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參數(shù)資料
| 型號(hào)： | LM3218SE |
| 廠(chǎng)商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, DSO8 |
| 封裝： | LTCC-8 |
| 文件頁(yè)數(shù)： | 5/16頁(yè) |
| 文件大?。?/td> | 4709K |
| 代理商： | LM3218SE |
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V
CON,ON
The output is disabled when V
CON is below 125 mV (typ.). It
is enabled when V
CON is above 150 mV (typ.). The threshold
has about 25 mV (typ.) of hysteresis.
Current Limiting
A current limit feature allows the LM3218 to protect itself and
external components during overload conditions. In PWM
mode, an 1100 mA (typ.) cycle-by-cycle current limit is nor-
mally used when V
CON is above 0.42V (typ.), and an 800 mA
(typ.) is used when V
CON is below 0.40V (typ.). If an excessive
load pulls the output voltage down to approximately 0.375V,
then the device switches to a timed current limit mode when
V
CON 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 LM3218 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The out-
put 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 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, promot-
ing longer battery life. See Setting the Output Voltage in the
Application Information section for further details. The
LM3218 moves into Pulse Skipping mode when duty cycle is
over 92% and the output voltage ripple increases slightly.
Thermal Overload Protection
The LM3218 has a thermal overload protection function that
operates to protect itself from short-term misuse and overload
conditions. When the junction temperature exceeds around
150°C, the device inhibits operation. Both the PFET and the
NFET are turned off in PWM mode. When the temperature
drops below 125°C, normal operation resumes. Prolonged
operation in thermal overload conditions may damage the de-
vice and is considered bad practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3218 features a pin-controlled variable output voltage
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 VCON
When V
CON is between 0.32V and 1.44V, the output voltage
will follow proportionally by 2.5 times of V
CON.
If V
CON is over 1.44V (VOUT = 3.6V), sub-harmonic oscillation
may occur because of insufficient slope compensation. If
V
CON voltage is less than 0.32V (VOUT = 0.8V), the output
voltage may not be regulated due to the required on-time be-
ing less than the minimum on-time (55 ns). The output voltage
can go lower than 0.8V providing a limited V
IN range is used.
Refer to datasheet curve (V
CON 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. 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 25 mV (typ.) hysteresis. The quiescent current
when (V
CON = 0V and VEN = Hi) is around 600 A.
ESTIMATION OF MAXIMUM OUTPUT CURRENT
CAPABILITY
Referring to Figure 3, the Inductor peak-to-peak ripple current
can be estimated by:
I
IND_PP = (VIN - VOUT ) × VOUT / (L1 × FSW × VIN)
Where, Fsw is switching frequency.
Therefore, maximum output current can be calculated by:
I
OUT_MAX = ILIM - 0.5 × IIND_PP
For the worst case calculation, the following parameters
should be used:
F
SW (Lowest switching frequency): 1.8 MHz
I
LIM (Lowest current limit value): 985 mA
L1 (Lowest inductor value): refer to inductor datasheet. Note
that inductance will drop with DC bias current and tempera-
ture. The worst case is typically at 85°C.
For example, V
IN = 4.2V, VOUT = 3.2V, L1 = 2.0 H (Induc-
tance value at 985 mA DC-bias current and 85°C), F
SW = 1.8
MHz , I
LIM = 985 mA.
I
IND_PP = 212 mA
I
OUT_MAX = 985 – 106 = 876 mA
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 ac-
count when calculating maximum output current. Special at-
tention 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, transient re-
sponse requirements may not be met for output current
greater than 650 mA.
INDUCTOR SELECTION
The inductor is an integrated LTCC 2.6 H substrate within
the LM3218 module and has a saturation current rating over
1200 mA. The integrated inductor’s low 1.2 mm maximum
height provides ease of use into small design constraints. In-
tegrating the inductor can eliminate layout issues associated
with DC/DC converters and reduce potential EMI problems.
CAPACITOR SELECTION
The LM3218 is designed for use with ceramic capacitors for
its input and output filters. Use a 10 F ceramic capacitor for
input and a 4.7 F ceramic capacitor for output. They should
maintain at least 50% capacitance at DC bias and tempera-
ture conditions. Ceramic capacitor types such as X5R, X7R
and B are recommended for both filters. Table 1 lists some
suggested part numbers and suppliers. DC bias characteris-
tics of the capacitors must be considered when selecting the
voltage rating and case size of the capacitor. If it is necessary
to choose a 0603-size capacitor for C
IN and COUT, the oper-
ation of the LM3218 should be carefully evaluated on the
system board. Use of multiple 2.2 F or 1 F capacitors in
parallel may also be considered.
13
www.national.com
LM3218
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相關(guān)代理商/技術(shù)參數(shù) |
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| LM3218SEE/NOPB | 功能描述:DC/DC 開(kāi)關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)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 |
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| LM3218SEX/NOPB | 功能描述:DC/DC 開(kāi)關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)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 |
| LM321AH | 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:LM221/LM321 Precision Preamplifiers |
| LM321H | 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:LM221/LM321 Precision Preamplifiers |
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