LM2731

SNVS217G – MAY 2004 – REVISED SEPTEMBER 2015

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8.2.2.2 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, TI recommends that they be paralleled with ceramic capacitors to reduce

ringing, switching losses, and output voltage ripple.

8.2.2.3 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. TI

recommends 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.

8.2.2.4 Feedforward Compensation

Although internally compensated, the feedforward capacitor Cf is required for stability (see Figure 26). 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 formula:

Cf = 1 / (2 ×

π X R1 × fz)

(1)

8.2.2.5 Selecting Diodes

The external diode used in the typical application should be a Schottky diode. TI recommends a 20-V diode such

as the MBR0520.

The MBR05XX series of diodes are designed to handle a maximum average current of 0.5 A. For applications

exceeding 0.5-A average but less than 1 A, a Microsemi UPS5817 can be used.

8.2.2.6 Setting the Output Voltage

The output voltage is set using the external resistors R1 and R2 (see Figure 26). A minimum value of 13.3 k

Ω is

recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula:

R1 = R2 × (VOUT/1.23 − 1)

(2)

8.2.2.7 Switching Frequency

The LM2731 is provided with two switching frequencies: the “X” version is typically 1.6 MHz, while the “Y” version

is typically 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 capacitors 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 frequently and become a larger percentage of total power loss. EMI is typically worse at higher

switching frequencies because more EMI energy will be seen in the higher frequency spectrum where most

circuits are more sensitive to such interference.

Figure 26. Basic Application Circuit

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