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參數(shù)資料
| 型號(hào)： | LM2717MTX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Step-down DC/DC Converter |
| 中文描述： | 3.2 A SWITCHING REGULATOR, 600 kHz SWITCHING FREQ-MAX, PDSO24 |
| 封裝： | TSSOP-24 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 673K |
| 代理商： | LM2717MTX |
Buck Operation
PROTECTION (BOTH REGULATORS)
The LM2717 has dedicated protection circuitry running dur-
ing normal operation to protect the IC. The Thermal Shut-
down circuitry turns off the power devices when the die
temperature reaches excessive levels. The UVP comparator
protects the power devices during supply power startup and
shutdown to prevent operation at voltages less than the
minimum input voltage. The OVP comparator is used to
prevent the output voltage from rising at no loads allowing
full PWM operation over all load conditions. The LM2717
also features a shutdown mode for each converter decreas-
ing the supply current to approximately 10μA (both in shut-
down mode).
CONTINUOUS CONDUCTION MODE
The LM2717 contains current-mode, PWM buck regulators.
A buck regulator steps the input voltage down to a lower
output voltage. In continuous conduction mode (when the
inductor current never reaches zero at steady state), the
buck regulator operates in two cycles. The power switch is
connected between V
IN
and SW1 and SW2.
In the first cycle of operation the transistor is closed and the
diode is reverse biased. Energy is collected in the inductor
and the load current is supplied by C
OUT
and the rising
current through the inductor.
During the second cycle the transistor is open and the diode
is forward biased due to the fact that the inductor current
cannot instantaneously change direction. The energy stored
in the inductor is transferred to the load and output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D' will be
required for design calculations.
DESIGN PROCEDURE
This section presents guidelines for selecting external com-
ponents.
SETTING THE OUTPUT VOLTAGE (ADJUSTABLE
REGULATOR)
The output voltage is set using the feedback pin and a
resistor divider connected to the output as shown in
Figure 1
.
The feedback pin voltage is 1.26V, so the ratio of the feed-
back resistors sets the output voltage according to the fol-
lowing equation:
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is
needed betwen the input pin and power ground. This capaci-
tor prevents large voltage transients from appearing at the
input. The capacitor is selected based on the RMS current
and voltage requirements. The RMS current is given by:
The RMS current reaches its maximum (I
/2) when
V
equals 2V
. This value should be calculated for both
regulators and added to give a total RMS current rating. For
an aluminum or ceramic capacitor, the voltage rating should
be at least 25% higher than the maximum input voltage. If a
tantalum capacitor is used, the voltage rating required is
about twice the maximum input voltage. The tantalum ca-
pacitor should be surge current tested by the manufacturer
to prevent being shorted by the inrush current. The minimum
capacitor value should be 47μF for lower output load current
applications and less dynamic (quickly changing) load con-
ditions. For higher output current applications or dynamic
load conditions a 68μF to 100μF low ESR capacitor is rec-
ommended. It is also recommended to put a small ceramic
capacitor (0.1μF to 4.7μF) between the input pins and
ground to reduce high frequency spikes.
INDUCTOR SELECTION
The most critical parameters for the inductor are the induc-
tance, peak current and the DC resistance. The inductance
is related to the peak-to-peak inductor ripple current, the
input and the output voltages (for 300kHz operation):
A higher value of ripple current reduces inductance, but
increases the conductance loss, core loss, and current
stress for the inductor and switch devices. It also requires a
bigger output capacitor for the same output voltage ripple
requirement. A reasonable value is setting the ripple current
to be 30% of the DC output current. Since the ripple current
increases with the input voltage, the maximum input voltage
is always used to determine the inductance. The DC resis-
tance of the inductor is a key parameter for the efficiency.
Lower DC resistance is available with a bigger winding area.
A good tradeoff between the efficiency and the core size is
letting the inductor copper loss equal 2% of the output
power.
OUTPUT CAPACITOR
The selection of C
is driven by the maximum allowable
output voltage ripple. The output ripple in the constant fre-
quency, PWM mode is approximated by:
The ESR term usually plays the dominant role in determining
the voltage ripple. Low ESR ceramic, aluminum electrolytic,
or tantalum capacitors (such as Taiyo Yuden MLCC, Nichi-
con PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX
TPS, and CDE polymer aluminum) is recommended. An
electrolytic capacitor is not recommended for temperatures
below 25C since its ESR rises dramatically at cold tem-
perature. Ceramic or tantalum capacitors have much better
ESR specifications at cold temperature and is preferred for
low temperature applications.
L
www.national.com
9
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| LM2717MTX/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 |
| LM2717MTX-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 |
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