參數(shù)資料
型號: LM2746MXA
廠商: NATIONAL SEMICONDUCTOR CORP
元件分類: 穩(wěn)壓器
英文描述: Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller
中文描述: SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO14
封裝: PLASTIC, TSSOP-14
文件頁數(shù): 11/24頁
文件大小: 1087K
代理商: LM2746MXA
Application Information
(Continued)
A desired delay time t
DELAY
between the startup of the
master supply output voltage and the LM2746 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 4
, the LM2746’s output voltage
has been represented symbolically as V
OUT2
, i.e. without
explicitly showing the power components.
V
is typically 1.08V and SR
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:
For example, if the master supply output voltage slew rate
were 1V/ms and the desired delay time between the startup
of the master supply and LM2746 output voltage were 5ms,
then the desired SD pin slew rate would be (1.08V/5ms) =
0.216V/ms. Due to the internal impedance of the SD pin, the
maximum recommended value for R
is 1k
. To achieve
the desired slew rate, R
would then be 274
. A timing
diagram for this example is shown in
Figure 5
.
SD PIN IMPEDANCE
When connecting a resistor divider to the SD pin of the
LM2746 some care has to be taken. Once the SD voltage
goes above V
, a 17 μA pull-up current is activated as
shown in
Figure 6
. This current is used to create the internal
hysteresis (
)
170mV); however, high external impedances
will affect the SD pin logic thresholds as well. The external
impedance used for the sequencing divider network should
preferably be a small fraction of the impedance of the SD pin
for good performance (around 1k
).
MOSFET GATE DRIVERS
The LM2746 has two gate drivers designed for driving
N-channel MOSFETs in a synchronous mode. Note that
unlike most other synchronous controllers, the bootstrap
capacitor of the LM2746 provides power not only to the
driver of the upper MOSFET, but the lower MOSFET driver
20147714
FIGURE 4. Sequencing Circuit
20147711
FIGURE 5. Delay for Sequencing
20147706
FIGURE 6. SD Pin Logic
L
www.national.com
11
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