LM2731XMF/HALF National Semiconductor, LM2731XMF/HALF Datasheet - Page 11

• Manufacturer Part Number: LM2731XMF/HALF
• Manufacturer: National Semiconductor

Specifications of LM2731XMF/HALF

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Application Hints
SELECTING THE EXTERNAL CAPACITORS
The best capacitors for use with the LM2731 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 di-
electric 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 provide
as little as 20% of rated capacitance in many typical applica-
tions. 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. If larger
amounts of capacitance are desired for improved line support
and transient response, tantalum capacitors can be used.
Aluminum electrolytics with ultra low ESR such as Sanyo Os-
con can be used, but are usually prohibitively expensive.
Typical AI electrolytic 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 capacitor with excessive ESR can also reduce phase
margin and cause instability.
In general, if electrolytics are used, it is recommended that
they be paralleled with ceramic capacitors to reduce ringing,
switching losses, and output voltage ripple.
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 nomi-
nal 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. The recommended frequency for the zero fz should
be approximately 6 kHz. Cf can be calculated using the for-
mula:
SELECTING DIODES
The external diode used in the typical application should be
a Schottky diode. A 20V diode such as the MBR0520 is rec-
ommended.
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
Cf = 1 / (2 X π X R1 X fz)
11
noise. All components must be as close as possible to the
LM2731 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:
Some additional guidelines to be observed:
1.
2.
3.
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 kΩ is recommended for R2 to establish a divider current
of approximately 92 µA. R1 is calculated using the formula:
SWITCHING FREQUENCY
The LM2731 is provided with two switching frequencies: the
“X” version is typically 1.6 MHz, while the “Y” version is typi-
cally 600 kHz. The best frequency for a specific application
must be determined based on the trade-offs involved:
Higher switching frequency means the inductors and capac-
itors 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 fre-
quently and become a larger percentage of total power loss.
EMI is typically worse at higher switching frequencies be-
cause more EMI energy will be seen in the higher frequency
spectrum where most circuits are more sensitive to such in-
terference.
Keep the path between L1, D1, and C2 extremely short.
Parasitic trace inductance in series with D1 and C2 will
increase noise and ringing.
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.
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.
Recommended PCB Component Layout
R1 = R2 X (V
OUT
/1.23 − 1)
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