AN2747 datasheet(11/44 Pages) STMICROELECTRONICS

11 / 44 page

AN2747

General circuit description

11/44

where

●

Ilamp = lamp current

●

IAV = average current

●

Ipeak = inductor peak current

●

∆I = inductor current ripple

The ripple current in a buck converter working in continuous mode is expressed as:

Equation 4

where

●

Vbus= DC bus voltage

●

L= inductance value

●

f= switching frequency

●

ζ= duty cycle

For the buck in continuous mode the duty cycle relation is:

Equation 5

Using the relation (Equation 4) in the equation (5) we obtain:

Equation 6

Assuming Vbus and f constant it is possible to write:

Equation 7

The relation (Equation 3) can be written as:

Equation 8

This relation is valid because the average current is equal to the lamp current. This formula

is implemented in the microcontroller in order to calculate the lamp current.

∆

bus

⋅

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

δ 1 δ

–

()

⋅⋅

lamp

bus

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

∆

-----

2fL V

bus

⋅⋅ ⋅

---------------------------------- V

lamp

bus

lamp

–

()

⋅⋅

2fL V

bus

⋅⋅ ⋅

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

lamp

peak

lamp

bus

lamp

–

()

⋅⋅

–

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