Datasheet

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L1 =

Vout[1] x (Vin(max)[3] - Vout[1])

0.4 x Fsw[6] x Vin(max)[3]

Rt =

135 x 10

-12

Fsw[5]

- 580 x 10

-9

Fsw(max) =

(Vout[1] + 0.6)

Vin(max)[3] x 8 x 10

-8

Fsw(max) =

Vin(min)[2] ± (Vout[1] + 0.6)

Vin(min)[2] x 5.5 x 10

-7

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Quick Guide Design Worksheet

Step 3. Selection of the operating frequency is a trade-off between the conversion efficiency and solution

size. Operating at a high frequency, with a relatively high input voltage will severely impact the efficiency

and consequently generate a lot of heat. In some applications, the selection of a high operating frequency

will limit the input voltage range. The recommended maximum operating frequency for applications using

LM5575 is 500 KHz. When using the LM25575 the operating frequency may be set as high as 1MHz.

Select a target operating frequency from 50 kHz to 1MHz (500kHz for the LM5575). Check to see if the

Vin(min) limits the

selected operating frequency:

(1)

The selected operating frequency must be less than Fsw(max) calculated above, if not reduce the

operating frequency. Check to see if the Vin(max) limits the selected operating frequency:

(2)

The selected operating frequency must be less than Fsw(max) calculated above, if not reduce the

operating frequency.

Selected operating frequency Fsw: Hz [5]

Step 4. Calculate the value of Rt for the selected operating frequency.

(3)

Selected value for R1: Ohms [6]

Step 5. Calculate the value of L1.

(4)

Select the nearest standard inductor value. During an overload condition the peak inductor current is

limited to 2.1A nominal (2.5A maximum). The selected inductor must be rated for peak current of at least

2.5 Amps.

Selected value of L1: Henrys [7]

Step 6. Calculate the value of C

RAMP

= L1[7] x 10

-5

(5)

Selected value of C

RAMP

: Farads [8]

Step 7. Set Rfb2 to 5kOhms if Vout[1] is less than or equal to 5 Volts. If Vout[1] is greater than 5V set

Rfb2 to 10K Ohms.

Selected value of Rfb2: Ohms [9]

SNVA211B–January 2007–Revised April 2013 AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575

and LM25575

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