f



(

)

1



+

OUT(MAX)

OUT1

OUT2

N2

I

I

I

0.1 A

N1

=

+

´

=

LM5019

SNVS788F – JANUARY 2012 – REVISED DECEMBER 2014

www.ti.com

8.2.2.1 Design Requirements

Selection of external components is illustrated through a design example. The design example specifications are

shown in Table 3.

Table 3. Buck Converter Design Specifications

DESIGN PARAMETERS

VALUE

Input Voltage Range

20 V to 95 V

Primary Output Voltage

10 V

Secondary (Isolated) Output Voltage

9.5 V

Maximum Output Current (Primary + Secondary)

100 mA

Maximum Power Output

1 W

Nominal Switching Frequency

750 kHz

8.2.2.2 Detailed Design Procedure

8.2.2.2.1

Transformer Turns Ratio

The transformer turns ratio is selected based on the ratio of the primary output voltage to the secondary

(isolated) output voltage. In this design example, the two outputs are nearly equal and a 1:1 turns ratio

transformer is selected. Therefore, N2 / N1 = 1.

If the secondary (isolated) output voltage is significantly higher or lower than the primary output voltage, a turns

ratio less than or greater than 1 is recommended. The primary output voltage is normally selected based on the

input voltage range such that the duty cycle of the converter does not exceed 50% at the minimum input voltage.

8.2.2.2.2

Total IOUT

The total primary referred load current is calculated by multiplying the isolated output load(s) by the turns ratio of

the transformer as shown in Equation 15.

(15)

8.2.2.2.3

RFB1, RFB2

value of 7.32 k

(16)

(17)

8.2.2.2.4

Frequency Selection

(18)

for this design to allow for second order effects at high switching frequency that are not included in Equation 1.

8.2.2.2.5

Transformer Selection

A coupled inductor or a flyback-type transformer is required for this topology. Energy is transferred from primary

to secondary when the low-side synchronous switch of the buck converter is conducting.

The maximum inductor primary ripple current that can be tolerated without exceeding the buck switch peak

current limit threshold (0.15 A minimum) is given by Equation 19.

18

Submit Documentation Feedback

Copyright © 2012–2014, Texas Instruments Incorporated

Product Folder Links: LM5019