lm2744mtcx National Semiconductor Corporation, lm2744mtcx Datasheet - Page 11

• Manufacturer Part Number: lm2744mtcx
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller With External Reference
• Manufacturer: National Semiconductor Corporation

A desired delay time t
supply output voltage and the LM2744 output voltage can be
set based on the SD pin low-to-high threshold V
slew rate of the voltage at the SD pin, SR
Note again, that in Figure 4, the LM2744’s output voltage has
been represented symbolically as V
showing the power components.
V
pin voltage. The values of the sequencing divider resistors
R
voltage slew rate, SR
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 LM2744 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 maxi-
mum recommended value for R
desired slew rate, R
for this example is shown in Figure 5.
SD-IH
S1
and R
is typically 1.08V and SR
S2
set the SR
FIGURE 4. Sequencing Circuit
t
S2
DELAY
DELAY
OUT1
would then be 274Ω. A timing diagram
SD
, using the following equation:
= V
based on the master supply output
between the startup of the master
SD-IH
SD
S2
/ SR
is the slew rate of the SD
OUT2
is 1kΩ. To achieve the
SD
, i.e. without explicitly
SD
:
SD-IH
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and the
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SD PIN IMPEDANCE
When connecting a resistor divider to the SD pin of the
LM2744 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 (
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 LM2744 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 LM2744 provides power not only to the driver of the
upper MOSFET, but the lower MOSFET driver too (both
drivers are ground referenced, i.e. no floating driver). To fully
turn the top MOSFET on, the BOOT voltage must be at least
one gate threshold greater than V
≊
FIGURE 5. Delay for Sequencing
170mV); however, high external impedances
SD-IH
FIGURE 6. SD Pin Logic
, a 17 µA pull-up current is activated as
IN
when the high-side drive
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