LM2586T-ADJ National Semiconductor, LM2586T-ADJ Datasheet - Page 23

• Manufacturer Part Number: LM2586T-ADJ
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2586T-ADJ

• Input Voltage: 40V
• Output Voltage: 37V
• No. Of Pins: 5
• Termination Type: Through Hole
• Mounting Type: Through Hole
• Output Current Max: 3A
• Peak Reflow Compatible (260 C): No
• Supply Voltage Max: 40V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Hints
stable or constant voltage supply to the LM2586, a storage
capacitor ( 100 µF) is required. If the input source is a recti-
fied DC supply and/or the application has a wide tempera-
ture range, the required rms current rating of the capacitor
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 typically 0.5V for Schottky diodes and 0.8V for
ultra-fast recovery diodes. In certain circuits, there exists a
voltage spike, V
voltage (see Figure 5 , waveform A). Usually, this voltage
spike is caused by the transformer leakage inductance
and/or the output rectifier recovery time. To “clamp” the volt-
age at the switch from exceeding its maximum value, a tran-
sient suppressor in series with a diode is inserted across the
transformer primary (as shown in the circuit in Figure 4 and
other flyback regulator circuits throughout the datasheet).
The schematic in Figure 43 shows another method of clamp-
ing 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 volt-
age 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 5). 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 LM2586 IC as well. When used in a flyback
F
is the forward biased voltage of the output diode,
V
SW(OFF)
IN
LL
(Max):
, superimposed on top of the steady-state
= V
IN
(Max) + (V
(Continued)
OUT
OUT
, and the maximum in-
+V
F
)/N
FIGURE 43. Flyback Regulator
IN
and
23
might be very large. This means a larger value of capaci-
tance 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.
regulator, the voltage at the Switch pin (pin 5) can go nega-
tive when the switch turns on. The “ringing” voltage at the
switch pin is caused by the output diode capacitance and the
transformer leakage inductance forming a resonant circuit at
the secondary(ies). The resonant circuit generates the “ring-
ing” voltage, which gets reflected back through the trans-
former to the switch pin. There are two common methods to
avoid this problem. One is to add an RC snubber around the
output rectifier(s), as in Figure 43 . The values of the resistor
and the 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 10 and 1 k , 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 in-
sert a Schottky diode clamp between pins 5 and 4 (ground),
also shown in Figure 43 . This prevents the voltage at pin 5
from dropping below −0.4V. The reverse voltage rating of the
diode must be greater than the switch off voltage.
DS012516-62
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