LM2585S-5.0/NOPB National Semiconductor, LM2585S-5.0/NOPB Datasheet - Page 23

IC MULTI CONFIG 5V 3A TO263-5

LM2585S-5.0/NOPB

Manufacturer Part Number
LM2585S-5.0/NOPB
Description
IC MULTI CONFIG 5V 3A TO263-5
Manufacturer
National Semiconductor
Series
SIMPLE SWITCHER®r
Type
Step-Up (Boost), Flyback, Forward Converterr
Datasheet

Specifications of LM2585S-5.0/NOPB

Internal Switch(s)
Yes
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
5V
Current - Output
3A
Frequency - Switching
100kHz
Voltage - Input
4 ~ 40 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
D²Pak, TO-263 (5 leads + tab)
For Use With
551011367-061 - BOARD WEBENCH LM2577,LM2585/87
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
Other names
*LM2585S-5.0
*LM2585S-5.0/NOPB
LM2585S-5.0
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 LM2585, 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.
In addition, a small bypass capacitor is required due to the
noise generated by the input current pulses. To eliminate the
noise, insert a 1.0 μF ceramic capacitor between V
ground as close as possible to the device.
SWITCH VOLTAGE LIMITS
In a flyback regulator, the maximum steady-state voltage ap-
pearing at the switch, when it is off, is set by the transformer
turns ratio, N, the output voltage, V
put voltage, V
where V
and is 0.5V for Schottky diodes and 0.8V for ultra-fast recov-
ery diodes (typically). In certain circuits, there exists a voltage
spike, V
(see
caused by the transformer leakage inductance and/or the
output rectifier recovery time. To “clamp” the voltage at the
switch from exceeding its maximum value, a transient sup-
pressor in series with a diode is inserted across the trans-
former primary (as shown in the circuit on the front page and
other flyback regulator circuits throughout the datasheet). The
schematic in
the switch voltage. A single voltage transient suppressor (the
SA51A) is inserted at the switch pin. This method clamps the
total voltage across the switch, not just the voltage across the
primary.
If poor circuit layout techniques are used (see the “Circuit
Layout Guideline” section), negative voltage transients may
appear on the Switch pin (pin 4). Applying a negative voltage
(with respect to the IC's ground) to any monolithic IC pin
causes erratic and unpredictable operation of that IC. This
holds true for the LM2585 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 4) can go negative
when the switch turns on. The “ringing” voltage at the switch
pin is caused by the output diode capacitance and the trans-
former leakage inductance forming a resonant circuit at the
secondary(ies). The resonant circuit generates the “ringing”
voltage, which gets reflected back through the transformer to
the switch pin. There are two common methods to avoid this
problem. One is to add an RC snubber around the output rec-
tifier(s), as in
capacitor must be chosen so that the voltage at the Switch
pin does not drop below −0.4V. The resistor may range in
value between 10Ω and 1 kΩ, and the capacitor will vary from
0.001 μF to 0.1 μF. Adding a snubber will (slightly) reduce the
efficiency of the overall circuit.
The other method to reduce or eliminate the “ringing” is to
insert a Schottky diode clamp between pins 4 and 3 (ground),
also shown in
from dropping below −0.4V. The reverse voltage rating of the
diode must be greater than the switch off voltage.
100 μF) is required. If the input source is a rectified DC
Figure
LL
F
, superimposed on top of the steady-state voltage
is the forward biased voltage of the output diode,
V
5, waveform A). Usually, this voltage spike is
SW(OFF)
Figure 39
IN
Figure
Figure
(Max):
Figure
= V
39. The values of the resistor and the
39. This prevents the voltage at pin 4
shows another method of clamping
IN
(Max) + (V
39). Both are required due to the
OUT
OUT
, and the maximum in-
+V
F
)/N
IN
and
23
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the max-
imum switch voltage minus a diode drop. In a flyback regula-
tor, the maximum output voltage is determined by the turns
ratio, N, and the duty cycle, D, by the equation:
The duty cycle of a flyback regulator is determined by the fol-
lowing equation:
Theoretically, the maximum output voltage can be as large as
desired—just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from in-
creasing to infinity. The physical limitations are capacitances
and inductances in the LM2585 switch, the output diode(s),
and the transformer—such as reverse recovery time of the
output diode (mentioned above).
NOISY INPUT LINE CONDITION
A small, low-pass RC filter should be used at the input pin of
the LM2585 if the input voltage has an unusual large amount
of transient noise, such as with an input switch that bounces.
The circuit in
with the capacitor placed from the input pin to ground and the
resistor placed between the input supply and the input pin.
Note that the values of R
are good enough for most applications, but some readjusting
might be required for a particular application. If efficiency is a
major concern, replace the resistor with a small inductor (say
10 μH and rated at 100 mA).
STABILITY
All current-mode controlled regulators can suffer from an in-
stability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic os-
cillations, a minimum value of inductance is required to en-
sure stability for all boost and flyback regulators. The
minimum inductance is given by:
where V
in the Characteristic Curves.
SAT
is the switch saturation voltage and can be found
Figure 40
FIGURE 40. Input Line Filter
V
OUT
demonstrates the layout of the filter,
N × V
IN
and C
IN
× D/(1 − D)
IN
shown in the schematic
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