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參數(shù)資料
| 型號(hào)： | LM3475 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Hysteretic PFET Buck Controller |
| 中文描述： | 遲滯PFET的降壓控制器 |
| 文件頁數(shù)： | 10/14頁 |
| 文件大?。?/td> | 305K |
| 代理商： | LM3475 |
Design Information
(Continued)
voltage regulated. The hysteretic control topology is well
suited to using ceramic output capacitors. However, ceramic
capacitors have a very low ESR, resulting in a 90 phase
shift of the output voltage ripple. This results in low operating
frequency and increased output ripple. To fix this problem a
low value resistor could be added in series with the ceramic
output capacitor. Although counter intuitive, this combination
of a ceramic capacitor and external series resistance provide
highly accurate control over the output voltage ripple. An-
other method is to add an external ramp at the FB pin as
shown in
Figure 3
. By proper selection of R1 and C2, the FB
pin sees faster voltage change than the output ripple can
cause. As a result, the switching frequency is higher while
the output ripple becomes lower. The switching frequency is
approximately:
Other types of capacitor, such as Sanyo POSCAP, OS-CON,
and Nichicon ’NA’ series are also recommended and may be
used without additional series resistance. For all practical
purposes, any type of output capacitor may be used with
proper circuit verification.
Capacitors with high ESL (equivalent series inductance) val-
ues should not be used. As shown in
Figure 1
, the output
ripple voltage contains a small step at both the high and low
peaks. This step is caused by and is directly proportional to
the output capacitor’s ESL. A large ESL, such as in an
electrolytic capacitor, can create a step large enough to
cause abnormal switching behavior.
INPUT CAPACITOR SELECTION
A bypass capacitor is required between V
IN
and ground. It
must be placed near the source of the external PFET. The
input capacitor prevents large voltage transients at the input
and provides the instantaneous current when the PFET turns
on. The important parameters for the input capacitor are the
voltage rating and the RMS current rating. Follow the manu-
facturer’s recommended voltage de-rating. RMS current and
power dissipation (PD) can be calculated with the equations
below:
DIODE SELECTION
The catch diode provides the current path to the load during
the PFET off time. Therefore, the current rating of the diode
must be higher than the average current through the diode,
which be calculated as shown:
I
D_AVE
= I
OUT
x (1 D)
The peak voltage across the catch diode is approximately
equal to the input voltage. Therefore, the diode’s peak re-
verse voltage rating should be greater than 1.3 times the
input voltage.
A Schottky diode is recommended, since a low forward
voltage drop will improve efficiency.
For high temperature applications, diode leakage current
may become significant and require a higher reverse voltage
rating to achieve acceptable performance.
P-CHANNEL MOSFET SELECTION
The PFET switch should be selected based on the maximum
Drain-Source voltage (VDS), Drain current rating (I
), maxi-
mum Gate-Source voltage (VGS), on resistance (R
),
and Gate capacitance. The voltage across the PFET when it
is turned off is equal to the sum of the input voltage and the
diode forward voltage. The VDS must be selected to provide
some margin beyond the sum of the input voltage and Vd.
Since the current flowing through the PFET is equal to the
current through the inductor, I
D
must be rated higher than the
maximum I
PK
. During switching, PGATE swings the PFET’s
gate from V
IN
to ground. Therefore, A PFET must be se-
lected with a maximum VGS larger than V
IN
. To insure that
the PFET turns on completely and quickly, refer to the
PGATE section.
20070120
FIGURE 3. External Ramp
L
www.national.com
10
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PDF描述 |
|---|---|
| LM3475MF | Hysteretic PFET Buck Controller |
| LM3475MFX | Hysteretic PFET Buck Controller |
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| LM3480IM3-3.3 | 100 mA, SOT-23, Quasi Low-Dropout Linear Voltage Regulator |
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3475EVAL | 功能描述:電源管理IC開發(fā)工具 LM3475 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評(píng)估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3475EVAL | 制造商:Texas Instruments 功能描述:EVAL BOARD, LM3475 HYSTERITIC STEP-DOWN |
| LM3475MF | 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 |
| LM3475MF/NOPB | 功能描述:DC/DC 開關(guān)控制器 HYSTERETIC PFET BUCK CONTROLLER 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 |
| LM3475MFX | 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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