lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 12

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

www.national.com
Application Information
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
the high-side drive goes high. This bootstrap voltage is
usually supplied from a local charge pump structure. But
looking at the Typical Application schematic, this also means
that the difference voltage V
bootstrap capacitor charges up to, must be always greater
than the maximum tolerance limit of the threshold voltage of
the upper MOSFET. Here V
across the bootstrap diode D1. This therefore may place
restrictions on the minimum input voltage and/or type of
MOSFET used.
Two things must be kept in mind here. First, the BOOT pin
has an absolute maximum rating of 21V. 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
ing any forward drop across the bootstrap diode). The boot-
strap capacitor is charged up by a given rail (called
V
rail can be the same as V
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
16V, and V
clearly if the V
pin is 16V + 5.5V = 21.5V. This is unacceptable, as it is in
excess of the rating of the BOOT pin. A V
acceptable in this case. Or the V
accordingly. There is also the option of deriving the bootstrap
DC rail from another 3V external rail, independent of V
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
high-side driver swings between V
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
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
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, 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
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
forward voltage drop across the bootstrap diode D1. This
may place restrictions on the minimum input voltage and/or
type of MOSFET used.
The basic bootstrap pump circuit can be built using one
Schottky diode and a small capacitor, as shown in Figure 7.
The capacitor C
between the top MOSFET gate and source to control the
device even when the top MOSFET is on and its source has
BOOT_DC
IN
) plus the voltage across the bootstrap capacitor (ignor-
BOOT_DC
here) whenever the upper MOSFET turns off. This
BOOT_DC
CC
BOOT
rail is 5.5V, the peak voltage on the BOOT
and Ground, whereas the output of the
is chosen to be the same as V
serves to maintain enough voltage
CC
D1
CC
- V
is the forward voltage drop
BOOT_DC
or it can be any external
D1
IN
, which is the voltage the
range must be reduced
BOOT_DC
IN
+ V
CC
to at least exceed
(Continued)
of 3V would be
BOOT_DC
rail.
CC
CC
D1
IN
, then
- V
when
is the
CC
IN
and
D1
.
is
,
12
risen up to the input voltage level. The charge pump circuitry
is fed from V
6.0V. Using this basic method the voltage applied to the
gates of both high-side and low-side MOSFETs is V
This method works well when V
gate drives will get at least 4.0V of drive voltage during the
worst case of V
MOSFETs generally specify their on-resistance at V
4.5V. When V
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 V
tend to have higher on-resistance. The circuit in Figure 7
works well for input voltages ranging from 1V up to 16V and
V
V
Note that the LM2746 can be paired with a low cost linear
regulator like the LM78L05 to run from a single input rail
between 6.0 and 16V. The 5V output of the linear regulator
powers both the V
efficient drive for logic level MOSFETs. An example of this
circuit is shown in Figure 8.
CC
CC
.
= 5V
FIGURE 7. Basic Charge Pump (Bootstrap)
GS
±
10%, because the drive voltage depends only on
= 2.5V, will work, but are more expensive, and
CC
CC
, which can operate over a range from 3.0V to
CC-MIN
= 3.3V
CC
= 4.5V and V
and the bootstrap circuit, providing
±
10%, the gate drive at worst case
CC
is 5V
D-MAX
±
= 0.5V. Logic level
10%, because the
CC
20147712
GS
- V
D
=
.