Datasheet

LM2745, LM2748
SNOSAL2E –APRIL 2005–REVISED APRIL 2013 www.ti.com
MOSFET GATE DRIVERS
The LM2745/8 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 LM2745/8 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 unacceptable, 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 desired, 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 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 V
BOOT_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 V
CC
- V
D1
, 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 MOSFET. Here V
D1
is the forward voltage drop across the bootstrap 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 25. The
capacitor C
BOOT
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 V
CC
, 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 V
CC
- V
D
. This method works well when V
CC
is
5V±10%, because the gate drives will get at least 4.0V of drive voltage during the worst case of V
CC-MIN
= 4.5V
and V
D-MAX
= 0.5V. Logic level MOSFETs generally specify their on-resistance at V
GS
= 4.5V. When V
CC
=
3.3V±10%, the gate drive at worst case could go as low as 2.5V. Logic level MOSFETs are not specified to turn
on, or may have much higher on-resistance at 2.5V. Sub-logic level MOSFETs, usually specified at V
GS
= 2.5V,
will work, but are more expensive, and tend to have higher on-resistance. The circuit in Figure 25 works well for
input voltages ranging from 1V up to 14V and V
CC
= 5V±10%, because the drive voltage depends only on V
CC
.
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