L6565

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of the converter's input voltage sensed through a dedicated pin (#3, VFF): the higher the input voltage, the lower

the setpoint. This is illustrated in the diagram of figure 17a that shows the relationship between the voltage at

The schematic in figure 17b shows also how the function is included in the control loop. With a proper selection

of the external divider R1-R2 it is possible to achieve the optimum compensation described by the lower curve

in the diagram of figure 16.

In applications where this function is not wanted, e.g. because of a narrow input voltage range, the VFF pin can

be simply grounded, thus saving the resistor divider. The overcurrent setpoint will be then fixed at the maximum

value of about 1.4V (1.5V max.).

Line Feedforward is also beneficial to other characteristics of quasi-resonant converters: it improves their input

ripple rejection ability and limits the variation of the power stage's small-signal gain versus the line voltage.

Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line Feedforward function block

0

0.5

1

1.5

2

2.5

3

3.5

0

0.5

1

1.5

E/A

4

23

Rs

PWM

VOLTAGE

FEED

FORWARD

+Vin

+

-

2.5V

1

DRIVER

7

R1

R2

INV

COMP

VFF

CS

GD

L6565

2 V

Hiccup

DISABLE

R

S

Q

5

ZCD

STARTER

ZCD

(reset-dominant)

starter STOP

a)

b)