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參數(shù)資料
| 型號(hào)： | LM1770TMFX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SOT-23 Synchronous Buck Controller |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO5 |
| 封裝： | SOT-23, 5 PIN |
| 文件頁(yè)數(shù)： | 10/14頁(yè) |
| 文件大?。?/td> | 687K |
| 代理商： | LM1770TMFX |
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Design Guide
(Continued)
profile unshielded inductor is recommended to reduce EMI.
This also helps prevent any spurious noise from picking up
on the feedback node resulting in unexpected tripping of the
feedback comparator.
OUTPUT CAPACITOR
One of the most important components to select with the
LM1770 is the output capacitor. This is because its size and
ESR have a direct effect on the stability of the loop. A
constant on-time control scheme works by sensing the out-
put voltage ripple and switching the FETs appropriately. The
output voltage ripple on a buck converter can be approxi-
mated by stating that the AC inductor ripple flows entirely
into the output capacitor and is created by the ESR of the
capacitor. This can be expressed in the following equation:
V
OUT
=
I
L
x R
ESR
To ensure stability, two constraints need to be met. The first
is that there is sufficient ESR to create enough voltage ripple
at the feedback pin. The recommendation is to have at least
10mV of ripple seen at the feedback pin. This can be calcu-
lated by multiplying the output voltage ripple by the gain
seen through the feedback resistors. This gain, H, can be
calculated below:
If the output voltage is fairly high, causing significant attenu-
ation through the feedback resistors, a feed-forward capaci-
tor can be used. This is actually recommended for most
circuits as it improves performance. See the feed-forward
capacitor section for more details.
The second criteria is to ensure that there is sufficient ripple
at the output that is in-phase with the switch. The problem
exists that there is actually ripple caused by the capacitor
charging and discharging, not only the ESR ripple. Since
these are effectively out of phase, problems can exist. To
avoid this issue it is recommended that the ratio of the two
ripples (
β
) is always greater than 5. To calculate the mini-
mum ESR value needed, the following equation can be
used.
In general the best capacitors to use are chemistries that
have a known and consistent ESR across the entire operat-
ing temperature range. Tantalum capacitors or similar chem-
istries such as Niobium Oxide perform well along with certain
families of Aluminum Electrolytics. Small value POSCAPs
and SP CAPs also work as they have sufficient ESR. When
used in conjunction with a low value inductor it is possible to
have an extremely stable design. The only capacitors that
require modification to the circuit are ceramic capacitors.
Ceramic capacitors cause problems meeting both criteria
because they have low ESR and low capacitance. There-
fore, if they are to be used, an external ESR resistor (RSNS)
should be added. This can be seen below in the following
circuit.
20166218
This circuit uses an additional resistor in series with the
inductor to add ripple at the output. It is placed in this location
and used in combination with the feed-forward capacitor
(C
FF
) to provide ripple to the feedback pin, without adding
ripple or a DC offset to the output. The benefit of using a
ceramic capacitor is still obtained with this technique. Be-
cause the addition of the resistor results in power loss, this
circuit implementation is only recommended for low currents
(2A and below). The power loss and rating of the resistor
should be taken into account when selecting this compo-
nent.
This circuit implementation utilizing the feed-forward capaci-
tor begins to experience limitations when the output voltage
is small. Previously the circuit relied on the C
FF
for all the
ripple at the feedback node by assuming that the resistor
divider was negligible. As V
OUT
decreases this can not be
assumed. The resistor divider contributes a larger amount of
ripple which is problematic as it is also out of phase. There-
fore the resistor location should be changed to be in series
with the output capacitor. This can be viewed as adding an
effective ESR to the output capacitor.
L
www.national.com
10
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM1770TMFX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM1770UMF | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM1770UMF/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM1770UMFX | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM1770UMFX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
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