- 您現(xiàn)在的位置：買賣IC網(wǎng) > PDF目錄358827 > LM2696MXAX (NATIONAL SEMICONDUCTOR CORP) 3A, Constant On Time Buck Regulator PDF資料下載
參數(shù)資料
| 型號： | LM2696MXAX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3A, Constant On Time Buck Regulator |
| 中文描述： | 6.4 A SWITCHING REGULATOR, 500 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | TSSOP-16 |
| 文件頁數(shù)： | 8/19頁 |
| 文件大?。?/td> | 833K |
| 代理商： | LM2696MXAX |
Application Information
(Continued)
limit. Following the termination of the on-time, a non-
resetable extended off timer is initiated. The length of the
off-time is proportional to the feedback voltage. When FB =
0V the off-time is preset to 20 μs. This condition is often a
result of in short circuit operation when a maximum amount
of off-time is required. This amount of time ensures safe
short circuit operation up to the maximum input voltage of
24V.
In cases of overload (not complete short circuit, FB
>
0V)
the current limit off-time is reduced. Reduction of the off-time
during smaller overloads reduces the amount of fold back.
This also reduces the initial startup time.
N-CHANNEL HIGH SIDE SWITCH AND DRIVER
The LM2696 utilizes an integrated N-Channel high side
switch and associated floating high voltage gate driver. This
gate driver circuit works in conjunction with an external
bootstrap capacitor and an internal diode. The minimum
off-time (165 ns) is set to ensure that the bootstrap capacitor
has sufficient time to charge.
THERMAL SHUTDOWN
An internal thermal sensor is incorporated to monitor the die
temperature. If the die temp exceeds 165
o
C then the sensor
will trip causing the part to stop switching. Soft-start will
restart after the temperature falls below 155
o
C.
COMPONENT SELECTION
As with any DC-DC converter, numerous trade-offs are
present that allow the designer to optimize a design for
efficiency, size and performance. These trade-offs are taken
into consideration throughout this section.
The first calculation for any buck converter is duty cycle.
Ignoring voltage drops associated with parasitic resistances
and non-ideal components, the duty cycle may be expressed
as:
A duty cycle relationship that considers the voltage drop
across the internal FET and voltage drop across the external
catch diode may be expressed as:
Where V
is the forward voltage of the external catch diode
(D
) and V
SW
is the voltage drop across the internal
FET.
FREQUENCY SELECTION
Switching frequency affects the selection of the output induc-
tor, capacitor, and overall efficiency. The trade-offs in fre-
quency selection may be summarized as; higher switching
frequencies permit use of smaller inductors possibly saving
board space at the trade-off of lower efficiency. It is recom-
mended that a nominal frequency of 300 kHz should be used
in the initial stages of design and iterated if necessary.
The switching frequency of the LM2696 is set by the resistor
connected to the RON pin. This resistor controls the current
flowing into the RON pin and is directly related to the on-time
pulse. Connecting a resistor from this pin to PVIN allows the
switching frequency to remain constant as the input voltage
changes. In normal operation this pin is approximately 0.65V
above GND. In shutdown, this pin becomes a high imped-
ance node to prevent current flow.
The on time may be exoressed as:
Where V
IN
is the voltage at the high side of the R
ON
resistor
(typically PV
IN
), V
D
is the diode voltage present at the RON
pin (0.65V typical), R
ON
is in k
, and k
ON
is a constant value
set internally (66 μA
μs nominal). This equation can be
re-arranged such that R
ON
is a function of switching fre-
quency:
Where f
SW
is in kHz.
In CCM the frequency may be determined using the relation-
ship:
(T
ON
is in μs)
Which is typically used to set the switching frequency.
Under no condition should a bypass capacitor be connected
to the R
ON
pin. Doing so couples any AC perturbations into
the pin and prevents proper operation.
INDUCTOR SELECTION
Selecting an inductor is a process that may require several
iterations. The reason for this is that the size of the inductor
influences the amount of ripple present at the output that is
critical to the stability of an adaptive on-time circuit. Typically,
an inductor is selected such that the maximum peak-to-peak
ripple current is equal to 30% of the maximum load current.
The inductor current ripple (
I
L
) may be expressed as:
Therefore, L can be initially set to the following by applying
the 30% guideline:
The other features of the inductor that should be taken into
account are saturation current and core material. A shielded
inductor or low profile unshielded inductor is recommended
to reduce EMI.
L
www.national.com
8
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM2696 | 3A, Constant On Time Buck Regulator |
| LM26CIM5-PHA | Accurate, Factory Preset Thermostat |
| LM26CIM5-SHA | Accurate, Factory Preset Thermostat |
| LM26CIM5X-ZHA | Accurate, Factory Preset Thermostat |
| LM26CIM5-KLA | Accurate, Factory Preset Thermostat |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| 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 |
| LM2698EVAL | 功能描述:電源管理IC開發(fā)工具 EVAL BOARDFOR LM2698 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 |
| LM2698MM-ADJ | 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 |
| LM2698MM-ADJ/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 1.35A BOOST REG 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 |
| LM2698MM-ADJ/NOPB | 制造商:Texas Instruments 功能描述:DC/DC Converter (DC-DC) / Switching Regu |
發(fā)布緊急采購，3分鐘左右您將得到回復(fù)。