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Revision 1.8

10 - 19

AS1310

Datasheet - Detailed Descri pti on

Energy stored during the on time is given by:

Joules (Region A in Figure 13)(EQ 6)

Watts

(EQ 7)

8.1.3 Output Loop Timing

The output loop consists of the main inductor, P-channel synchronous switch (or diode if fitted), output capacitor and load. When the input loop is

interrupted, the voltage on the LX pin rises (Lenz’s Law). At the same time a comparator enables the synchronous switch, and energy stored in

the inductor is transferred to the output capacitor and load. Inductor peak current supports the load and replenishes the charge lost from the

output capacitor. The magnitude of the current from the inductor is monitored, and as it approaches zero, the synchronous switch is turned off.

Output power is composed of the dc component (Region C in Figure 13):

(EQ 8)

Output power is also composed of the inductor component (Region B in Figure 13), neglecting efficiency loss:

(EQ 9)

Total power delivered to the load is the sum of (EQ 8) and (EQ 9):

(EQ 10)

From (EQ 3) (using nominal values) peak current is given by:

(EQ 11)

Substituting (EQ 11) into (EQ 10) and re-arranging:

(EQ 12)

Output power in terms of regulated output voltage and load resistance is:

(EQ 13)

Combining (EQ 12) and (EQ 13):

(EQ 14)

Symbol

η reflects total energy loss between input and output and is approximately 0.8 for these calculations. Use (EQ 14) to plot duty cycle

E

()

2

=

()

2

T

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

=

2

--------

T

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

()

2

T

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

2

--------

T

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

()

2

T

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

+

=

L

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

=

V

2

2TL

---------------------- 0.9T

()

=

V

2

OUT

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

=

V

2

OUT

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

V

2

2TL

---------------------- 0.9T

()η

=