VB408 / VB408B / VB408FI

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1

Figure 1: Typical application diagram

OPERATION DESCRIPTIO N

The VB408, VB408B, VB408FI are positive voltage

regulators continuously adjustable from 1.25V to the input

voltage minus a dropout of 30V through an external

divider. In order to set the proper internal reference

voltage, a typical 70

µA current has to be drawn from the

ADJ pin. Considering that, it results:

the term

will be affected by an error <5%. The current set resistor

regulator rather than near the load. This eliminates line

drops from appearing in series with the reference and

degrading regulation. The output voltage can also be set

by a zener diode put between the adjustment pin and

ground (Figure 2). The biasing current of the zener is

the ripple rejection and reduces the value of the worst

case output voltage error. In this case the output voltage

is given by:

In order to reduce excessive output ringing, a minimum

will improve loop stability and output impedance. The

VB408/VB408B/VB408FI can be connected both to a DC

source or to a 285V r.m.s. line by a rectifier diode that

prevents a negative voltage to be applied to the device. In

to supply to the input pin the minimum allowed voltage

more constant the input voltage is and consequently

smaller the output voltage ripple. For the right choice of

the input capacitor, you have also to remember that

increasing its value you will obtain an increased power

dissipation, and a heatsink could be required. The right

heatsink is determined by the load current, input voltage

and ambient temperature. In case of pulse or A.C.

operation the junction temperature is limited by the

thermal capacitances since the package and the heatsink

masses are able

to absorb heat. If the junction

temperature reaches the thermal shutdown limit the

output stage of the regulator is turned-off so that only the

biasing current of the device can flow into the input pin.

Thermal hysteresis is added just to prevent oscillations.

The device is able to provide minimum 40 mA for a time

that is function of dissipated power and consequently of

the used heatsink. In general the device is also suitable in

electronically

switched

motor

housekeeping

supply,

consumer equipments controls power supply or constant

current source.

APPLICATION EXAMPLE

The most common application for the device is in the DC/

DC converters with an input voltage up to 400 VDC and a

Using an external rectifier an AC/DC converter can be

easily implemented, in this case the device can operate

with an AC voltage up to 285VRMS. In this case must be

used a minimum 1

µF input capacitor to provide the load

current during the negative half cycle of the main. Another

important circuit that can be implemented is the start up

function for low voltage input PWM ICs directly from a

high voltage source (see figure 3).

VB408

R1

R2

C2

C1

FC00420