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參數(shù)資料
| 型號： | LM3743MMX-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)： | 15/23頁 |
| 文件大?。?/td> | 1145K |
| 代理商： | LM3743MMX-1000 |
Application Information
(Continued)
limit cycles occur in between two current limit events). Hic-
cup mode will be discussed in further detail in the “Hiccup
Mode and Internal Soft-Start” section.
HIGH-SIDE COARSE CURRENT LIMIT
The LM3743 employs a comparator to monitor the voltage
across the high-side MOSFET when it is on. This provides
protection for short circuits from switch node to ground or the
case when the inductor is shorted, which the low side current
limit cannot detect. A 200 ns blanking time period after the
high-side FET turns on is used to prevent switching transient
voltages from tripping the high-side current limit without
cause. If the difference between V
CC
pin and SW pin voltage
exceeds 500 mV, the LM3743 will immediately enter hiccup
mode (see Hiccup Mode section).
OUTPUT UNDER-VOLTAGE PROTECTION (UVP)
After the end of soft-start the output UVP comparator is
activated. The threshold is 50% of the feedback voltage.
Once the comparator indicates UVP for more than 7 μs typ.
(glitch filter time), the IC goes into hiccup mode.
HICCUP MODE AND INTERNAL SOFT-START
Hiccup protection mode is designed to protect the external
components of the circuit (output inductor, FETs, and input
voltage source) from thermal stress. During hiccup mode,
the LM3743 disables both the high-side and low-side FETs
and begins a cool down period of 5.5 ms. At the conclusion
of this cool down period, the regulator performs an internal
3.6 ms soft-start. There are three distinct conditions under
which the IC will enter the hiccup protection mode:
1.
The low-side current sensing threshold has exceeded
the current limit threshold for fifteen sampled cycles, see
Figure 6
. Each cycle is sampled at the start of each off
time (t
OFF
). The low-side current limit counter is reset
when 32 consecutive non-current limit cycles occur in
between two current limit events.
2.
The high-side current limit comparator has sensed a
differential voltage larger than 500 mV.
3.
The voltage at the FB pin has fallen below 0.4V, and the
UVP comparator has sensed this condition for 7 μs
(during steady state operation).
The band gap reference, the external soft-start, and internal
hiccup soft-start of 3.6 ms (typ) connect to the non-inverting
input of the error amplifier through a multiplexer. The lowest
voltage of the three connects directly to the non-inverting
input. Hiccup mode will not discharge the external soft-start,
only UVLO or shut-down will. When in hiccup mode the
internal 5.5 ms timer is set, and the internal soft-start capaci-
tor is discharged. After the 5.5 ms timeout, the internal 3.6
ms soft-start begins, see
Figure 7
. During soft-start, only
low-side current limit and high side current limit can put the
LM3743 into hiccup mode.
For example, if the low-side current limit is 10A, then once in
overload the low-side current limit controls the valley current
and only allows an average amount of 10A plus the ripple
current to pass through the inductor and FETs for 15 switch-
ing cycles. In such an amount of time, the temperature rise is
very small. Once in hiccup mode, the average current
through the high-side FET is:
I
HSF-AVE
= (I
CLIM
+
I) x [ D(15 cycles x T
SW
) ] / 5.5 ms
equals 71 mA. With an arbitrary D = 60%, ripple current of
3A, and a 300 kHz switching frequency.
The average current through the low-side FET is:
I
LSF-AVE
= (I
CLIM
+
I) x [ (1–D) x (15 cycles x T
SW
) ] / 5.5 ms
equals 47 mA,
And the average current through the inductor is:
I
L-AVE
= (I
CLIM
+
I) x [ (15 cycles x T
SW
) ] / 5.5 ms
equals 118 mA.
DESIGN CONSIDERATIONS
The following is a design procedure for selecting all the
components in the Typical Application circuit on the front
page. This design converts 5V (V
IN
) to 1.8V (V
OUT
) at a
maximum load of 10A with an efficiency of 90% and a
switching frequency of 300 kHz. The same procedures can
20177452
FIGURE 6. Entering Hiccup Mode
20177406
FIGURE 7. Hiccup Time-Out and Internal Soft-Start
L
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15
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| 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 拓撲結(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-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 拓撲結(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-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 拓撲結(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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