lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 11

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

Application Information
A desired delay time t
master supply output voltage and the LM2746 output voltage
can be set based on the SD pin low-to-high threshold V
and the slew rate of the voltage at the SD pin, SR
Note again, that in Figure 4, the LM2746’s output voltage
has been represented symbolically as V
explicitly showing the power components.
V
pin voltage. The values of the sequencing divider resistors
R
output voltage slew rate, SR
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
the desired slew rate, R
diagram for this example is shown in Figure 5.
SD-IH
S1
and R
is typically 1.08V and SR
S2
FIGURE 4. Sequencing Circuit
set the SR
t
DELAY
DELAY
= V
S1
SD
OUT1
would then be 274Ω. A timing
SD-IH
based on the master supply
between the startup of the
SD
, using the following equa-
/ SR
is the slew rate of the SD
S2
SD
is 1kΩ. To achieve
OUT2
(Continued)
, i.e. without
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SD
:
SD-IH
11
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
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 1kΩ).
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
FIGURE 5. Delay for Sequencing
SD-IH
FIGURE 6. SD Pin Logic
, a 17 µA pull-up current is activated as
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