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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)： | 13/24頁 |
| 文件大?。?/td> | 1087K |
| 代理商： | LM2746 |
Application Information
(Continued)
Figure 9
shows a second possibility for bootstrapping the
MOSFET drives using a doubler. This circuit provides an
equal voltage drive of
V
- 3V
+ V
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 LM2746 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.
POWER GOOD SIGNAL
The open drain output on the Power Good pin needs a
pull-up resistor to a low voltage source. The pull-up resistor
should be chosen so that the current going into the Power
Good pin is less than 1 mA. A 100 k
resistor is recom-
mended for most applications.
The Power Good signal is an OR-gated flag which takes into
account both output overvoltage and undervoltage condi-
tions. If the feedback pin (FB) voltage is 18% above its
nominal value (118% x V
FB
= 0.708V) or falls 28% below that
value (72 %x V
FB
= 0.42V) the Power Good flag goes low.
The Power Good flag can be used to signal other circuits that
the output voltage has fallen out of regulation, however the
switching of the LM2746 continues regardless of the state of
the Power Good signal. The Power Good flag will return to
logic high whenever the feedback pin voltage is between
72% and 118% of 0.6V.
UVLO
The 2.76V turn-on threshold on V
has a built in hysteresis
of about 300mV. If V
drops below 2.42V, the chip enters
UVLO mode. UVLO consists of turning off the top and bot-
tom MOSFETS and remaining in that condition until V
rises above 2.76V. As with shutdown, the soft-start capacitor
is discharged through an internal MOSFET, ensuring that the
next start-up will be controlled by the soft-start circuitry.
CURRENT LIMIT
Current limit is realized by sensing the voltage across the
low-side MOSFET while it is on. The R
of the MOSFET
is a known value; hence the current through the MOSFET
can be determined as:
V
DS
= I
OUT
* R
DSON
The current through the low-side MOSFET while it is on is
also the falling portion of the inductor current. The current
limit threshold is determined by an external resistor, R
CS
,
connected between the switching node and the I
SEN
pin. A
constant current of 40 μA is forced through R
CS
, causing a
fixed voltage drop. This fixed voltage is compared against
V
DS
and if the latter is higher, the current limit of the chip has
been reached. To obtain a more accurate value for R
CS
you
must consider the operating values of R
and I
at
their operating temperatures in your application and the
effect of slight parameter differences from part to part. R
can be found by using the following equation using the
R
value of the low side MOSFET at it’s expected hot
temperature and the absolute minimum value expected over
the full temperature range for the for the I
SEN-TH
which is 25
μA:
R
CS
= R
DSON-HOT
x I
LIM
/ 40 μA
For example, a conservative 15A current limit in a 10A
design with a minimum R
of 10m
would require a 6k
resistor. The minimum value for R
in any application is 1
k
. Because current sensing is done across the low-side
MOSFET, no minimum high-side on-time is necessary. The
LM2746 enters current limit mode if the inductor current
exceeds the current limit threshold at the point where the
high-side MOSFET turns off and the low-side MOSFET turns
on. (The point of peak inductor current, see
Figure 10
). Note
that in normal operation mode the high-side MOSFET al-
ways turns on at the beginning of a clock cycle. In current
limit mode, by contrast, the high-side MOSFET on-pulse is
skipped. This causes inductor current to fall. Unlike a normal
operation switching cycle, however, in a current limit mode
switching cycle the high-side MOSFET will turn on as soon
20147713
FIGURE 8. LM78L05 Feeding Basic Charge Pump
20147719
FIGURE 9. Charge Pump with Added Gate Drive
L
www.national.com
13
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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 |
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