LM2587T-3.3 National Semiconductor, LM2587T-3.3 Datasheet - Page 22

• Manufacturer Part Number: LM2587T-3.3
• Description: IC, FLYBACK REGULATOR, TO-220-5
• Manufacturer: National Semiconductor

Specifications of LM2587T-3.3

• Input Voltage: 40V
• Output Current: 5A
• Output Voltage: 3.3V
• No. Of Outputs: 1
• Supply Voltage Range: 4V To 40V
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +125°C
• Termination Type: Through Hole
• Filter Terminals: Through Hole
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Hints
PROGRAMMING OUTPUT VOLTAGE
(SELECTING R
Referring to the adjustable regulator in
voltage is programmed by the resistors R
lowing formula:
Resistors R
can be compared with the 1.23V internal reference. With R
between 1k and 5k, R
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
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 ex-
ternally 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 5A.
V
R
OUT
1
= R
= V
2
1
(V
and R
REF
OUT
1
AND R
(1 + R
/V
2
Figure
divide the output voltage down so that it
REF
1
is:
1
2
− 1)
/R
)
41), current flows directly from the
2
)
where V
where V
Figure
1
and R
REF
REF
41, the output
= 1.23V
= 1.23V
FIGURE 41. Boost Regulator
2
by the fol-
2
22
In a flyback regulator application
dard transformers, the LM2587 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 capaci-
tors needed in a flyback regulator; one for energy storage and
one for filtering (see
inherent operation of a flyback regulator. To keep a stable or
constant voltage supply to the LM2587, a storage capacitor
(
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 of the input capacitor. The stor-
age capacitor will also attenuate noise which may interfere
with other circuits connected to the same input supply voltage.
≥
100 μF) is required. If the input source is a recitified DC
Figure
42). Both are required due to the
(Figure
1231626
42), using the stan-