LM2731XMF/HALF National Semiconductor, LM2731XMF/HALF Datasheet - Page 4

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

Specifications of LM2731XMF/HALF

• Lead Free Status / Rohs Status: Supplier Unconfirmed

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Block Diagram
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 mΩ 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 (propor-
tional 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
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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 refer-
ence 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 com-
parator.
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