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VB409 / VB409SP

1

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

2a). When the threshold pin voltage gets over

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:

conduction angle. As the comparator also

the trilinton would switch off. But doing this, the

capacitor would discharge through the load so

consequence the trilinton reaches a stable

condition limiting the current to a value

sufficient to supply the load and hold the

figures 2b and 2c).

conduction angle. In this case the trilinton

doesn’t reduce the current, but hold it to a

constant

value

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.