Cirrus-logic AN372 Uživatelský manuál stáhnout pdf (Strana 26)

AN372

26 AN372REV1

For practical winding reasons an integer turns ratio is preferred giving the option of multifilar winding taking

advantage of tight coupling. The maximum FET voltage V

is calculated using Equation 44.

Step 5) Select the Full Brightness Switching Frequency

Minimum and maximum duty cycles, for the tapped buck, are calculated using Equation 9:

expecting a slightly better efficiency

Step 6) Determine the Buck Nominal Timing T1 and T2

At maximum boost voltage V

BST

time T1 is calculated:

and time T2 is calculated:

At minimum boost voltage V

BST

time T1 is calculated:

and time T2 is calculated:

Step 7) Calculate Peak Current on the Buck Primary-side

Calculate average current I

Calculate the peak current I

PK(FB)

at minimum boost voltage V

BST

Step 8) Calculate R

Sense

(R21)

The CS1613 current sense threshold is set to 1.4V. The voltage drop across the sense resistor R

Sense

must

equal 1.4V when a maximum current of 0.15A flows through resistor R

Sense

. Use Equation 14 to solve for

Sense

(R23).

BST max

OUT max

+ 445V 4 25.2 V+ 546V===

[Eq. 44]



Cmin/max

OUT min

N1+

OUT min

V

BST max

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

23.8V 5

4 23.8V  445.5V+

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

= 22%==

[Eq. 45]



Cmax/min

OUT min

N1+

OUT min

V

BST max

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

25.2V 5

4 25.2V  365V+

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

27%==

[Eq. 46]

OUT



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

9.6W

0.95

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

10.1W==

[Eq. 47]

T1 

Cmin/max

TT 0.22 8s 1.76s===

[Eq. 48]

T2 TT T1– 8s1.76s– 6.24s== =

[Eq. 49]

T1 

Cmax/min

TT 0.27 8s 2.16s===

[Eq. 50]

T2 TT T1– 8s2.16s– 5.84s== =

[Eq. 51]

BST min

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

10.1W

405V 40.5V–

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

== 28mA=

[Eq. 52]

PK FB





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

228mA

0.27

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

207mA==

[Eq. 53]

Sense

1.4V

PK FB

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

1.4V

0.207A

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

6.76===

[Eq. 54]

1 2 ... 21 22 23 24 25 26 27 28 29 30 31 32

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