參數(shù)資料
型號(hào)： LM2744
廠商： National Semiconductor Corporation
英文描述： Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference
中文描述： 低壓N溝道MOSFET同步降壓穩(wěn)壓控制器與外部參考
文件頁(yè)數(shù)： 12/22頁(yè)
文件大?。?/td> 663K
代理商： LM2744
Application Information
(Continued)
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
IN
when
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
- V
, which is the voltage the
bootstrap capacitor charges up to, must be always greater
than the maximum tolerance limit of the threshold voltage of
the upper MOSFET. Here V
is the forward voltage drop
across the bootstrap diode D1. This therefore may place
restrictions on the minimum input voltage and/or type of
MOSFET used.
The most basic charge bootstrap pump circuit can be built
using one Schottky diode and a small capacitor, as shown in
Figure 7
. The capacitor C
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
, 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 MOS-
FETs is
V
CC
- V
D
. This method works well when V
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 guaranteed to turn on, or may have much
higher on-resistance at 2.5V. Sub-logic level MOSFETs, usu-
ally specified at V
GS
= 2.5V, will work, but are more expen-
sive, and tend to have higher on-resistance. The circuit in
Figure 7
works well for input voltages ranging from 1V up to
16V and V
= 5V
±
10%, because the drive voltage de-
pends only on V
CC
.
Note that the LM2744 can be paired with a low cost linear
regulator like the LP8340 to run from a single input rail
between 6.0 and 16V. The 5V output of the linear regulator
powers both the V
and the bootstrap circuit, providing
efficient drive for logic level MOSFETs. An example of this
circuit is shown in
Figure 8
.
Figure 9
shows a second possibility for bootstrapping the
MOSFET drives using a doubler. This circuit provides an
equal voltage drive of
V
CC
- 3V
D
+ V
IN
to both the high-side
and low-side MOSFET drives. This method should only be
used in circuits that use 3.3V for both V
and V
. Even with
V
= V
= 3.0V (10% lower tolerance on 3.3V) and V
=
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 LM2744 IC will quickly be reached if this
circuit is used with V
CC
or V
IN
voltages over 5V.
All the gate drive circuits shown in the above figures typically
use 100nF ceramic capacitors in the bootstrap locations.
20106012
FIGURE 7. Basic Charge Pump (Bootstrap)
20106013
FIGURE 8. LP8340 Feeding Basic Charge Pump
20106019
FIGURE 9. Charge Pump with Added Gate Drive
L
www.national.com
12
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