LM2586S-12 National Semiconductor, LM2586S-12 Datasheet - Page 22

• Manufacturer Part Number: LM2586S-12
• Description: SIMPLE SWITCHER 3A Flyback Regulator with Shutdown
• Manufacturer: National Semiconductor

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Application Hints
FREQUENCY SYNCHRONIZATION
Another feature of the LM2586 is the ability to synchronize
the switching frequency to an external source, using the
sync pin (pin 6). This feature allows the user to parallel mul-
tiple devices to deliver more output power.
A negative falling pulse applied to the sync pin will synchro-
nize the LM2586 to an external oscillator (see Figures 40,
41 ).
Use of this feature enables the LM2586 to be synchronized
to an external oscillator, such as a system clock. This opera-
PROGRAMMING OUTPUT VOLTAGE
(SELECTING R1 AND R2)
Referring to the adjustable regulator in Figure 42 , the output
voltage is programmed by the resistors R1 and R2 by the fol-
lowing formula:
Resistors R1 and R2 divide the output voltage down so that
it can be compared with the 1.23V internal reference. With
R2 between 1k and 5k, R1 is:
For best temperature coefficient and stability with time, use
1% metal film resistors.
SHORT CIRCUIT CONDITION
Due to the inherent nature of boost regulators, when the out-
put is shorted (see Figure 42 ), current flows directly from the
input, through the inductor and the diode, to the output, by-
passing the switch. The current limit of the switch does not
limit the output current for the entire circuit. To protect the
load and prevent damage to the switch, the current must be
V
R1 = R2 (V
OUT
FIGURE 40. Frequency Synchronization
= V
REF
OUT
(1 + R1/R2)
/V
REF
− 1)
(Continued)
where V
where V
REF
REF
FIGURE 42. Boost Regulator
= 1.23V
= 1.23V
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22
tion allows multiple power supplies to operate at the same
frequency,
problems.
The scope photo in Figure 41 shows a LM2586 12V Boost
Regulator synchronized to a 200 kHz signal. There is a 700
ns delay between the falling edge of the sync signal and the
turning on of the switch.
externally limited, either by the input supply or at the output
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 be-
fore the switch turns back on. Hence, the switch turns on ini-
tially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the de-
vice.
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
FIGURE 41. Waveforms of a Synchronized
thus
12V Boost Regulator
eliminating
DS012516-61
frequency-related
DS012516-69
noise