LM2585SADJNOPB NATIONAL, LM2585SADJNOPB Datasheet - Page 22

• Manufacturer Part Number: LM2585SADJNOPB
• Manufacturer: NATIONAL

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Application Hints
PROGRAMMING OUTPUT VOLTAGE
(SELECTING R
Referring to the adjustable regulator in Figure 38, the output
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 Figure 38), current flows directly from the
V
R
OUT
1
= R
= V
2
1
(V
and R
REF
OUT
1
AND R
(1 + R
/V
2
divide the output voltage down so that it
REF
1
is:
1
2
− 1)
/R
)
2
)
where V
where V
1
and R
REF
REF
= 1.23V
= 1.23V
FIGURE 39. Flyback Regulator
FIGURE 38. Boost Regulator
2
by the fol-
2
22
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 3A.
In a flyback regulator application (Figure 39), using the stan-
dard transformers, the LM2585 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.
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