SO16N datasheet(26/51 Pages) STMICROELECTRONICS

26 / 51 page

Application information

L6566BH

26/51

Doc ID 16610 Rev 2

Cycle-by-cycle current limitation is realized with a second comparator (OCP comparator)

that senses the voltage across the current sense resistor Rs as well and compares this

voltage to a reference value V

CSX. Its output is OR-ed with that of the PWM comparator (see

the circuit schematic in

Figure 17). In this way, if the programming signal delivered by the

feedforward block and sent to the PWM comparator exceeds V

CSX, it is the OCP comparator

to reset first the PWM latch instead of the PWM comparator. The value of Vcsx, thereby,

determines the overcurrent setpoint along with the sense resistor Rs.

The power that QR flyback converters with a fixed overcurrent setpoint (like fixed-frequency

systems) are able to deliver changes considerably with the input voltage. With wide-range

mains, at maximum line it can be more than twice the value at minimum line, as shown by

the upper curve in the diagram of

Figure 16. The device has the line feedforward function

available to solve this issue.

It acts on the overcurrent setpoint V

CSX, so that it is a function of the converter’s input voltage

Vin sensed through a dedicated pin (15, VFF): the higher the input voltage, the lower the

setpoint. This is illustrated in the diagram on the left-hand side of

Figure 17: it shows the

relationship between the voltage on the pin VFF and V

CSX (with the error amplifier saturated

high in the attempt to obtain output voltage regulation):

Equation 5

Figure 16.

Typical power capability change vs. input voltage in QR flyback

converters

Note:

If the voltage on the pin exceeds 3 V, switching ceases but the soft-start capacitor is not

discharged. The schematic in Figure 17 shows also how the function is included in the

control loop.

With a proper selection of the external divider R1-R2, i.e. of the ratio k = R2 / (R1+R2), it is

possible to achieve the optimum compensation described by the lower curve in the diagram

Figure 16.

The optimum value of k, kopt, which minimizes the power capability variation over the input

voltage range, is the one that provides equal power capability at the extremes of the range.

The exact calculation is complex, and non-idealities shift the real-world optimum value from

Vin

VFF

csx

−

AM11492v1

11.5

2.5

33.5

0.5

1.5

2.5

system optimally

compensated

system not

compensated

k = 0

k = kopt

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