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參數(shù)資料
| 型號： | LM2696 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3A, Constant On Time Buck Regulator |
| 中文描述： | 第3A，恒定時間降壓穩(wěn)壓器 |
| 文件頁數(shù)： | 11/19頁 |
| 文件大?。?/td> | 833K |
| 代理商： | LM2696 |
Application Information
(Continued)
While C
OUT
and V
OUT
control the slew rate of the output
voltage, the total amount of time the LM2696 takes to startup
is dependent on two other terms. See the “Startup” section
for more information.
EXTV
CC
CAPACITOR
External V
is a 3.65V rail generated by an internal sub-
regulator that powers the parts internal circuitry. This rail
should be bypassed with a 1 μF ceramic capacitor (X5R or
equivalent dielectric). Although EXTV
is for internal use, it
can be used as an external rail for extremely light loads (
<
50
μA). If EXTV
is accidentally shorted to GND the part is
protected by a 5 mA current limit. This rail also has an
under-voltage lockout that will prevent the part from switch-
ing if the EXTV
CC
voltage drops.
SHUTDOWN
The state of the shutdown pin enables the device or places
it in a sleep state. This pin has an internal pull-up and may be
left floating or connected to a high logic level. Connecting
this pin to GND will shutdown the part. Shutting down the
part will prevent the part from switching and reduce the
quiescent current drawn by the part. This pin must be by-
passed with a 1 nF ceramic capacitor (X5R or Y5V) to
ensure proper logic thresholds.
CBOOT CAPACITOR
The purpose of an external bootstrap capacitor is to turn the
FET on by using the SW node as a pedestal. This allows the
voltage on the CBOOT pin to be greater than V
IN
. Whenever
the catch diode is conducting and the SW node is at GND,
an internal diode will conduct that charges the CBOOT ca-
pacitor to approximately 4V. When the SW node rises, the
CBOOT pin will rise to approximately 4V above the SW
node. For optimal performance, a 0.1 μF ceramic capacitor
(X5R or equivalent dielectric) should be used.
PGOOD RESISTOR
The PGOOD resistor is used to pull the PGOOD pin high
whenever a steady state operating range is achieved. This
resistor needs to be sized to prevent excessive current from
flowing into the PGOOD pin whenever the open drain FET is
turned on. The recommendation is to use a 10 k
–100 k
resistor. This range of values is a compromise between rise
time and power dissipation.
CATCH DIODE
The catch or freewheeling diode acts as the bottom switch in
a non-synchronous buck switcher. Because of this, the diode
has to handle the full output current whenever the FET is not
conducting. Therefore, it must be sized appropriately to
handle the current. The average current through the diode
can be calculated by the equation:
I
D_AVG
= I
OUT
(1–D)
Care should also be taken to ensure that the reverse voltage
rating of the diode is adequate. Whenever the FET is con-
ducting the voltage across the diode will be approximately
equal to V
. It is recommended to have a reverse rating that
is equal to 120% of V
to ensure adequate guard banding
against any ringing that could occur on the switch node.
Selection of the catch diode is critical to overall switcher
performance. To obtain the optimal performance, a Schottky
diode should be used due to their low forward voltage drop
and fast recovery.
BYPASS CAPACITOR
A bypass capacitor must be used on the AV
line to help
decouple any noise that could interfere with the analog
circuitry. Typically, a small (1 μF) ceramic capacitor is placed
as close as possible to the AVIN pin.
EXTERNAL OPERATION STARTUP
The total startup time, from the initial V
rise to the time
V
OUT
reaches its nominal value is determined by three sepa-
rate steps. Upon the rise of V
IN
, the first step to occur is that
the EXTV
CC
voltage has to reach its nominal output voltage
of 3.65V before the internal circuitry is active. This time is
dictated by the output capacitance (1 μF) and the current
limit of the regulator (5 mA typical), which will always be on
the order of 730 μs. Upon reaching its steady state value, an
internal delay of 200 μs will occur to ensure stable operation.
Upon completion the LM2696 will begin switching and the
output will rise. The rise time of the output will be governed
by the soft-start capacitor. To highlight these three steps a
timing diagram please refer to
Figure 3
.
L
www.national.com
11
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| LM2696EVAL | 功能描述:電源管理IC開發(fā)工具 LM2696 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM2696MXA | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM2696MXA/NOPB | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 3A,CONSTANT ON TIME BUCK REG RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
| LM2696MXAX | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 4.5V to 24V 16-Pin TSSOP EP T/R |
| LM2696MXAX/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
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