LM2587T-ADJ/NOPB National Semiconductor, LM2587T-ADJ/NOPB Datasheet - Page 23

• Manufacturer Part Number: LM2587T-ADJ/NOPB
• Description: IC REG SIMPLE SWITCHER TO-220-5
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Up (Boost), Flyback, Forward Converterr

Specifications of LM2587T-ADJ/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 5A
• Frequency - Switching: 100kHz
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Through Hole
• Package / Case: TO-220-5 (Bent and Staggered Leads)
• Current, Input Bias: 125 nA
• Current, Output: 5 A
• Current, Supply: 11 mA
• Frequency, Oscillator: 100 kHz
• Package Type: TO-220
• Regulation, Line: 20 mV
• Regulation, Load: 20 mV
• Regulator Type: Boost (Step-Up)
• Resistance, Thermal, Junction To Case: 2 °C/W
• Temperature, Operating, Range: -40 to +125 °C
• Transconductance: 3.2
• Voltage, Gain: 670 V/V
• Voltage, Input: 4 to 40 V
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Voltage: 37V
• Output Current: 5A
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Filter Terminals: Through Hole
• Rohs Compliant: Yes
• For Use With: 551011367-061 - BOARD WEBENCH LM2577,LM2585/87LM2587EVAL - EVALUATION BOARD FOR LM2587
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device
• Other names: *LM2587T-ADJ; *LM2587T-ADJ/NOPB; LM2587T-ADJ

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 LM2587 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
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 42
IN
(Max):
= V
shows another method of clamping
IN
(Max) + (V
OUT
OUT
, and the maximum in-
+V
F
)/N
FIGURE 42. Flyback Regulator
IN
and
23
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.
Figure
Figure
FIGURE 43. Input Line Filter
42. The values of the resistor and the
42. This prevents the voltage at pin 4
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