L6566B can't be re-enabled without taking Vcc pin below 7.6V?

[treez]

Hello,
We wish to use L6566B for a 60W flyback. We will supply its Vcc pin with an external 18V supply.
We need to be able to disable the L6566B to stop it switching.
We can do this by either
1...Taking "DIS" pin to >4.5V
2....Grounding the "AC_OK" pin with a BJT

However, if we do the above, does this mean that it cannot subsequently be re-enabled unless the Vcc pin is taken below 7.6V?

The datasheet is not clear about this, since it simply assumes that one will use the internal startup current source and then a bias winding to power it.

L6566B datasheet
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf

 

[treez]

So, does anyone believe that L6566B pwm controller cannot be simply powered by an external 18V bias supply? (instead of via an auxiliary coil)?

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Also, relating to the L6566B pwm controller, please may I enquire as to the value of the R(pullup) resistor in feedback loop calculation?...as follows....

We are using the L6566B for a 60W offline flyback as in figure 30 (page 45) of the L6566B datasheet

L6566B datasheet:
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf

As you know, there is a gain term in the error amplifier transfer function equal to

[R(pullup)/R(LED)] * CTR

where:-
CTR = CTR of opto
R(led) = resistance in series with opto diode (as seen in fig 14, page 26, LHS).
R(pullup) = the resistor in series with the common-emitter connected opto transistor, that connects to the COMP pin.

Is the value of “R(pullup)” equal to 25k?, as depicted by “dynamic resistance” on page 14 of the datasheet?

 

[KlausST]

Hi,

Datasheet says it is "non latching". --> Re-enabling should work.


AC_OK
Brownout protection input. A voltage below 0.45 V shuts down (not latched) the IC,
lowers its consumption and clears the latch set by latched protections (DIS > 4.5 V,
SS > 6.4 V, VFF > 6.4 V). IC’s operation is re-enabled as the voltage exceeds
0.45 V. The comparator is provided with current hysteresis: an internal 15 μA
current generator is ON as long as the voltage on the pin is below 0.45 V and is
OFF if this value is exceeded. Bypass the pin with a capacitor to GND (pin 3) to
reduce noise pick-up. Tie to Vcc with a 220 to 680 kΩ resistor if the function is not
used

Is there any possibility that an undervoltage or overvoltage occurs to pins: 6, 7, 8, 10, 14, 12, 13
This could cause a "latch up" condition to the complete chip or parts of the chip.
Consider protection circuits if you are not sure..

Klaus

 

[treez]

Also relating to L6566B, please could you tell why the L6566B shuts down (latching shutdown) if the COMP pin gets pulled to ground?

….As you know, this is highly unusual, and is confirmed on page 9 of the L6566B datasheet..

L6566B datasheet:
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf

This is surely non-sensical, as in cases of no-load, the secondary side error amplifier would likely saturate and draw maximum current through the optocoupler…this indeed would result in the COMP pin voltage going to zero volts, and thus, an unwanted shutdown……nobody wants an SMPS to shut down and latch off when it goes into no-load or light load, so why does the L656B do this?

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Thanks Klaus...

Hi,

Datasheet says it is "non latching". --> Re-enabling should work.


AC_OK
Brownout protection input. A voltage below 0.45 V shuts down (not latched) the IC,
lowers its consumption and clears the latch set by latched protections (DIS > 4.5 V,
SS > 6.4 V, VFF > 6.4 V). IC’s operation is re-enabled as the voltage exceeds
0.45 V. The comparator is provided with current hysteresis: an internal 15 μA
current generator is ON as long as the voltage on the pin is below 0.45 V and is
OFF if this value is exceeded. Bypass the pin with a capacitor to GND (pin 3) to
reduce noise pick-up. Tie to Vcc with a 220 to 680 kΩ resistor if the function is not
used

Is there any possibility that an undervoltage or overvoltage occurs to pins: 6, 7, 8, 10, 14, 12, 13
This could cause a "latch up" condition to the complete chip or parts of the chip.
Consider protection circuits if you are not sure..

Klaus

yes I understand what you mean, but I think that when the AC_OK pin is released, in order to re-enable the L6566B, the VCC pin still needs to go below 5V for the L6566B to get up and running again?..i mean, the datasheet shows the re-enablement of the chip following DIS>5V by the AC_OK pin being pulled low then going high....but this is done by the VCC pin going below 5v and then coming back up again via the internal HVS source....Also, the datasheet does not say what is the current consumption of the L6566B when the AC_OK pin has been pulled to ground in order to shut the L6566B off.