(bǔ)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(bǔ)償控制器（高于全?15V的到為2.5V?4V變換））
文件頁(yè)數(shù)： 7/10頁(yè)
文件大小： 248K
代理商： ML4895
ML4895
7
MOSFET SELECTION
The switching MOSFETs must be logic level types with
the ON resistance specified at V
GS
= 4.5V. In general, the
ON resistance - gate charge product provides a good
figure of merit by which to compare various MOSFETs,
the lower the figure the better. The internal gate drivers of
the ML4895 can drive over 100nC of total gate charge,
but 60nC to 70nC is a more practical limit to ensure good
switching times.
The drain-source breakdown voltage rating is determined
by the input voltage. For input voltages up to 10V, a drain
to source rating of 20V is acceptable. For input voltages
up to 15V, a drain to source rating of 30V is
recommended. For a more reliable design, look for
MOSFETs that are avalanche rated.
In high current applications, the MOSFET’s power
dissipation often becomes a major design factor. The I
2
R
losses generate the largest portion of heat in the MOSFET
package. Make sure that the MOSFETs are within their
rated junction temperature at the maximum ambient
temperature by calculating the temperature rise using the
thermal resistance specifications.
The worst case power dissipation for the P-MOS switch
occurs at the minimum input voltage and is determined as
follows:
P
I
R
P MOS
-
RMS ON
DS ON
(
=
(
)
)
2
(10)
where:
I
V
V
I
I
I
I
RMS ON
OUT
IN MAX
(
SENSE(MAX
SENSE(MAX
L PEAK MAX
(
3
L PEAK MAX
(
(
)
)
)
)
(
))
(
))
2
2
2
=
+
+
For reliable operation, the inductor current rating should
exceed the value calculated by 10%-20%.
For future reference, determine the peak inductor current
at the minimum input voltage:
I
I
I
L PEAK MIN
(
SENSE MIN
V
SENSE MIN
L MIN
(
(
))
(
)
)
=
+
D
5
(7)
I
R
I
L PEAK MIN
SENSE
V
L MIN
5
7
5
=
+
D
I
R
I
L PEAK MIN
SENSE
L MIN
5
7
5
=
+
006
.
D
Now the sense resistor’s power rating can be determined.
The sense resistor must be able to carry the peak current
in the inductor during the OFF-time:
P
I
R
R
RMS OFF
(
SENSE
SENSE
=
)
2
(8)
where:
I
V
V
I
I
I
I
RMS OFF
(
OUT
IN MAX
(
SENSE(MAX
SENSE(MAX
L PEAK MAX
(
L PEAK MAX
(
)
)
)
)
(
))
(
))
2
2
2
1
3
=
-
+
+
The final parameter that should be specified is the
winding resistance of the inductor. In general, the winding
resistance should be as low as possible, preferably in the
low m
W
range. Since the inductor is in series with the
load at all times, the copper losses can be approximated
by:
P
I
R
Cu
OUT
L
=
2
(9)
A good rule of thumb is to allow 2 m
W
of winding
resistance per
m
H of inductance.
Figure 5. Buck Regulator Inductor Current
t
I
L
I
L
I
LPK
I
SENSE
=
V
SENSE
R
SENSE
I
OUT
= I
SENSE
+ 1/2
I
L
T
ON
V
IN –
V
OUT
L
T
OFF
V
OUT
L
DESIGN CONSIDERATIONS
(Continued)
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