L4984D datasheet(24/35 Pages) STMICROELECTRONICS

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L4984D

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both surges and drops. A capacitor CFF and a resistor RFF, connected from the VFF pin to

ground, complete an internal peak-holding circuit that provides a DC voltage equal to the

peak of the voltage applied on the MULT pin. In this way, in case of sudden line voltage rise,

CFF is rapidly charged through the low impedance of the internal diode; in case of line

voltage drop, an internal “mains drop” detector enables a low impedance switch that

suddenly discharges CFF, therefore reducing the settling time needed to reach the new

voltage level. The discharge of CFF is stopped when either its voltage equals the voltage on

the MULT pin or the voltage on the VFF pin falls below 0.88 V, to prevent the “brownout

protection” function from being improperly activated (see Section 12: Power management

and housekeeping functions

). With this functionality, an acceptably low steady-state ripple of

the VFF voltage (and, then, low current distortion) can be achieved with a limited

undershoot or overshoot on the pre-regulator output during line transients.

The twice-mains-frequency (2

⋅ fL) ripple appearing across CFF is triangular with peak-to-

peak amplitude that, with good approximation, is given by:

Equation 13

where fL is the line frequency. The amount of 3rd harmonic distortion introduced by this

ripple, related to the amplitude of its 2

⋅ fL component, is:

Equation 14

Figure 32

shows a diagram that helps choose the time constant RFF·CFF based on the

amount of maximum desired 3rd harmonic distortion. Note, however, that there is a

minimum value for the time constant RFF·CFF below which improper activation of the VFF

fast discharge may occur. In fact, the twice-mains-frequency ripple across CFF under

steady-state conditions must be lower than the minimum line drop detection threshold

(

ΔVVFF_min = 40 mV). Therefore:

Equation 15

Always connect RFF and CFF to the pin; the IC does not work properly if the pin is left

floating or may be damaged if connected directly to ground.

MULTpk

100

min

VFF

max

MULTpk

−

⋅

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