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LM2731XMF_NOPB Datasheet(PDF) 10 Page - Texas Instruments
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LM2731XMF_NOPB Datasheet(HTML) 10 Page - Texas Instruments
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SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
THEORY OF OPERATION
The LM2731 is a switching converter IC that operates at a fixed frequency (0.6 or 1.6 MHz) for fast transient
response over a wide input voltage range and incorporates pulse-by-pulse current limiting protection. Because
this is current mode control, a 33 m
Ω sense resistor in series with the switch FET is used to provide a voltage
(which is proportional to the FET current) to both the input of the pulse width modulation (PWM) comparator and
the current limit amplifier.
At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a
voltage (proportional to this current) is summed with the ramp coming from the ramp generator and then fed into
the input of the PWM comparator. When this voltage exceeds the voltage on the other input (coming from the
Gm amplifier), the latch resets and turns the FET off. Since the signal coming from the Gm amplifier is derived
from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets
the correct peak current through the FET to keep the output voltage in regulation.
Q1 and Q2 along with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation.
The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to
maintain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at
the FB node "multiplied up" by the ratio of the output resistive divider.
The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches
the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit
input terminates the pulse regardless of the status of the output of the PWM comparator.
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 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 provide 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. 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 Oscon 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 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
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Product Folder Links: LM2731
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