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參數(shù)資料
| 型號： | LM2641-ADJMDC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | DUAL SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, UUC |
| 封裝： | DIE |
| 文件頁數(shù)： | 9/22頁 |
| 文件大?。?/td> | 531K |
| 代理商： | LM2641-ADJMDC |
Design Procedure (Continued)
Solid Tantalum capacitors are best in applications which
must operate over a wide temperature range. A good quality
Tantalum will typically exhibit less than 2:1 change in ESR
over the temperature range of +125C to 40C. Recom-
mended types are Sprague 593D, Sprague 594D, and AVX
TPS series.
Selecting An Output Capacitor
The required value of output capacitance is directly related
to the specification for the maximum amount of output volt-
age ripple allowed in the application. Since ESR effects the
ripple voltage, it is important to have a guideline for ESR.
The maximum allowed ESR can be calculated as follows.
V
RIPPLE =IRIPPLE *ESR(max)
Using V = Ldi/dt
V
OUT =L *IRIPPLE/{(1D)TS}=L *IRIPPLE *FS/(1D)
I
RIPPLE =VOUT*(1D)/)L *FS)
ESR(max) = V
RIPPLE/IRIPPLE
A reasonable value for C
OUT can be obtained by choosing
capacitors with net ESR less than 12 of ESR(max).
Hence,
ESR(max) = V
RIPPLE*L* FS/{VOUT(1D)}
The value of C
OUT necessary to meet the voltage ripple
specification can be found using the approximation:
Where:
I
RIPPLE is the inductor ripple current.
V
RIPPLE is the output ripple voltage.
ESR is the equivalent series resistance of the output capaci-
tor.
F is the switching frequency, F
S.
T
S = 1/FS.
D = Duty Cycle.
The ESR term predominates in determining output ripple
voltage. Good quality Tantalum capacitors have guaranteed
maximum specifications for ESR, but the typical values for
ESR are usually considerably lower than the maximum limit.
POWER MOSFETs
Two N-channel logic-level MOSFETs are required for each
output. The voltage rating should be at least 1.2 times the
maximum input voltage.
Maximizing efficiency for a design requires selecting the right
FET. The ON-resistance of the FET determines the ON-state
(conduction) losses, while gate charge defines the losses
during switch transitions. These two parameters require a
trade-off, since reducing ON-resistance typically requires
increasing gate capacitance (which increases the charge
required to switch the FET). Improved FETs are currently
being released which are designed specifically for optimized
ON-resistance and gate charge characteristics.
The V
IN and VOUT for a specific application determines the
ON time of each switch. In some cases where one FET is on
most of the time, efficiency may be improved slightly by
selecting a low ON-resistance FET for one of the FET
switches and a different type with lower gate charge require-
ment for the other FET switch. However, for most applica-
tions this would give no measurable improvement.
CURRENT SENSE RESISTOR
A sense resistor is placed between the inductor and the
output capacitor to measure the inductor current. The value
of this resistor is set by the current limit voltage of the
LM2641 (see Electrical Characteristics) and the maximum
(peak) inductor current. The value of the sense resistor can
be calculated from:
Where:
V
CL(MIN) is the minimum specified current limit voltage (see
Electrical Characteristics).
I
MAX is the maximum output current for the application.
I
RIPPLE is the inductor ripple current for the application.
TOL is the tolerance (in %) of the sense resistor.
The physical placement of the sense resistors should be as
close as possible to the LM2641 to minimize the lead length
of the connections to the CSH and CSL pins. Keeping short
leads on these connections reduces the amount of switching
noise conducted into the current sense circuitry of the
LM2641.
EXTERNAL DIODES
FET Diodes
Both of the low-side MOSFET switches have an external
Schottky diode connected from drain to source. These di-
odes are electrically in parallel with the intrinsic body diode
present inside the FET. These diodes conduct during the
dead time when both FETs are off and the inductor current
must be supplied by the catch diode (which is either the body
diode or the Schottky diode).
Converter efficiency is improved by using external Schottky
diodes. Since they have much faster turn-off recovery than
the FET body diodes, switching losses are reduced.
The voltage rating of the Schottky must be at least 25%
higher than the maximum input voltage. The average current
rating of the diode needs to be only about 30% of the output
current, because the duty cycle is low.
The physical placement of the Schottky diode must be as
close as possible to the FET, since any parasitic (lead)
inductance in series with the Schottky will slow its turn-ON
and cause current to flow through the FET body diode.
Bootstrap Diodes
As shown in the block diagram for the LM2641, the CBOOT
pin has an internal diode which is connected to the 5V
internal rail (which is also connected to the LIN pin). This
diode charges up the bootstrap capacitor to about 5V when
the low-side FET switch turns ON and pulls its drain down to
ground. The internal diode works well until the pulse widths
get extremely narrow, and then the charge applied to the
bootstrap capacitor can become insufficient to fully turn ON
the gate of the FET.
For this reason, an external diode should be used which
connects directly between the bootstrap capacitor and the
external capacitor connected to the LIN pin (C17). A
fast-recovery silicon diode should be used which has an
average current rating
≥ 50 mA, with voltage rating > 30V.
LM2641
www.national.com
17
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2641MTC-ADJ | 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 |
| LM2641MTC-ADJ/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 |
| LM2641MTCX-ADJ | 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 |
| LM2641MTCX-ADJ/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 |
| LM2642 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述: |
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