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LM2733 Arkusz danych(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Numer części LM2733
Szczegółowy opis  0.6/1.6 MHz Boost Converters With 40V Internal FET
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Producent  NSC [National Semiconductor (TI)]
Strona internetowa  http://www.national.com
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LM2733 Arkusz danych(HTML) 9 Page - National Semiconductor (TI)

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Application Hints
SELECTING THE EXTERNAL CAPACITORS
The best capacitors for use with the LM2733 are multi-layer
ceramic capacitors. They have the lowest ESR (equivalent
series resistance) and highest resonance frequency which
makes them optimum for use with high frequency switching
converters.
When selecting a ceramic capacitor, only X5R and X7R
dielectric types should be used. Other types such as Z5U
and Y5F have such severe loss of capacitance due to effects
of temperature variation and applied voltage, they may pro-
vide as little as 20% of rated capacitance in many typical
applications. Always consult capacitor manufacturer’s data
curves before selecting a capacitor. High-quality ceramic
capacitors can be obtained from Taiyo-Yuden, AVX, and
Murata.
SELECTING THE OUTPUT CAPACITOR
A single ceramic capacitor of value 4.7 µF to 10 µF will
provide sufficient output capacitance for most applications.
For output voltages below 10V, a 10 µF capacitance is
required. If larger amounts of capacitance are desired for
improved line support and transient response, tantalum ca-
pacitors can be used in parallel with the ceramics. Aluminum
electrolytics with ultra low ESR such as Sanyo Oscon can be
used, but are usually prohibitively expensive. Typical AI elec-
trolytic capacitors are not suitable for switching frequencies
above 500 kHz due to significant ringing and temperature
rise due to self-heating from ripple current. An output capaci-
tor with excessive ESR can also reduce phase margin and
cause instability.
SELECTING THE INPUT CAPACITOR
An input capacitor is required to serve as an energy reservoir
for the current which must flow into the coil each time the
switch turns ON. This capacitor must have extremely low
ESR, so ceramic is the best choice. We recommend a
nominal value of 2.2 µF, but larger values can be used. Since
this capacitor reduces the amount of voltage ripple seen at
the input pin, it also reduces the amount of EMI passed back
along that line to other circuitry.
FEED-FORWARD COMPENSATION
Although internally compensated, the feed-forward capacitor
Cf is required for stability (see Basic Application Circuit).
Adding this capacitor puts a zero in the loop response of the
converter. Without it, the regulator loop can oscillate. The
recommended frequency for the zero fz should be approxi-
mately 8 kHz. Cf can be calculated using the formula:
Cf = 1/(2X
π XR1Xfz)
SELECTING DIODES
The external diode used in the typical application should be
a Schottky diode. If the switch voltage is less than 15V, a
20V diode such as the MBR0520 is recommended. If the
switch voltage is between 15V and 25V, a 30V diode such as
the MBR0530 is recommended. If the switch voltage ex-
ceeds 25V, a 40V diode such as the MBR0540 should be
used.
The MBR05XX series of diodes are designed to handle a
maximum average current of 0.5A. For applications exceed-
ing 0.5A average but less than 1A, a Microsemi UPS5817
can be used.
LAYOUT HINTS
High frequency switching regulators require very careful lay-
out of components in order to get stable operation and low
noise. All components must be as close as possible to the
LM2733 device. It is recommended that a 4-layer PCB be
used so that internal ground planes are available.
As an example, a recommended layout of components is
shown:
20055422
Recommended PCB Component Layout
Some additional guidelines to be observed:
1.
Keep the path between L1, D1, and C2 extremely short.
Parasitic trace inductance in series with D1 and C2 will
increase noise and ringing.
2.
The feedback components R1, R2 and CF must be kept
close to the FB pin of U1 to prevent noise injection on
the FB pin trace.
3.
If internal ground planes are available (recommended)
use vias to connect directly to ground at pin 2 of U1, as
well as the negative sides of capacitors C1 and C2.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the external resistors R1 and
R2 (see Basic Application Circuit). A value of approximately
13.3 k
Ω is recommended for R2 to establish a divider current
of approximately 92 µA. R1 is calculated using the formula:
R1=R2 X (V
OUT/1.23 − 1)
SWITCHING FREQUENCY
The LM2733 is provided with two switching frequencies: the
“X” version is typically 1.6 MHz, while the “Y” version is
typically 600 kHz. The best frequency for a specific applica-
tion must be determined based on the tradeoffs involved:
Higher switching frequency means the inductors and capaci-
tors can be made smaller and cheaper for a given output
voltage and current. The down side is that efficiency is
slightly lower because the fixed switching losses occur more
frequently and become a larger percentage of total power
loss. EMI is typically worse at higher switching frequencies
because more EMI energy will be seen in the higher fre-
quency spectrum where most circuits are more sensitive to
such interference.
www.national.com
9


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