LM27313, LM27313-Q1

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SNVS487E – DECEMBER 2006 – REVISED JANUARY 2015

Typical Applications (continued)

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Selecting the External Capacitors

The LM27313 requires ceramic capacitors at the input and output to accommodate the peak switching currents

the part needs to operate. Electrolytic capacitors have resonant frequencies which are below the switching

frequency of the device, and therefore can not provide the currents needed to operate. Electrolytics may be used

in parallel with the ceramics for bulk charge storage which will improve transient response.

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.

8.2.1.2.2

Selecting the Output Capacitor

A single ceramic capacitor of value 4.7 µF to 10 µF provides sufficient output capacitance for most applications.

For output voltages below 10 V, 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.

8.2.1.2.3

Selecting the Input Capacitor

An input capacitor is required to serve as an energy reservoir for the current which must flow into the inductor

each time the switch turns ON. This capacitor must have extremely low ESR and ESL, so ceramic must be used.

We recommend a nominal value of 2.2 µF, but larger values can be used. Because 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.

8.2.1.2.4

Feed-Forward Compensation

Although internally compensated, the feed-forward capacitor Cf is required for stability (see Equation 1). 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)

8.2.1.2.5

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 15 V and 25 V, a 30-

V 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 500 mA. For applications

with load currents to 800 mA, a Microsemi UPS5817 can be used.

8.2.1.2.6

Setting the Output Voltage

The output voltage is set using the external resistors R1 and R2 (see Equation 2). A 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:

(2)

8.2.1.2.7

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:

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