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AN1317 - APPLICATION NOTE

Figure 3: Switching cycle in continuous mode in low input voltage condition

and

By using these two equations:

Finally, by introducing the duty cycle expression in continuous mode:

The minimum output current mandatory to keep the output voltage under control is given by:

To prevent these disturbances resulting in possible output overvoltage or incorrect start up, a 15V zener

diode DZ1 is added. It allows a current to flow at the output, insuring a correct start up and clamps any

possible overvoltage. Nevertheless, as shown in the above last formula, the current flowing in this zener

can be very high when the input voltage approaches the output one. Section 5 describes a schematic

modification to overcome this issue.

Fig. 4 presents the operation of the free wheeling diode in this condition: Actually it is always blocked, as

the voltage on the cathode never becomes negative. Also on this figure, it can be observed that the

I

DD

1

2T

s

⋅

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

I

P

I

0

+

() T

S

t

on

–

()

⋅⋅

=

I

out

1

2T

s

⋅

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

I

p

I

0

+

() t

on

⋅⋅

=

I

out

t

on

I

DD

T

s

t

on

–

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

⋅

=

d

t

on

T

s

------

V

in

V

ou t

-----------

==

I

ou t

I

DD

V

out

V

in

V

out

–

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

⋅

=

Charge

Discharge

t

P

P