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參數(shù)資料
| 型號(hào)： | ML4902 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | High Current Synchronous Buck Controller For DC/DC conversion(用于DC/DC變換的大電流同步補(bǔ)償控制器（5V到1.8V～3.5V變換）) |
| 中文描述： | 高電流同步降壓控制器，用于直流/直流轉(zhuǎn)換（用于直流/直流變換的大電流同步補(bǔ)償控制器（5V的到1.8?3.5V的變換）） |
| 文件頁(yè)數(shù)： | 9/12頁(yè) |
| 文件大小： | 280K |
| 代理商： | ML4902 |
ML4902
9
DESIGN CONSIDERATIONS
(Continued)
The power handling requirement for R
SENSE
is given by:
!
For example, for a 14A output, R
SENSE
should be:
P
I
V
V
R
D
OUT MAX
(
OUT
IN
SENSE
=
′
-
′
"
$
)
2
1
(5)
R
mV
14
A
m
m
SENSE
=
-
|
.
105
′
=
@
|
.
87
592
6
W
W
The maximum dissipation in R
SENSE
for a 5.0V input
occurs at 1.80V out, where P
DISS
is
@
0.94W.
R
SENSE
must be a low inductance part, such as Dale/
Vishay’s type WSL-2512 series (WSL–2512–.006±1%).
Using a PCB trace as a current sense element is not
recommended due to the high temperature coefficient of
copper, and due to etching and plating tolerances which
can occur from board to board.
If a current sense resistor is not employed for overcurrent
protection, the voltage drop across (Q3||Q4)’s channel
during its conducting interval (the synchronous
rectification interval) is used to monitor the inductor
current. Ignoring the AC component of the current in the
buck inductor, the voltage across (Q3||Q4) will be:
V
I
R
SENSE
Q Q
3
DS ON Q Q
=
4
3
4
||
||
1
6
6
(6)
R
DS(ON)
is typically specified at a MOSFET junction
temperature (T
j
) of 25oC, but its value at other junction
temperatures can either be found graphically in the
MOSFET data sheet, or can be estimated by:
R
R
T
C
DS ON T
(
DS ON
(
C
)(
)
)(
o )
.
o
2
25
1007
2
25
=
-
0
5
(7)
With a nominal threshold of -97mV for the I
SENSE
comparator, the current limit threshold is then:
I
mV
R
LIMIT
DS ON T
(
=
-
97
2
)(
)
(8)
For Pentium Pro and Pentium II applications, the
continuous current may be as high as 14A, so the current
limit threshold should be set for a minimum value of 16A
at the (Q3||Q4)’s highest anticipated T
j
. If necessary, the
voltage across the channel of (Q3||Q4) may be divided
using two moderately-valued resistors (use R5 = 100
W
)
and presented after that division to the ML4902.
The R and C values connected to the PROTECT pin for
setting the current limit delay and the off-time of the
hiccup mode are 1M
W
and 220nF, respectively. These
values will protect the external power components and
the power source from overheating during an overcurrent
condition. If it is necessary to change the ratio of on and
off times during overcurrent conditions, this can be done
by selecting a different value for C12. Larger values of
C12 will increase the delay between retry attempts (the
length of the “hiccup”), and smaller values will reduce
the delay.
HIGHER CURRENT LEVELS
Next generation processor chips will require currents of up
to 20A. Additionally, it is often desirable in larger
systems to distribute all power from one 5V buss,
regulating it down to other voltages as needed at the
points of use. These applications are readily met by the
ML4902. For instance, the circuit shown in Figure 1 will
deliver an output current of 20A with only three changes:
As I
OUT
increases, the ripple current through the input
capacitor bank will also increase. Add at least one
1500μF, 6.3V input capacitor in parallel with the three
shown (C1 - C3).
Synchronous rectifier transistors Q3 and Q4 will see a
significantly greater RMS drain current at 20A output
than at 14A. Therefore, the use of lower R
DS(ON)
parts
such as Siliconix’ Si4420DY is required.
The value of R1 may require adjustment, depending
upon factors such as the specific MOSFET type chosen
for Q3 and Q4, and the required operating ambient
temperature.
In dealing with circuits handling greater than 50W, it is
always important to pay attention to thermal issues.
When the circuit of Figure 1 is modified for >20A
applicatons, a key consideration is that it be provided
with adequate heatsinking. Ideally, the system should
provide 100 linear feet per minute (LFM) of airflow as
specified in Intel’s standards relating to VRMs. Micro
Linear does not recommend using the sense resistor
method of overcurrent protection at high output current
levels, as this does not provide the inherent thermal
foldback of I
OUT(MAX)
which is obtained by directly
sensing the V
DS(ON)
of the rectifier MOSFETs.
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