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參數(shù)資料
| 型號： | LM3205TL |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers |
| 中文描述： | 1.2 A SWITCHING REGULATOR, 2300 kHz SWITCHING FREQ-MAX, PBGA8 |
| 封裝： | LEAD FREE, MICRO, SMD-8 |
| 文件頁數(shù)： | 13/18頁 |
| 文件大?。?/td> | 1285K |
| 代理商： | LM3205TL |
Current Limiting
(Continued)
mode, an 1200mA (max.) cycle-by-cycle current limit is nor-
mally used. If an excessive load pulls the output voltage
down to approximately 0.375V, then the device switches to a
timed current limit mode. In timed current limit mode the
internal PFET switch is turned off after the current compara-
tor trips and the beginning of the next cycle is inhibited for
3.5us to force the instantaneous inductor current to ramp
down to a safe value. The synchronous rectifier is off in
timed current limit mode. Timed current limit prevents the
loss of current control seen in some products when the
output voltage is pulled low in serious overload conditions.
Dynamically Adjustable Output
Voltage
The LM3205 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The
output can be set from 0.8V(typ.) to 3.6V(typ.) by changing
the voltage on the analog V
pin. This feature is useful in
PA applications where peak power is needed only when the
handset is far away from the base station or when data is
being transmitted. In other instances the transmitting power
can be reduced. Hence the supply voltage to the PA can be
reduced, promoting longer battery life. See
Setting the Out-
put Voltage
in the
Application Information section
for further
details.
Thermal Overload Protection
The LM3205 has a thermal overload protection function that
operates to protect itself from short-term misuse and over-
load conditions. When the junction temperature exceeds
around 150C, the device inhibits operation. Both the PFET
and the NFET are turned off in PWM mode. When the
temperature drops below 125C, normal operation resumes.
Prolonged operation in thermal overload conditions may
damage the device and is considered bad practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3205 features a pin-controlled variable output volt-
age to eliminate the need for external feedback resistors. It
can be programmed for an output voltage from 0.8V (typ.) to
3.6V (typ.) by setting the voltage on the V
CON
pin, as in the
following formula:
V
OUT
= 2.5 x V
CON
When V
is between 0.32V and 1.44V, the output voltage
will follow proportionally by 2.5 times of V
CON
.
If V
is over 1.44V (V
= 3.6V), sub-harmonic oscilla-
tion may occur because of insufficient slope compensation. If
V
voltage is less than 0.32V (V
= 0.8V), the output
voltage may not be regulated due to the required on-time
being less than the minimum on-time (50ns). The output
voltage can go lower than 0.8V providing a limited V
range
is used. Refer to datasheet curve (V
Voltage vs Output
Voltage) for details. This curve is for a typical part and there
could be part-to-part variation for output voltages less than
0.8V over the limited V
IN
range.
INDUCTOR SELECTION
A 3.3μH inductor with saturation current rating over 1200mA
and low inductance drop at the full DC bias condition is
recommended for almost all applications. The inductor’s DC
resistance should be less than 0.2
for good efficiency. For
low dropout voltage, lower DCR inductors are advanta-
geous. The lower limit of acceptable inductance is 2.3μH at
1200mAover the operating temperature range. Full attention
should be paid to this limit, because some small inductors
show large inductance drops at high DC bias. These can not
be used with the LM3205. Taiyo-Yuden NR3015T3R3M is an
example of an inductor with the lowest acceptable limit (as of
Nov./05).
Table 1
suggests some inductors and suppliers.
TABLE 1. Suggested inductors and their suppliers
Model
Size (WxLxH) [mm]
3.0 x 3.0 x 1.5
3.3 x 3.3 x 1.4
Vendor
Taiyo-Yuden
Coilcraft
NR3015T3R3M
DO3314-332MXC
If a smaller inductance inductor is used in the application, the
LM3205 may become unstable during line and load tran-
sients and V
CON
transient response times may get affected.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise-critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with footprints accommodating both
types for design flexibility. This allows substitution of a low-
noise toroidal inductor, in the event that noise from low-cost
bobbin models is unacceptable. Saturation occurs when the
magnetic flux density from current through the windings of
the inductor exceeds what the inductor’s core material can
support with a corresponding magnetic field. This can cause
poor efficiency, regulation errors or stress to a DC-DC con-
verter like the LM3205.
CAPACITOR SELECTION
The LM3205 is designed for use with ceramic capacitors for
its input and output filters. Use a 10μF ceramic capacitor for
input and a 4.7μF ceramic capacitor for output. They should
maintain at least 50% capacitance at DC bias and tempera-
ture conditions. Ceramic capacitors types such as X5R, X7R
are recommended for both filters. These provide an optimal
balance between small size, cost, reliability and performance
for cell phones and similar applications.
Table 2
lists some
suggested part numbers and suppliers. DC bias character-
istics of the capacitors must be considered when selecting
the voltage rating and case size of the capacitor. A few
manufactures can supply 4.7μF capacitors in the 0805 case
size which maintain at least 50% of their value, but TDK is
currently the only manufacturer which can provide such
capacitors in the 0603 case size. As of November, 2005, no
manufacture can supply 10μF capacitors in the 0603 case
size which maintain 50% of their value. If it is necessary to
choose a 0603-size capacitor for V
, the operation of the
LM3205 should be carefully evaluated on the system board.
Output capacitors with smaller case sizes mitigate piezo
electric vibrations when the output voltage is stepped up and
down at fast rates. However, they have a larger percentage
drop in value with dc bias. Use of multiple 2.2μF or 1μF
capacitors in parallel may also be considered.
TABLE 2. Suggested capacitors and their suppliers
Model
Vendor
AVX
TDK
TDK
0805ZD475KA 4.7μF, 10V
C1608X5R0J475M, 4.7μF, 6.3V
C2012X5R0J106M,10μF, 6.3V
L
www.national.com
13
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| LM3205TL/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 安裝風格: 封裝 / 箱體:CPAK |
| LM3205TLEV | 功能描述:電源管理IC開發(fā)工具 LM3205 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 |
| LM3205TLX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers |
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| LM3206 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:650mA Miniature, Step-Down DC-DC Converter for Portable Applications |
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