Duty Cycle =

LM2733

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SNVS209F – NOVEMBER 2002 – REVISED DECEMBER 2014

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. For output voltages below 10V, a 10 µF capacitance is required. If larger amounts of capacitance

are desired for improved line support and transient response, tantalum capacitors can be used in parallel with the

ceramics. 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.

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. 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.

8.2.2.4 Feed-Forward Compensation

Although internally compensated, the feed-forward capacitor Cf is required for stability (see Figure 24). Adding

this capacitor puts a zero in the loop response of the converter. Without it, the regulator loop can oscillate. The

recommended frequency for the zero fz should be approximately 8 kHz. Cf can be calculated using the formula:

Cf = 1 / (2 X

π X R1 X fz)

(1)

8.2.2.5 Selecting Diodes

The external diode used in the typical application should be a Schottky diode. If the switch voltage is less than 15

V, a 20 V diode such as the MBR0520 is recommended. If the switch voltage is between 15 V and 25 V, a 30-V

diode such as the MBR0530 is recommended. If the switch voltage exceeds 25 V, a 40-V diode such as the

MBR0540 should be used.

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 24). A value of approximately 13.3

k

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

formula:

(2)

8.2.2.7 Switching Frequency

The device options provide for two fixed frequency operating conditions 1.6 MHz, and 600 kHz. Chose the

operating frequency required noting the following trade-offs:

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.

8.2.2.8 Duty Cycle

The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input

voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost

application is defined as:

(3)

This applies for continuous mode operation.

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