L6565D データシートの表示（PDF） - STMicroelectronics

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L6565D

QUASI-RESONANT SMPS CONTROLLER

STMicroelectronics 

L6565

It acts on the clamp level of the control voltage Vcsx, that is on the overcurrent setpoint, so that it is a function

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 pin VFF and Vcsx (with the error amplifier saturated high in the attempt of keeping output voltage regulation).

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

Vcsx [V]

1.5

VCOMP = Upper clamp

1

a)

0.5

0

0

0.5

1

1.5

2

2.5

3

3.5

VVFF [V]

+Vin

R1

R2

Rs

COMP

2

INV 1

-

E/A

+

VFF

CS

3

4

VOLTAGE

FEED

FORWARD

ZCD

5

starter STOP

STARTER

ZCD

PWM

S

Q

R

DRIVER

(reset-dominant)

7

GD

2.5V

DISABLE

Hiccup

2V

L6565

b)

10/17

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