viper31sp STMicroelectronics, viper31sp Datasheet - Page 11

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

An external resistance R
the negative voltage generated by the winding. As
long as the transformer is delivering some energy
on secondary side, the negated EOD signal
remains in the high state and the mosfet switch Q
is on. The duration of this state is noted tonsec
and corresponds to the time where the secondary
current is flowing through D
the demagnetisation function, refer to figure 6.
The average output current can be expressed as:
I
Where :
I
t
T
Taking into account the transformer ratio n
between primary and secondaryside, I
be expressed versus primary peak current I
I
The value of the capacitor C is sufficiently high to
consider the voltage Uc as constant. This
capacitor is submitted to a charging current and
discharging current at the rhythm of the switching
frequency. As these currents are in the range of a
few mA (Iref is typically 1 mA), a 470 nF is a
suited value for a switching frequency of 60 kHz.
In steady state, it can be written that the charge is
equal to the discharge :
I
It comes :
U
As U
can be also expressed as :
I
Combining (1), (2), (3) and (4) :
I
This last expression shows that the average
output current doesn’t depend any more neither
on the output voltage, nor on the duty cycle, nor
on the input voltage. The only parameters which
are setting its value are :
The transformer ratio n.
The sense resistor value R
OUT
S
ONSEC
S
REF
P
OUT
SW
C
is the peak secondary current.
C
x T
n x
U
R
is the switching period.
R
C
S
is the conduction time on secondary side.
I
can be considered as a constant voltage,
n
2
2
S
x I
SW
I
x
P
X
REF
R x I
t
ONSEC
T
t
x
R
SW
ONSEC
S
t
REF
ONSEC
T
SW
1
S
is needed to withstand
U
R
1
C
. For details about
(4)
I
REF
S
x t
(2)
can also
ONSEC
P
(3)
:
(1)
The product R x I
This product corresponds to a voltage which is
noted Vreg in the specification tables. Figure 5
shows the test fixture for measuring it : The
DSENSE pin is held in the high state (In fact, it is
left open, as an internal pull up current source is
internally connected on this pin) and the mosfet
switch Q is always in the high state. In this case,
the voltage on the CREF pin establishes at
R x I
Note that the oscillator must be running for the
demagnetisation block to sample correctly the
DSENSE pin.
As V
current can be finally written as :
I
A sense resistor of 1.3
of 6 gives a typical output current of about 800
mA.
The
compensation on the CSENSE pin with the two
resistances R5 and R7. These resistances are
connected on the Vin input voltage and are
providing an offset on the current sense pin. The
higher is the input voltage, and the higher is this
offset current. The purpose of this compensation
is to cancel the effect of the current control
propagation time td, which induces an extra
current on top of the theoretical peak current Ip
given by (4).
The
compensation can be seen on figure 11. The
typical ”flatness” is about +/-2.5 %, including the
input voltage variation from 100 VDC to 400 VDC.
If less accuracy is needed, these two resistances
can be omitted.
CONSTANT VOLTAGE OPERATION
An another part of the circuit is in charge of the
regulation of the output voltage, and generates
the vertical characteristic of figure 11. It consists
of a primary feedback regulation, with a
conventional
operational amplifier with an internal voltage
reference of 2.6 V is configured in error amplifier
and defines the duty cycle of the power mosfet
switch by comparison with the oscillator sawtooth
(See block diagram on page 1).
As it is a primary feedback, the accuracy of the
output
transformer coupling quality. This is especially
OUT
reg
REF
schematics
output
n x
has a typical value of 350 mV, the output
voltage
.
0.175
R
voltage
S
current
REF
depends
of
figure
mode
with a transformer ratio
obtained
closely
10
control
VIPer31SP
with
shows
on
:
11/16
this
the
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