LM2747
Application Information
(Continued)
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 LM2747 output voltage was 5 ms,
then the desired SD pin slew rate would be (1.08V/5 ms) =
0.216V/ms. Due to the internal impedance of the SD pin, the
maximum recommended value for R
S2
is 1 kΩ. To achieve
the desired slew rate, R
S1
would then be 274Ω. A timing
diagram for this example is shown in
20150906
FIGURE 8. SD Pin Logic
MOSFET GATE DRIVERS
The LM2747 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 LM2747 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).
Two things must be kept in mind here. First, the BOOT pin
has an absolute maximum rating of 18V. This must never be
exceeded, even momentarily. Since the bootstrap capacitor
is connected to the SW node, the peak voltage impressed on
the BOOT pin is the sum of the input voltage (V
IN
) plus the
voltage across the bootstrap capacitor (ignoring any forward
drop across the bootstrap diode). The bootstrap capacitor is
charged up by a given rail (called V
BOOT_DC
here) whenever
the upper MOSFET turns off. This rail can be the same as
V
CC
or it can be any external ground-referenced DC rail. But
care has to be exercised when choosing this bootstrap DC
rail that the BOOT pin is not damaged. For example, if the
desired maximum V
IN
is 14V, and V
BOOT_DC
is chosen to be
the same as V
CC
, then clearly if the V
CC
rail is 6V, the peak
voltage on the BOOT pin is 14V + 6V = 20V. This is unac-
ceptable, as it is in excess of the rating of the BOOT pin. A
V
CC
of 3V would be acceptable in this case. Or the V
IN
range
must be reduced accordingly. There is also the option of
deriving the bootstrap DC rail from another 3V external rail,
independent of V
CC
.
The second thing to be kept in mind here is that the output of
the low-side driver swings between the bootstrap DC rail
level of V
BOOT_DC
and Ground, whereas the output of the
high-side driver swings between V
IN
+ V
BOOT_DC
and
Ground. To keep the high-side MOSFET fully on when de-
sired, the Gate pin voltage of the MOSFET must be higher
than its instantaneous Source pin voltage by an amount
equal to the ’Miller plateau’. It can be shown that this plateau
is equal to the threshold voltage of the chosen MOSFET plus
a small amount equal to Io/g. Here Io is the maximum load
current of the application, and g is the transconductance of
this MOSFET (typically about 100 for logic-level devices).
That means we must choose V
BOOT_DC
to at least exceed
20150911
FIGURE 7. Delay for Sequencing
SD PIN IMPEDANCE
When connecting a resistor divider to the SD pin of the
LM2747 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
This current is used to create the internal
hysteresis ()170 mV); 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 1 kΩ).
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