Datasheet

LM2733

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SNVS209E –NOVEMBER 2002–REVISED APRIL 2013

Application Hints

SELECTING THE EXTERNAL CAPACITORS

The best capacitors for use with the LM2733 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. 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.

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.

FEED-FORWARD COMPENSATION

Although internally compensated, the feed-forward capacitor Cf is required for stability (see Figure 26). 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)

SELECTING DIODES

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

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

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

MBR0540 should be used.

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

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

LAYOUT HINTS

High frequency switching regulators require very careful layout of components in order to get stable operation

and low noise. All components must be as close as possible to the LM2733 device. It is recommended that a 4-

layer PCB be used so that internal ground planes are available.

As an example, a recommended layout of components is shown:

Product Folder Links: LM2733