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參數資料
| 型號： | LM1770TMFX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩壓器 |
| 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 |
| 文件頁數： | 11/14頁 |
| 文件大小： | 687K |
| 代理商： | LM1770TMFX |
Design Guide
(Continued)
20166219
FEED-FORWARD CAPACITOR
The feed-forward capacitor is used across the top feedback
resistor to provide a lower impedance path for the high
frequency ripple without degrading the DC accuracy. Typi-
cally the value for this capacitor should be small enough to
prevent load transient errors because of the discharging
time, but large enough to prevent attenuation of the ripple
voltage. In general a small ceramic capacitor in the range of
1nF to 10nF is sufficient.
If C
is used then it can be assumed that the ripple voltage
seen at the feedback pin is the same as the ripple voltage at
the output. The attenuation factor H no longer needs to be
used. However, in these conditions, it is recommended to
have a minimum of 20mV ripple at the feedback pin. The use
of a C
FF
capacitor is recommended as it improves the regu-
lation and stability of the design. However, its benefit is
diminished as V
OUT
starts approaching V
REF
, therefore it is
not needed in this situation.
INPUT CAPACITOR
The dominating factor that usually sets an input capacitors’
size is the current handling ability. This is usually determined
by the package size and ESR of the capacitor. If these two
criteria are met then there usually should be enough capaci-
tance to prevent impedance interactions with the source. In
general it is recommended to use a ceramic capacitor for the
input as they provide a low impedance and small footprint.
One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over
temperature performance and also minimize the DC voltage
derating that occurs on Y5V capacitors. To calculate the
input capacitor RMS current, the equation below can be
used:
which can be approximated by,
MOSFET Selection
The two FETs used in the LM1770 requires attention to
selection of parameters to ensure optimal performance of
the power supply. The high side FET should be a PFET and
the low side an NFET. These can be integrated in one
package or as two separate packages. The criteria that
matter in selection are listed below:
VDS VOLTAGE RATING
The first selection criteria is to select FETs that have suffi-
cient V
voltage ratings to handle the maximum voltage
seen at the input plus any transient spikes that can occur
from parasitic ringing. In general most FETs available for this
application will have ratings from 8V to 20V. If a larger
voltage rating is used then the performance will most likely
be degraded because of higher gate capacitance.
RDSON
The R
DS(ON)
specification is important as it determines sev-
eral attributes of the FET and the overall power supply. The
first is that it sets the maximum current of the FET for a given
package. A lower R
will permit a higher allowable
current and reduce conduction losses, however, it will in-
crease the gate capacitance and the switching losses.
GATE DRIVE
The next step is to ensure that the FETs are capable of
switching at the low Vin supplies used by the LM1770. The
FET should have the Rdson specified at either 1.8V or 2.5V
to ensure that it can switch effectively as soon as the
LM1770 starts up.
GATE CHARGE
Because the LM1770 utilizes a fixed dead-time scheme to
prevent cross conduction, the FET transitions must occur in
this time. The rise and fall time of the FETs gate can be
influenced by several factors including the gate capacitance.
L
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11
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相關代理商/技術參數 |
參數描述 |
|---|---|
| LM1770TMFX/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770UMF | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770UMF/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770UMFX | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770UMFX/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
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