Datasheet

LM2586
SNVS121D MAY 1996REVISED APRIL 2013
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PROGRAMMING OUTPUT VOLTAGE (SELECTING R1 AND R2)
Referring to the adjustable regulator in Figure 56, the output voltage is programmed by the resistors R1 and R2
by the following formula:
V
OUT
= V
REF
(1 + R1/R2)
where
V
REF
= 1.23V (1)
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:
R1 = R2 (V
OUT
/V
REF
1)
where
V
REF
= 1.23V (2)
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 output is shorted (see Figure 56), current flows directly
from the input, through the inductor and the diode, to the output, bypassing 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 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 57), using the standard 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.
FLYBACK REGULATOR INPUT CAPACITORS
A flyback regulator draws discontinuous pulses of current from the input supply. Therefore, there are two input
capacitors needed in a flyback regulator—one for energy storage and one for filtering (see Figure 57). 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 temperature range, the required rms current rating of the capacitor might be very large.
This means a larger value of capacitance 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.
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