LM2731YMFX/NOPB National Semiconductor, LM2731YMFX/NOPB Datasheet - Page 10

• 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

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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
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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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
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