Cirrus-logic AN372 Manual do Utilizador descarregar pdf (Página 6)

AN372

6 AN372REV1

3.3 Buck Stage Design

Figure 2 illustrates the steps for designing the buck stage.

Figure 2. Buck Stage Design

Buck Specification

Determine N, F

Reflected

, and V

CLAMP

Estimate T3

Calculate TT

Calculate R

Sense

FBG AIN

, and Buck

Inductance

Sense

FBG AIN

Fit?

Yes

Calculate T1,

T2, and I

PK(FB )

Calculate RMS Current

and Output Capacitor

Buck Inductor

Specification

Steps for the Buck Design

1. Select a buck topology

2. Set the boost output voltage, V

BST

3. Select a FET that aligns with the quality standards of the

designer’s company.

4. Determine the inductor turns ratio from the

BST

, FET

voltage, and reflected voltage, V

Reflected

5. Use the nominal switching frequency and an initial

estimate for time T3 to determine the value of time TT at

full brightness.

6. Use V

BST

, TT, and V

Reflected

to determine times T1 and T2.

7. Use times T2 and TT, turns ratio N, and load current to

determine the value of the peak primary current, I

PK(FB)

8. Use I

PK(FB)

to determine R

Sense

9. Calculate the buck inductance L using time T1.

10. Calculate second stage gain resistor R

FBGAIN

using full

load conditions. Ensure linearity of the load versus the dim

curve.

11. Calculate RMS currents in the two windings of L using

peak current I

PK(FB)

and duty cycle.

12. Select an output capacitor.

13. Determine the buck inductor specifications.

14. Determine if the buck inductor fits into a specified form

factor after designing and constructing the buck inductor.

Repeat steps 3 to 12 until form factor criteria is met.

15. Refinements to the circuit with final buck inductor design.

16. Validate that the system meets the operating criteria.

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