LM2731YMFX/NOPB National Semiconductor, LM2731YMFX/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM2731YMFX/NOPB
• Description: IC BOOST ADJ 1.8A SOT23-5
• Manufacturer: National Semiconductor
• Type: Step-Up (Boost)r

Specifications of LM2731YMFX/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 22 V
• Current - Output: 1.8A
• Frequency - Switching: 600kHz
• Voltage - Input: 2.7 ~ 14 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• For Use With: LM2731YEVAL - BOARD EVALUATION LM2731Y
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2731YMFX

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
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:
Recommended PCB Component Layout
Cf = 1 / (2 X
π
X R1 X fz)
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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.
R1 = R2 X (V
OUT
/1.23 − 1)
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