- 您現(xiàn)在的位置：買賣IC網(wǎng) > PDF目錄385477 > LM2746 (National Semiconductor Corporation) Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller PDF資料下載
參數(shù)資料
| 型號： | LM2746 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller |
| 中文描述： | 低壓N溝道MOSFET同步降壓穩(wěn)壓控制器 |
| 文件頁數(shù)： | 12/24頁 |
| 文件大?。?/td> | 1087K |
| 代理商： | LM2746 |
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.
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
) plus the voltage across the bootstrap capacitor (ignor-
ing any forward drop across the bootstrap diode). The boot-
strap capacitor is charged up by a given rail (called
V
here) whenever the upper MOSFET turns off. This
rail can be the same as V
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
16V, and V
BOOT_DC
is chosen to be the same as V
CC
, then
clearly if the V
CC
rail is 5.5V, the peak voltage on the BOOT
pin is 16V + 5.5V = 21.5V. 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
and Ground, whereas the output of the
high-side driver swings between V
+ V
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
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, 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.
The basic 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 MOSFETs 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, usually speci-
fied at V
GS
= 2.5V, will work, but are more expensive, 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
CC
= 5V
±
10%, because the drive voltage depends only on
V
CC
.
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
and the bootstrap circuit, providing
efficient drive for logic level MOSFETs. An example of this
circuit is shown in
Figure 8
.
20147712
FIGURE 7. Basic Charge Pump (Bootstrap)
L
www.national.com
12
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM2746MXA | Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller |
| LM2746MXAX | Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller |
| LM2753SD | High Power Switched Capacitor Voltage Convertor/Flash LED Driver |
| LM2753 | High Power Switched Capacitor Voltage Convertor/Flash LED Driver |
| LM2753SDX | High Power Switched Capacitor Voltage Convertor/Flash LED Driver |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2746MXA | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller |
| LM2746MXA/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 14TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
| LM2746MXAX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller |
| LM2746MXAX/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 14TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:100% 電源電壓:8.2 V ~ 30 V 降壓:無 升壓:無 回掃:是 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:是 工作溫度:0°C ~ 70°C 封裝/外殼:8-DIP（0.300"，7.62mm） 包裝:管件 產(chǎn)品目錄頁面:1316 (CN2011-ZH PDF) |
| LM2747 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization |
發(fā)布緊急采購，3分鐘左右您將得到回復(fù)。