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LM2748MTC View Datasheet(PDF) - National ->Texas Instruments

Part NameLM2748MTC National-Semiconductor
National ->Texas Instruments National-Semiconductor
DescriptionSynchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization
LM2748MTC Datasheet PDF : 23 Pages
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Application Information (Continued)
the Miller plateau level. This may therefore affect the choice
of the threshold voltage of the external MOSFETs, and that
in turn may depend on the chosen VBOOT_DC rail.
So far, in the discussion above, the forward drop across the
bootstrap diode has been ignored. But since that does affect
the output of the driver somewhat, it is a good idea to include
this drop in the following examples. Looking at the Typical
Application schematic, this means that the difference voltage
VCC - VD1, which is the voltage the bootstrap capacitor
charges up to, must always be greater than the maximum
tolerance limit of the threshold voltage of the upper MOS-
FET. Here VD1 is the forward voltage drop across the boot-
strap diode D1. This may place restrictions on the minimum
input voltage and/or type of MOSFET used.
A basic bootstrap circuit can be built using one Schottky
diode and a small capacitor, as shown in Figure 9. The
capacitor CBOOT serves to maintain enough voltage between
the top MOSFET gate and source to control the device even
when the top MOSFET is on and its source has risen up to
the input voltage level. The charge pump circuitry is fed from
VCC, which can operate over a range from 3.0V to 6.0V.
Using this basic method the voltage applied to the gates of
both high-side and low-side MOSFETs is VCC - VD. This
method works well when VCC is 5V±10%, because the gate
drives will get at least 4.0V of drive voltage during the worst
case of VCC-MIN = 4.5V and VD-MAX = 0.5V. Logic level
MOSFETs generally specify their on-resistance at VGS =
4.5V. When VCC = 3.3V±10%, the gate drive at worst case
could go as low as 2.5V. Logic level MOSFETs are not
guaranteed to turn on, or may have much higher on-
resistance at 2.5V. Sub-logic level MOSFETs, usually speci-
fied at VGS = 2.5V, will work, but are more expensive, and
tend to have higher on-resistance. The circuit in Figure 9
works well for input voltages ranging from 1V up to 14V and
VCC = 5V±10%, because the drive voltage depends only on
VCC.
powers both the VCC and the bootstrap circuit, providing
efficient drive for logic level MOSFETs. An example of this
circuit is shown in Figure 10.
20137413
FIGURE 10. LM78L05 Feeding Basic Charge Pump
Figure 11 shows a second possibility for bootstrapping the
MOSFET drives using a doubler. This circuit provides an
equal voltage drive of VCC - 3VD + VIN to both the high-side
and low-side MOSFET drives. This method should only be
used in circuits that use 3.3V for both VCC and VIN. Even with
VIN = VCC = 3.0V (10% lower tolerance on 3.3V) and VD =
0.5V both high-side and low-side gates will have at least
4.5V of drive. The power dissipation of the gate drive cir-
cuitry is directly proportional to gate drive voltage, hence the
thermal limits of the LM2745/8 IC will quickly be reached if
this circuit is used with VCC or VIN voltages over 5V.
20137412
FIGURE 9. Basic Charge Pump (Bootstrap)
Note that the LM2745/8 can be paired with a low cost linear
regulator like the LM78L05 to run from a single input rail
between 6.0 and 14V. The 5V output of the linear regulator
20137419
FIGURE 11. Charge Pump with Added Gate Drive
All the gate drive circuits shown in the above figures typically
use 100 nF ceramic capacitors in the bootstrap locations.
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