VB409SP View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
VB409SP Datasheet PDF : 9 Pages
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VB409 / VB409SP
OPERATION DESCRIPTION
The VB409, VB409SP contain two separate
stages, as shown in the block diagram. The first
stage is a preregulator that translates the high
rectified mains voltage to a low voltage and
charges an external electrolytic capacitor. The
second stage is a simple 5V regulator. The typical
operating waveforms are shown in Figure 2. The
device may be driven by a half wave (110 or 230
Vr.m.s.) or by a full wave using a bridge rectifier.
Current flow through the preregulator stage is
provided by the trilinton only during a conduction
angle, at both the start and the end of each half
cycle. This angle is set by adjusting the external
resistor divider (R1 and R2), in order to set the
time t1 at which voltage at the threshold pin
reaches the internal threshold Vref1 (see Figure
2a). When the threshold pin voltage gets over
Vref1, the series trilinton is switched off and
remains off until voltage at the threshold pin again
drops below the internal threshold. Using this
technique, energy is drawn from the AC mains
only during the low voltage portions of each
positive half cycle, thus reducing the dissipation in
the first stage. During the conduction angle,
current provided by the trilinton is used to supply
the load and to charge the capacitor C1. In such a
way, when the trilinton switches off, the load
receives the required current by the capacitor
discharge. For this reason it is important to set
properly the conduction angle: during this period
C1 has to reach a sufficient charge to guarantee
that, at the end of discharging, the voltage drop
between the capacitor and the output pin is over
2V. Assuming that conduction angle has been set,
two different possibilities can occur:
1) C1 value is such to reach Vcap(max) within the
conduction angle. As the comparator also
senses C1 voltage, when Vcap gets over Vref1,
the trilinton would switch off. But doing this, the
capacitor would discharge through the load so
reducing its voltage. As soon as Vcap drops
below Vref1, the trilinton switches on. As
consequence the trilinton reaches a stable
condition limiting the current to a value
sufficient to supply the load and hold the
capacitor voltage just below Vcap(max) (see
figures 2b and 2c).
2) C1 value is such to reach Vcap(max) outside the
conduction angle. In this case the trilinton
doesn’t reduce the current, but hold it to a
constant value (ICL(in)) during the whole
conduction angle (see figures 3a and 3b).
As there are two conduction angles for each half
cycle, the capacitor is recharged twice during each
period. In such a way the capacitor voltage has a
small ripple and, consequently, it needs a small
current to regenerate its charge. The device has
integrated current limit and thermal shutdown
protections. The thermal shutdown turns the low
voltage stage off, if the die temperature exceeds a
predetermined value. Hysteresis in the thermal
sense circuit holds the device off until the die
temperature cools down.
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