AN1061 APPLICATION NOTE

4/21

The oscillator peak-to-valley voltage is proportional to the supply voltage, and the voltage

feedforward is operative from 8V to 55V of input supply.

Also the

∆V/∆t of the sawtooth is directly proportional to the supply voltage. As Vcc increases,

the Ton time of the power transistor decreases in such a way to provide to the chocke, and

finally also the load, the product Voltxsec constant.

Figure 6 shows how the duty cycle varies as a result of the change on the

∆V/∆t of the sawtooth

with the Vcc.

Figure 6. Voltage Feedforward Function

The output of the error amplifier doesn’t change in order to maintain the output voltage con-

stant and in regulation.

With this function on board, the output response time is greatly reduced in presence of an

abrupt change on the supply voltage, and the output ripple voltage at the mains frequency is

greatly reduced too.

In fact, the slope of the ramp is modulated by the input ripple voltage, generally present in the

order of some tens of Volt, for both off-line and dc-dc converters using mains transformers.

The charge and discharge time are approximable to:

The maximum duty cycle is a function of Tch, Tdis and an internal delay and is expressed by

the equation:

and is represented in figure 7.

∆V

osc

V

CC

1

–

6

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

=

V1

V2-3

D97IN684

Vi=30V

Vi=15V

Vc

t

Vi=30V

Vi=15V

t

T

ch

R

osc

C

osc

In

6

5

---





⋅⋅

=

T

dis

100 C

osc

⋅

=

Dmax

R

osc

C

osc

In

6

5

---





80 10

9

–

⋅

–

⋅⋅

R

osc

C

osc

In

6

5

---





100 C

osc

⋅

+

⋅⋅

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

=