AN2835 datasheet(7/21 Pages) STMICROELECTRONICS

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AN2835

The selected solution

Doc ID 15073 Rev 1

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realized by controlling the peak inductor current of the dual-buck converter. Assume the

input voltage of the buck converter is Vbi, the output voltage is Vbo, the duty cycle is D,

input peak current is Iin_pk, as the buck converter is working at a critical discontinuous

mode, and the average input current is:

Equation 1

And the duty cycle is:

Equation 2

The input power becomes:

Equation 3

Thus the relationship between the input power (Pin), input peak current (Iin_pk) and output

voltage (Vbo) is:

Equation 4

If the lamp is operating with a constant current source, once input peak current (Iin_pk) is

selected, we observe the input power (Pin) is proportional to the output voltage (Vbo).

Despite the power losses, the output power is also proportional to the output voltage.

Figure 5.

Input power, output voltage and input peak current

In Figure 5, there are three different Iin_pk curves, this helps to choose the proper input peak

current according to the different types of lamp. After warming-up, the lamp voltage

increases slowly to the minimum value of the rated power, the duty cycle increases

accordingly. And then the input peak current decreases. In order to power up the lamp in

steady-state, the circuit changes from constant current control function to constant power

control.

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inpk

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