L6566B datasheet(39/51 Pages) STMICROELECTRONICS

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L6566B

Application information

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Figure 26.

Voltage sensing techniques to implement brownout protection with the

L6566B

It is typically convenient to use a single divider to bias both the AC_OK and the VFF pins, as

shown in Figure 26: this is possible because in all practical cases the voltage on the VFF pin

is lower than that on the AC_OK pin. Once RH and RL have been found as suggested above,

and kopt, either calculated from (6) or (8) or experimentally found, RL will be split as:

Equation 18

Circuit a) senses the input voltage bus (across the bulk capacitor, downstream the bridge

rectifier); in this case, for a proper operation of the brownout function, VsenON must be lower

than the peak voltage at minimum mains and VsenOFF lower than the minimum voltage on

the input bulk capacitor at minimum mains and maximum load considering, in case, holdup

requirements during mains missing cycles as well. Brownout level will be load-dependent. In

case of latched shutdown, when the input source is removed it is necessary to wait until the

bulk capacitor voltage falls below the start voltage of the HV generator VHVstart in order for

the unit to restart, which may take even several seconds.

Circuit b) senses the mains voltage directly, upstream the bridge rectifier. It can be

configured either for half-wave sensing (only the line/neutral wire is sensed) or full-wave

sensing (both neutral and line are sensed); in the first case, assuming CF is large enough,

the sensed voltage will be equal to 1/

π the peak mains voltage, while in the second case it

will be equal to 2/

π the peak mains voltage. C

F needs to be quite a big capacitor (in the uF)

to have small residual ripple superimposed on the dc level; as a rule-of-thumb, use a time

constant RL ·CF at least 4-5 times the maximum line cycle period in case of half-wave

sensing, 2-3 times in case of full-wave sensing. Then fine tune if needed, considering also

transient conditions such as mains missing cycles. Brownout level will not depend on the

load. When the input source is removed CF will be discharged after some ten ms then this

circuit is suitable to have a quick restart after a latched shutdown.

The AC_OK pin is a high impedance input connected to high value resistors, thus it is prone

to pick up noise, which might alter the OFF threshold when the converter is running or give

origin to undesired switch-off of the device during ESD tests. It is possible to bypass the pin

to ground with a small film capacitor (e.g. 1-10 nF) to prevent any malfunctioning of this kind.

The voltage on the pin is clamped upwards at about 3.15 V; then, if the function is not used

the pin has to be connected to Vcc through a resistor (220 to 680 k

Ω).

HV Input bus

AC_OK

VFF

Optionalfor

OVPsettings

RL1

RL2

L6566B

AC mains (L/N)

AC_OK

VFF

RL1

RL2

L6566B

Optionalfor

OVP settings

AC mains (N/L)

(

)

opt

;

−

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