UM0670 datasheet(37/43 Pages) STMICROELECTRONICS

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Design calculation

UM0670

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Doc ID 15327 Rev 2

Appendix A

Design calculation

The aim of this section is to demonstrate how the components for the modified buck

converter are calculated. Design calculation follows precisely the equations used in

application note AN2928 (Section 11: References and related materials: 2.). Therefore,

please refer to this application note for more information.

A.1

Design specifications for a modified buck convertor

●

VIN = 400 V

●

VLED = 80 V

●

IAVR = 1 A

●

IMAX = 1.4 A

●

IMIN = 0.6 A

●

f = 50 kHz

●

TA = 30 °C

●

TJMAX_MOSFET = 70 °C

Modified buck converter working with duty cycle (output LED current is 1 A):

Equation 1

Calculated OFF time for selected switching frequency of 50 kHz is:

Equation 2

Now, the FOT network should be calculated. First, resistor R203 is selected:

R203 = 3900 Ω

Two capacitors in parallel (C204 and C206) are used for the 1 A output LED current (jumper

JP3 is connected) and their size is determined using the following equation:

Equation 3

Therefore the capacitors C204 and C206 have the following size:

C204 = 390 pF

C206 = 1.5 nF

LED

--------------

400

----------

0.2

OFF

–

()

------------------

10.2

–

()

50000

----------------------

μs

204

206

OFF

2.1

203

---------------------------

–

2.1

3900

----------------------------

1.95nF

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