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參數資料
| 型號： | MBR0520LTI |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | DB-LM3S102 Daughterboard |
| 中文描述： | 同步降壓控制器，帶有預偏置啟動和可選時鐘同步 |
| 文件頁數： | 10/23頁 |
| 文件大小： | 1002K |
| 代理商： | MBR0520LTI |
Application Information
(Continued)
target values at the same time. For this case, the equation
governing the values of the tracking divider resistors R
T1
and
R
T2
is:
The current through R
should be about 4 mA for precise
tracking. The final voltage of the SS/TRACK pin should be
set higher than the feedback voltage of 0.6V (say about
0.65V as in the above equation). If the master supply voltage
was 5V and the LM2747 output voltage was 1.8V, for ex-
ample, then the value of R
needed to give the two supplies
identical soft-start times would be 150
.Atiming diagram for
the equal soft-start time case is shown in
Figure 4
.
TRACKING A VOLTAGE SLEW RATE
The tracking feature can alternatively be used not to make
both rails reach regulation at the same time but rather to
have similar rise rates (in terms of output dV/dt). This
method ensures that the output voltage of the LM2747 al-
ways reaches regulation before the output voltage of the
master supply. In this case, the tracking resistors can be
determined based on the following equation:
For the example case of V
= 5V and V
= 1.8V, with
R
set to 150
as before, R
is calculated from the above
equation to be 265
. A timing diagram for the case of equal
slew rates is shown in
Figure 5
.
SEQUENCING
The start up/soft-start of the LM2747 can be delayed for the
purpose of sequencing by connecting a resistor divider from
the output of a master power supply to the SD pin, as shown
in
Figure 6
.
A desired delay time t
between the startup of the
master supply output voltage and the LM2747 output voltage
can be set based on the SD pin low-to-high threshold V
SD-IH
and the slew rate of the voltage at the SD pin, SR
SD
:
t
DELAY
= V
SD-IH
/ SR
SD
Note again, that in
Figure 6
, the LM2747’s output voltage
has been represented symbolically as V
OUT2
, i.e. without
explicitly showing the power components.
V
SD-IH
is typically 1.08V and SR
SD
is the slew rate of the SD
pin voltage. The values of the sequencing divider resistors
R
and R
set the SR
based on the master supply
output voltage slew rate, SR
OUT1
, using the following equa-
tion:
20150908
FIGURE 4. Tracking with Equal Soft-Start Time
20150910
FIGURE 5. Tracking with Equal Slew Rates
20150914
FIGURE 6. Sequencing Circuit
L
www.national.com
10
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