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參數(shù)資料
| 型號： | 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)： | 12/23頁 |
| 文件大小： | 1145K |
| 代理商： | LM3743MM-1000 |
Application Information
THEORY OF OPERATION
The LM3743 is a voltage mode PWM buck controller featur-
ing synchronous rectification at 300 kHz or 1 MHz. In steady
state operation the LM3743 is always synchronous even at
no load, thus simplifying the compensation design. The
LM3743 ensures a smooth and controlled start-up to support
pre-biased outputs. Two levels of current limit protection
enhance the robustness of the power supply and requires no
current sense resistor in the power path. The primary level of
protection is the low side current limit and is achieved by
sensing the voltage V
across the low side MOSFET. The
second level of protection is the high side current limit, which
protects power components from extremely high currents,
caused by switch node short to ground.
NORMAL OPERATION
While in normal operation, the LM3743 IC controls the output
voltage by controlling the duty cycle of the power FETs. The
DC level of the output voltage is determined by a pair of
feedback resistors using the following equation:
(Designators refer to the Typical Application Circuit in the
front page)
For synchronous buck regulators, the duty ratio D is approxi-
mately equal to:
START UP
The LM3743 IC begins to operate when the COMP/EN pin is
released from a clamped condition and the voltage at the
V
pin has exceeded 2.84V. Once these two conditions
have been met the internal 10μA current source begins to
charge the soft-start capacitor connected at the SS/TRACK
pin. During soft-start the voltage on the soft-start capacitor is
connected internally to the non-inverting input of the error
amplifier. The soft-start period lasts until the voltage on the
soft-start capacitor exceeds the LM3743 reference voltage
of 0.8V. At this point the reference voltage takes over at the
non-inverting error amplifier input. The capacitance deter-
mines the length of the soft-start period, and can be approxi-
mated by:
C4 = (t
SS
x 10 μA) / 0.8V
Where t
is the desired soft-start time. In the event of either
V
falling below UVLO or COMP/EN pin being pulled below
0.45V, the soft-start pin will discharge C4 to allow the output
voltage to recover smoothly.
START UP WITH PRE-BIAS
Apre-bias output is a condition in which current from another
source has charged up the output capacitor of the switching
regulator before it has been turned on. The LM3743 features
a proprietary glitch free monotonic pre-bias start-up method
designed to ramp the output voltage from a pre-biased rail to
the target nominal output voltage. The IC limits the on time of
the low-side FET to 150 ns (typ) during soft-start, while
allowing the high-side FET to adjust it’s time according to
soft-start voltage, V
, and the internal voltage ramp. Any
further commutation of the load current is carried by the body
diode of the low-side FET or an external Schottky diode, if
used. The low side current limit is active during soft-start
while allowing the asynchronous switching. When soft-start
is completed, the on-time of the low-side FET is allowed to
increase in a controlled fashion up to the steady state duty
cycle determined by the control loop. A plot of the LM3743
starting up into a pre-biased condition is shown in the Typical
Performance Characteristics section.
Note that the pre-bias voltage must not be greater than the
target output voltage of the LM3743, otherwise the LM3743
will pull the pre-bias supply down during steady state opera-
tion.
TRACKING WITH EQUAL SOFT-START TIME
The LM3743 can track the output of a master power supply
during soft-start by connecting a resistor divider to the SS/
TRACK pin. In this way, the output voltage slew rate of the
LM3743 will be controlled by the master supply for loads that
require precise sequencing. When the tracking function is
used, no soft-start capacitor should be connected to the
SS/TRACK pin. However in all other cases, a capacitor
value (C4) of at least 560 pF should be connected between
the soft-start pin and ground.
One way to use the tracking feature is to design the tracking
resistor divider so that the master supply’s output voltage
(V
OUT1
) and the LM3743’s output voltage (represented sym-
bolically in
Figure 1
as V
OUT2
, i.e. without explicitly showing
the power components) both rise together and reach their
target values at the same time. For this case, the equation
governing the values of the tracking divider resistors R
T1
and
R
T2
is:
The top resistance R
must be set to 1 k
in order to limit
current into the LM3743 during UVLO or shutdown. The final
voltage of the SS/TRACK pin should be slightly higher than
the feedback voltage of 0.8V, say about 0.85V as in the
above equation. The 50 mV difference will ensure the
LM3743 to reach regulation slightly before the master sup-
ply. If the master supply voltage was 5V and the LM3743
20177430
FIGURE 1. Tracking Circuit
L
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12
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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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