SO16N datasheet(38/51 Pages) STMICROELECTRONICS

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Application information

L6566BH

38/51

Doc ID 16610 Rev 2

Figure 26.

Voltage sensing techniques to implement brownout protection with the

L6566BH

It is usually 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 is split as:

Equation 18

Circuit a) senses the input voltage bus (across the bulk capacitor, downstream of 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, if necessary,

holdup requirements during mains missing cycles as well. Brownout level is 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 up to several seconds.

Circuit b) senses the mains voltage directly, upstream of 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 is equal to 1/

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

equal to 2/

π the peak mains voltage. CF 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 does not depend on the load.

When the input source is removed, CF is discharged after some ten ms then this circuit is

suitable for a quick restart after a latched shutdown.

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

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

lead 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.

AM11502v1

HV Input bus

AC_OK

VFF

Optional for

OVP settings

RL1

RL2

L6566BH

AC mains (L/N)

AC_OK

VFF

RL1

RL2

L6566BH

Optionalfor

OVP settings

AC mains (N/L)

()

opt

;

−

SO16N Datasheet(PDF) 38 Page - STMicroelectronics

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