LM2585SX-ADJ National Semiconductor, LM2585SX-ADJ Datasheet - Page 20

• Manufacturer Part Number: LM2585SX-ADJ
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BIPOLAR,SIP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2585SX-ADJ

• Rohs Compliant: NO

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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
it can be compared with the 1.23V internal reference. With
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
input, through the inductor and the diode, to the output, by-
2
V
between 1k and 5k, R
R
OUT
1
= R
= V
2
1
(V
REF
and R
OUT
1
(1 + R
AND R
/V
2
REF
divide the output voltage down so that
1
− 1)
2
/R
1
)
2
is:
)
where V
where V
1
and R
REF
REF
FIGURE 39. Flyback Regulator
FIGURE 38. Boost Regulator
= 1.23V
= 1.23V
2
by the fol-
20
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
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 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 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
device.
DS012515-56
DS012515-55