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參數(shù)資料
| 型號： | LM2599T-ADJ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator, with Features |
| 中文描述： | 7.5 A SWITCHING REGULATOR, 173 kHz SWITCHING FREQ-MAX, PZFM7 |
| 封裝： | TO-220, 7 PIN |
| 文件頁數(shù)： | 22/31頁 |
| 文件大?。?/td> | 812K |
| 代理商： | LM2599T-ADJ |
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Application Information
(Continued)
By allowing the percentage of inductor ripple current to in-
crease for low load currents, the inductor value and size can
be kept relatively low.
When operating in the continuous mode, the inductor current
waveform ranges from a triangular to a sawtooth type of
waveform (depending on the input voltage), with the average
value of this current waveform equal to the DC output load
current.
Inductors are available in different styles such as pot core,
toroid, E-core, bobbin core, etc., as well as different core ma-
terials, such as ferrites and powdered iron. The least expen-
sive, the bobbin, rod or stick core, consists of wire wound on
a ferrite bobbin. This type of construction makes for an inex-
pensive inductor, but since the magnetic flux is not com-
pletely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus caus-
ing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope read-
ings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.
When multiple switching regulators are located on the same
PC board, open core magnetics can cause interference be-
tween two or more of the regulator circuits, especially at high
currents. A torroid or E-core inductor (closed magnetic struc-
ture) should be used in these situations.
The inductors listed in the selection chart include ferrite
E-core construction for Schott, ferrite bobbin core for Renco
and Coilcraft, and powdered iron toroid for Pulse Engineer-
ing.
Exceeding an inductor’s maximum current rating may cause
the inductor to overheat because of the copper wire losses,
or the core may saturate. If the inductor begins to saturate,
the inductance decreases rapidly and the inductor begins to
look mainly resistive (the DC resistance of the winding). This
can cause the switch current to rise very rapidly and force
the switch into a cycle-by-cycle current limit, thus reducing
the DC output load current. This can also result in overheat-
ing of the inductor and/or the LM2599. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
may be a better choice. It would use an inductor that would
be physically smaller, and would need only one half to one
third the inductance value needed for a continuous mode de-
sign. The peak switch and inductor currents will be higher in
a discontinuous design, but at these low load currents (1A
and below), the maximum switch current will still be less than
the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The out-
put pin (switch) waveform can have some damped sinusoi-
dal ringing present. (See Typical Performance Characteris-
tics photo titled Discontinuous Mode Switching Waveforms)
This ringing is normal for discontinuous operation, and is not
caused by feedback loop instabilities. In discontinuous op-
eration, there is a period of time where neither the switch or
the diode are conducting, and the inductor current has
dropped to zero. During this time, a small amount of energy
can circulate between the inductor and the switch/diode
parasitic capacitance causing this characteristic ringing. Nor-
mally this ringing is not a problem, unless the amplitude be-
comes great enough to exceed the input voltage, and even
then, there is very little energy present to cause damage.
Different inductor types and/or core materials produce differ-
ent amounts of this characteristic ringing. Ferrite core induc-
tors have very little core loss and therefore produce the most
ringing. The higher core loss of powdered iron inductors pro-
duce less ringing. If desired, a series RC could be placed in
parallel with the inductor to dampen the ringing. The com-
puter aided design software
Switchers Made Simple
(ver-
sion 4.3) will provide all component values for continuous
and discontinuous modes of operation.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply operating in
the continuous mode will contain a sawtooth ripple voltage at
the switcher frequency, and may also contain short voltage
spikes at the peaks of the sawtooth waveform.
The output ripple voltage is a function of the inductor saw-
tooth ripple current and the ESR of the output capacitor. A
typical output ripple voltage can range from approximately
0.5% to 3% of the output voltage. To obtain low ripple volt-
age, the ESR of the output capacitor must be low, however,
caution must be exercised when using extremely low ESR
capacitors because they can affect the loop stability, result-
ing in oscillation problems. If very low output ripple voltage is
needed (less than 20 mV), a post ripple filter is recom-
DS012582-36
FIGURE 18. (
I
) Peak-to-Peak Inductor Ripple
Current (as a Percentage of the
Load Current) vs Load Current
DS012582-37
FIGURE 19. Post Ripple Filter Waveform
L
www.national.com
22
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