ALED6000 datasheet(36/45 Pages) STMICROELECTRONICS

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Table 8. Transfer function singularities

ωZ

ωRHPZ

ωLC

Buck boost

1C0⋅N⋅Rd

1−D2⋅V0

L⋅D⋅ILED

1−DL⋅C0

Floating boost

1C0⋅N⋅Rd

1−D2⋅V0

L⋅ILED

1−DL⋅C0

Table 9. Transfer function parameters

Buck boost

C0L⋅1−D⋅N⋅Rd

D⋅ 1−D ⋅ RSNS

RSNS+N⋅Rd

V0V0+VIN

Floating boost

C0L⋅1−D⋅N⋅Rd

V01−D⋅ RSNS

RSNS+N⋅Rd

1−VINV0

This simplified model is based on the assumption that the output capacitor ESR is negligible compared to LED

dynamic resistance, Rd, and LED current sensing resistor, RSNS. Further, RSNS is assumed negligible compared

to the total LED dynamic resistance, N*Rd.

The closed loop transfer function is still given by Eq. (18), assuming for the power section the model summarized

in Eq. (39) .

The singularity ωRHPZ, computed at maximum load and minimum input voltage, is the limitation in the loop

bandwidth design (fBW). Typically the maximum bandwidth is designed to be lower than:

fBW=14⋅ωRHPZ2π

(40)

In case ωZ and ωLC are lower than the target bandwidth, a type II compensation network is necessary for loop

stabilization, following the compensation strategy described in Section 6.3 Compensation strategy.

Otherwise, in case a very low bandwidth design is suitable, a simple type I compensation network is required.

This approach is feasible if the target bandwidth is ¼ of fLC or lower.

Figure 34. Type I compensation network

COMP

V REF

V SNS

RSNS

ALED6000

Compensation strategy for alternative topologies

DS13018 - Rev 4

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