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L6598

to a transconductance amplifier, as shown in fig. 7. Thus this voltage signal is converted in a growing current

start is equal to:

[1]

where

[2]

At the start-up (t=0) the oscillator frequency is set by:

[3]

[4]

Since the second term of eq.1 is equal to zero, we have:

[5]

[6]

Start Timing Constant) is 0.15 s/

µF.

to compute an approximate value of the oscillator frequency in normal operation is:

[7]

The degree of approximation depends on the frequency value, but it remains very good in the range from 30kHz

to 100kHz (figg.9-13)

I

osc

I

fmin

I

fstart

g

()

–

()

+

I

fm in

I

fstart

g

C

ss

--------------t

–





+

==

I

fmin

V

REF

R

fm in

-------------- I

fsart

,

V

REF

R

fs tart

---------------- V

RE F

,

2V

==

=

I

()

I

fm in

I

fstart

+

V

REF

1

R

fmin

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

1

R

fstart

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

+





==

I

()

I

fm in

V

REF

R

fm in

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

==

I

fsta rt

g

C

ss

--------------T

SS

–

0T

SS

→

C

g

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

==

I

SS

I

fstart

K

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

T

SS

→

C

g

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

T

SS

→

C

ss

g

-----------

T

SS

k

–

→

==

=

f

min

1.41

R

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

=