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LM2743 Datasheet(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part # LM2743
Description  N-Channel FET Synchronous Buck Regulator Controller for Conversion from 3.3V
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2743 Datasheet(HTML) 11 Page - National Semiconductor (TI)

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Application Information (Continued)
A desired delay time t
DELAY between the startup of the
master supply output voltage and the LM2743 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 =VSD-IH /SRSD
Note again, that in Figure 4, the LM2743’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 SRSD is the slew rate of the SD
pin voltage. The values of the sequencing divider resistors
R
S1 and RS2 set the SRSD 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
was 1V/ms and the desired delay time between the startup
of the master supply and LM2743 output voltage was 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
S2 is 1k
Ω. To achieve
the desired slew rate, R
S2 would then be 274
Ω. 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
LM2743 some care has to be taken. Once the SD voltage
goes above V
SD-IH, 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
Ω).
MOSFET GATE DRIVERS
The LM2743 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 LM2743 provides power not only to the
driver of the upper MOSFET, but the lower MOSFET driver
20095214
FIGURE 4. Sequencing Circuit
20095211
FIGURE 5. Delay for Sequencing
20095206
FIGURE 6. SD Pin Logic
www.national.com
11


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