viper31sp STMicroelectronics, viper31sp Datasheet - Page 10

• Manufacturer Part Number: viper31sp
• Description: Battery Charger Primary I.c.
• Manufacturer: STMicroelectronics

VIPer31SP
OPERATION DESCRIPTION :
This device is intended to be used in off line
AC/DC adapter where the desired output
characteristic
characteristic. For output voltage values lower
than a fixed value, the average output current
must be constant, whatever are the input or
output voltages. If the output current consumed
by the load is lower than the previous constant
current value, the output voltage value must be
limited. In addition, the device provides protection
against output short circuits and overtemperature
events.
The two modes of operation are described in the
following paragraphs. Figure 10 presents a typical
application of which the output characteristic can
be seen on figure 11.
CONSTANT OUTPUT CURRENT
The power topology to be used with this device is
a simple discontinuous flyback, as shown on
figure 10. The average output current of such a
10/16
Figure 12: Constant Current Operation
C2
Uc
CREF
VCC
must
C
Ic
Iref
GND
D2
R
present
CS ENSE
Demag.
+
-
DSENSE
EOD
R1
VIPer31
PWM
Latch
a
R2
Oscillator
R
SOURCE
rectangular
Q
DRAIN
+Vin
RS
Ip
n
T
Is
topology cannot be easily kept constant, as it
depends on the output voltage. Actually, if the
peak primary current is fixed, the converter
behaves as a constant power generator.
Therefore, a modulation of the peak primary
current versus output voltage must be done in
order to
characteristic. A conventional way consists to use
an optocoupler between primary and secondary,
with additional circuitry on secondary side
(Reference, error amplifier and current sense
resistor).
This device avoids the use of all the secondary
circuitry by controlling from primary side the
secondary average output current. Figure 12
presents the internal constitution of the current
control function. It is built around a constant
current source Iref, and a mosfet switch driven
with the complemented signal EOD, in series with
a resistance R. The middle point of these
elements is available on the CREF pin.
The
demagnetisation function, which is monitoring the
voltage of the main transformer auxiliary winding.
D1
n.
Iref -
VCREF
VCREF
RS
RS
EOD
Iref
Uc
R
Ic
Ip
Is
1
0
EOD
C1
IOUT
GND
get the constant output current
signal
tonsec
Tsw
is
SC12140
generated
t
t
t
t
by
the