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參數(shù)資料
| 型號(hào)： | LM3743MM-1000 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | N-Channel FET Synchronous Buck Controller for Low Output Voltages |
| 中文描述： | SWITCHING CONTROLLER, 1150 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 17/23頁 |
| 文件大?。?/td> | 1145K |
| 代理商： | LM3743MM-1000 |
Application Information
(Continued)
Where duty cycle D = V
/V
. The worst-case ripple for a
buck converter occurs during full load and when the duty
cycle (D) is 0.5.
When multiple capacitors of the same type and value are
paralleled, the power dissipated by each input capacitor is:
where n is the number of paralleled capacitors, and ESR is
the equivalent series resistance of each capacitor. The equa-
tion above indicates that power loss in each capacitor de-
creases rapidly as the number of input capacitors increases.
For this 5V to 1.8V design the duty cycle is 0.36. For a 10A
maximum load the RMS current is 4.8A.
Connect one or two 22 μF MLCC as close as possible across
the drain of the high-side MOSFET and the source of the
low-side MOSFET, this will provide high frequency decou-
pling and satisfy the RMS stress. A bulk capacitor is recom-
mended in parallel with the MLCC in order to prevent switch-
ing frequency noise from reflecting back into the input line,
this capacitor should be no more than 1inch away from the
MLCC capacitors.
MOSFETs
Selection of the power MOSFETs is governed by a trade-off
between cost, size, and efficiency. One method is to deter-
mine the maximum cost that can be endured, and then
select the most efficient device that fits that price. Using a
spreadsheet to estimate the losses in the high-side and
low-side MOSFETs is one way to determine relative efficien-
cies between different MOSFETs. Good correlation between
the prediction and the bench result is not guaranteed.
Losses in the high-side MOSFET can be broken down into
conduction loss, gate charging loss, and switching loss.
Conduction, or I
2
R loss, is approximately:
For the high side FET:
P
C
= D (I
OUT2
x R
DSON-HI
x 1.3)
For the low side FET:
P
C
= (1 - D) x (I
OUT2
x R
DSON-LO
x 1.3)
In the above equations the factor 1.3 accounts for the in-
crease in MOSFET R
due to heating. Alternatively, the
1.3 can be ignored and the R
of the MOSFET estimated
using the R
vs. Temperature curves in the MOSFET
manufacturer datasheet.
Gate charging loss results from the current driving the gate
capacitance of the power MOSFETs, and is approximated
as:
P
GC
= (V
CC
) x Q
G
x f
SW
V
CC
is the driving voltage (see MOSFET Gate Driver sec-
tion) and Q
G
is the gate charge of the MOSFET. If multiple
devices will be placed in parallel, their gate charges can
simply be summed to form a cumulative Q
G
.
Switching loss occurs during the brief transition period as the
high-side MOSFET turns on and off, during which both cur-
rent and voltage are present in the channel of the MOSFET.
It can be approximated as:
P
SW
= 0.5 x V
IN
x I
OUT
x (t
r
+ t
f
) x f
SW
where t
and t
are the rise and fall times of the MOSFET.
Switching loss occurs in the high-side MOSFET only.
For this example, the maximum drain-to-source voltage ap-
plied to either MOSFET is 5.5V. The maximum drive voltage
at the gate of the high-side MOSFET is 5.0V, and the maxi-
mum drive voltage for the low-side MOSFET is 5.5V. For
designs between 5A and 10A, single MOSFETs in SO-8
provide a good trade-off between size, cost, and efficiency.
V
CC
Filtering
To ensure smooth DC voltage for the chip supply a 1 μF
(C3), X5R MLCC type or better must be placed as close as
possible to the V
and GND pin. Together with a small 1 to
4.99
resistor placed between the input rail and the V
pin,
a low pass filter is formed to filter out high frequency noise
from injecting into the V
CC
rail. Since V
CC
is also the sense
pin for the high-side current limit, the resistor should connect
close to the drain of the high-side MOSFET to prevent IR
drops due to trace resistance. A second design consider-
ation is the low pass filter formed by C3 and R6 on the V
CC
pin, a fast slew rate, large amplitude load transient may
cause a larger voltage droop on C
IN
than on V
CC
pin. This
may lead to a lower current at which high-side protection
may occur. Thus increase the bulk input capacitor if the
high-side current limit is engaging due to a dynamic load
transient behavior as explained above.
Bootstrap Diode (D1)
The MBR0520 and BAT54 work well as a bootstrap diode in
most designs. Schottky diodes are the preferred choice for
the bootstrap circuit because of their low forward voltage
drop. For circuits that will operate at high ambient tempera-
ture the Schottky diode datasheet must be read carefully to
ensure that the reverse current leakage at high temperature
does not increase enough to deplete the charge on the
bootstrap capacitor while the high side FET is on. Some
Schottky diodes increase their reverse leakage by as much
as 1000 times at high temperatures. Fast rectifier and PN
junction diodes maintain low reverse leakage even at high
ambient temperature. These diode types have higher for-
ward voltage drop but can still be used for high ambient
temperature operation.
Control Loop Compensation
The LM3743 uses voltage-mode (‘VM’) PWM control to cor-
rect changes in output voltage due to line and load tran-
sients. VM requires careful small signal compensation of the
control loop for achieving high bandwidth and good phase
margin.
The control loop is comprised of two parts. The first is the
power stage, which consists of the duty cycle modulator,
output inductor, output capacitor, and load. The second part
is the error amplifier, which for the LM3743 is a 30 MHz
op-amp used in the classic inverting configuration.
Figure 8
shows the regulator and control loop components.
L
www.national.com
17
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參數(shù)描述 |
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| LM3743MM-1000/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MM-300 | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MM-300/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MMX-1000 | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MMX-1000/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
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