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參數(shù)資料
| 型號： | LM3208TLX |
| 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, 2200 kHz SWITCHING FREQ-MAX, PBGA8 |
| 封裝： | ROHS COMPLIANT, THIN MICRO SMD, 8 PIN |
| 文件頁數(shù)： | 14/17頁 |
| 文件大?。?/td> | 1651K |
| 代理商： | LM3208TLX |
R
DSON(P)
Management
(Continued)
device uses only a small part of the PFET to minimize drive
loss of the PFET. When V
is greater than 0.42V (typ.),
the entire PFET is used to minimize R
loss. This
threshold has about 20mV (typ.) of hysteresis.
V
CON,ON
The output is disabled when V
CON
is below 125mV (typ.). It
is enabled when V
CON
is above 150mV (typ.). The threshold
has about 25mV (typ.) of hysteresis.
Current Limiting
A current limit feature allows the LM3208 to protect itself and
external components during overload conditions. In PWM
mode, an 1100mA (typ.) cycle-by-cycle current limit is nor-
mally used when V
is above 0.42V (typ.), and an 800mA
(typ.) is used when V
is below 0.40V (typ.). If an exces-
sive load pulls the output voltage down to approximately
0.375V, then the device switches to a timed current limit
mode when V
is above 0.42V (typ.). In timed current limit
mode the internal PFET switch is turned off after the current
comparator 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 LM3208 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The
output can be set from 0.8V to 3.6V by changing the voltage
on the analog V
CON
pin. This feature is useful in PA applica-
tions where peak power is needed only when the handset is
far away from the base station or when data is being trans-
mitted. In other instances, the transmitting power can be
reduced. Hence the supply voltage to the PA can be re-
duced, promoting longer battery life. See
Setting the Output
Voltage
in the
Application Information section
for further
details. The LM3208 moves into Pulse Skipping mode when
duty cycle is over 92% and the output voltage ripple in-
creases slightly.
Thermal Overload Protection
The LM3208 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 LM3208 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 to 3.6V
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 (55ns). 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
range. When the control pin voltage
is more than 0.15V (typ.), the switches are turned on. When
it is less than 0.125V (typ.), the switches are turned off. This
on/off function has 25mV (typ.) hysteresis. The quiescent
current when (V
CON
= 0V and V
EN
= Hi) is around 600μA.
ESTIMATION OF MAXIMUM OUTPUT CURRENT
CAPABILITY
Referring to
Figure 3
, the Inductor peak to peak ripple cur-
rent can be estimated by:
I
IND_PP
= (V
IN
- V
OUT
) x V
OUT
/ (L1 x F
SW
x V
IN
)
Where, Fsw is switching frequency.
Therefore, maximum output current can be calculated by:
I
OUT_MAX
= I
LIM
- 0.5 x I
IND_PP
For the worst case calculation, the following parameters
should be used:
F
SW
(Lowest switching frequency): 1.8MHz
I
LIM
(Lowest current limit value): 985mA
L1 (Lowest inductor value): refer to inductor data-sheet.
Note that inductance will drop with DC bias current and
temperature. The worst case is typically at 85C.
For example, V
= 4.2V, V
= 3.2V, L1 = 2.0μH (Induc-
tance value at 985mA DC bias current and 85C), F
SW
=
1.8MHz , I
LIM
= 985mA.
I
IND_PP
= 212mA
I
OUT_MAX
= 985 – 106 = 876mA
The effects of switch, inductor resistance and dead time are
ignored. In real application, the ripple current would be 10%
to 15% higher than ideal case. This should be taken into
account when calculating maximum output current. Special
attention needs to be paid that a delta between maximum
output current capability and the current limit is necessary to
satisfy transient response requirements. In practice, tran-
sient response requirements may not be met for output
current greater than 650mA.
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 recom-
mended. The lower limit of acceptable inductance is 1.7μH
at 1200mA over the operating temperature range. Full atten-
tion should be paid to this limit, because some small induc-
tors show large inductance drops at high DC bias. These
cannot be used with the LM3208. FDK MIPW3226D3R0M is
an example of an inductor with the lowest acceptable limit
(as of Oct./05).
Table 1
suggests some inductors and suppli-
ers.
L
www.national.com
14
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3208TLX/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 |
| LM3208UR/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 |
| LM3208URX/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 |
| LM3209-G3 | 制造商:TI 制造商全稱:Texas Instruments 功能描述:LM3209-G3 PRODUCT BRIEF Seamless-Transition Buck-Boost Converter for Battery- Powered 3G/4G RF Power Amplifiers |
| LM3209TLE/NOPB | 功能描述:低壓差穩(wěn)壓器 - LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動電壓（最大值）:307 mV 輸出電流:1 A 負載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
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