參數(shù)資料
型號(hào)： LM2585-12MDC
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： 7 A SWITCHING REGULATOR, 125 kHz SWITCHING FREQ-MAX, UUC
封裝： DIE
文件頁(yè)數(shù)： 14/29頁(yè)
文件大小： 708K
代理商： LM2585-12MDC
Application Hints (Continued)
FLYBACK REGULATOR INPUT CAPACITORS
A flyback regulator draws discontinuous pulses of current
from the input supply. Therefore, there are two input capaci-
tors needed in a flyback regulator; one for energy storage
and one for filtering (see
Figure 39). Both are required due to
the inherent operation of a flyback regulator. To keep a
stable or constant voltage supply to the LM2585, a storage
capacitor (
≥100 F) is required. If the input source is a
rectified DC supply and/or the application has a wide tem-
perature range, the required rms current rating of the capaci-
tor might be very large. This means a larger value of capaci-
tance or a higher voltage rating will be needed of the input
capacitor. The storage capacitor will also attenuate noise
which may interfere with other circuits connected to the
same input supply voltage.
In addition, a small bypass capacitor is required due to the
noise generated by the input current pulses. To eliminate the
noise, insert a 1.0 F ceramic capacitor between V
IN and
ground as close as possible to the device.
SWITCH VOLTAGE LIMITS
In a flyback regulator, the maximum steady-state voltage
appearing at the switch, when it is off, is set by the trans-
former turns ratio, N, the output voltage, V
OUT, and the
maximum input voltage, V
IN (Max):
V
SW(OFF) =VIN (Max) + (VOUT +VF)/N
where V
F is the forward biased voltage of the output diode,
and is 0.5V for Schottky diodes and 0.8V for ultra-fast recov-
ery diodes (typically). In certain circuits, there exists a volt-
age spike, V
LL, superimposed on top of the steady-state
voltage (see
Figure 5, waveform A). Usually, this voltage
spike is caused by the transformer leakage inductance
and/or the output rectifier recovery time. To “clamp” the
voltage at the switch from exceeding its maximum value, a
transient suppressor in series with a diode is inserted across
the transformer primary (as shown in the circuit on the front
page and other flyback regulator circuits throughout the
datasheet). The schematic in
Figure 39 shows another
method of clamping the switch voltage. A single voltage
transient suppressor (the SA51A) is inserted at the switch
pin. This method clamps the total voltage across the switch,
not just the voltage across the primary.
If poor circuit layout techniques are used (see the “Circuit
Layout Guideline” section), negative voltage transients may
appear on the Switch pin (pin 4). Applying a negative voltage
(with respect to the IC’s ground) to any monolithic IC pin
causes erratic and unpredictable operation of that IC. This
holds true for the LM2585 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 4) can go nega-
tive when the switch turns on. The “ringing” voltage at the
switch pin is caused by the output diode capacitance and the
transformer leakage inductance forming a resonant circuit at
the secondary(ies). The resonant circuit generates the “ring-
ing” voltage, which gets reflected back through the trans-
former to the switch pin. There are two common methods to
avoid this problem. One is to add an RC snubber around the
output rectifier(s), as in
Figure 39. The values of the resistor
and the capacitor must be chosen so that the voltage at the
Switch pin does not drop below 0.4V. The resistor may
range in value between 10
and1k, and the capacitor will
vary from 0.001 F to 0.1 F. Adding a snubber will (slightly)
reduce the efficiency of the overall circuit.
The other method to reduce or eliminate the “ringing” is to
insert a Schottky diode clamp between pins 4 and 3
(ground), also shown in
Figure 39. This prevents the voltage
at pin 4 from dropping below 0.4V. The reverse voltage
rating of the diode must be greater than the switch off
voltage.
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the
maximum switch voltage minus a diode drop. In a flyback
regulator, the maximum output voltage is determined by the
turns ratio, N, and the duty cycle, D, by the equation:
V
OUT ≈ NxVIN xD/(1D)
The duty cycle of a flyback regulator is determined by the
following equation:
Theoretically, the maximum output voltage can be as large
as desired — just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from
increasing to infinity. The physical limitations are capaci-
tances and inductances in the LM2585 switch, the output
diode(s), and the transformer — such as reverse recovery
time of the output diode (mentioned above).
NOISY INPUT LINE CONDITION
A small, low-pass RC filter should be used at the input pin of
the LM2585 if the input voltage has an unusual large amount
of transient noise, such as with an input switch that bounces.
The circuit in
Figure 40 demonstrates the layout of the filter,
with the capacitor placed from the input pin to ground and
the resistor placed between the input supply and the input
pin. Note that the values of R
IN and CIN shown in the
schematic are good enough for most applications, but some
readjusting might be required for a particular application. If
efficiency is a major concern, replace the resistor with a
small inductor (say 10 H and rated at 100 mA).
STABILITY
All current-mode controlled regulators can suffer from an
instability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic
oscillations, a minimum value of inductance is required to
ensure stability for all boost and flyback regulators. The
minimum inductance is given by:
where V
SAT is the switch saturation voltage and can be
found in the Characteristic Curves.
DS012515-57
FIGURE 40. Input Line Filter
LM2585
www.national.com
21
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LM2585S-12/NOPB 功能描述:直流/直流開(kāi)關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開(kāi)關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT
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