LM2586S-12/NOPB National Semiconductor, LM2586S-12/NOPB Datasheet - Page 26

• Manufacturer Part Number: LM2586S-12/NOPB
• Description: IC REG SIMPLE SWITCHER TO-263-7
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Up (Boost), Flyback, Forward Converterr

Specifications of LM2586S-12/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 12V
• Current - Output: 3A
• Frequency - Switching: 100kHz ~ 200kHz
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: D²Pak, TO-263 (7 leads + tab)
• For Use With: 551011367-051 - BOARD WEBENCH BUILD IT LM2586/88
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2586S-12; *LM2586S-12/NOPB; LM2586S-12

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Application Hints
with an external current limit circuit. The external limit should
be set to the maximum switch current of the device, which is
3A.
In a flyback regulator application (Figure 43), using the stan-
dard transformers, the LM2586 will survive a short circuit to
the main output. When the output voltage drops to 80% of its
nominal value, the frequency will drop to 25 kHz. With a
lower frequency, off times are larger. With the longer off
times, the transformer can release all of its stored energy
before the switch turns back on. Hence, the switch turns on
initially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the
device.
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 µF ceramic capacitor between V
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
maximum input voltage, V
where V
and is typically 0.5V for Schottky diodes and 0.8V for ultra-
fast recovery diodes. In certain circuits, there exists a volt-
age spike, V
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 in Figure 4
and other flyback regulator circuits throughout the
F
is the forward biased voltage of the output diode,
V
SW(OFF)
LL
, superimposed on top of the steady-state
= V
IN
IN
(Max) + (V
(Max):
(Continued)
OUT
+V
F
)/N
OUT
FIGURE 43. Flyback Regulator
, and the
IN
and
26
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 43). Both are required due to
the inherent operation of a flyback regulator. To keep a
stable or constant voltage supply to the LM2586, a storage
capacitor (≥100 µF) 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 for the input
capacitor. The storage capacitor will also attenuate noise
which may interfere with other circuits connected to the
same input supply voltage.
datasheet). The schematic in Figure 43 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 5). 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 LM2586 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 5) 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 43. 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
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