VB408 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
VB408 Datasheet PDF : 8 Pages
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VB408 / VB408B / VB408FI
Figure 1: Typical application diagram
VIN
C1
VIN
VOUT
VB408
ADJ
VOUT
VREF R1 C2
R2
FC00420
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 70µA current has to be drawn from the
ADJ pin. Considering that, it results:
VOUT = VREF (1+R2/R1) + IADJR2
To simplify this formula, neglecting IADJ with respect to
the term
VREF /R1
R1 must be chosen so to obtain a minimum 1.2mA current
flowing through the divider. In such a way the VOUT value
will be affected by an error <5%. The current set resistor
R1 should be tied directly to the output terminal of the
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
properly chosen by R1 resistor. The zener diode improves
the ripple rejection and reduces the value of the worst
case output voltage error. In this case the output voltage
is given by:
VOUT = VREF+VZ
In order to reduce excessive output ringing, a minimum
output capacitor C2 of 0.1µF is suggested. This capacitor
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
the first case the input capacitor C1 is not needed,
whereas in the second case a high value of C1 (> 1µF)
must be chosen. In fact, in this kind of application, C1 has
to supply to the input pin the minimum allowed voltage
during the negative half wave. Obviously bigger C1 value,
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
C2 suggested value of 0,1µF.
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 1µF 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).
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