TI Confidential - NDA Restrictions

LM2731

SNVS217F – MAY 2004 – REVISED NOVEMBER 2012

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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.

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 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

circuitry.

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