viper31sp STMicroelectronics, viper31sp Datasheet - Page 12

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

VIPer31SP
true for low output current where the output
voltage can reach high values, as shown on
figure 11 : 20 V can be reached for a nominal
regulated one of 14.5 V, with a typical
transformer. But a simple clamping zener can
limit it to about 17 V with a reasonable dissipated
power. The 10 % to 100 % output load regulation
is better than +/-7 %.
COMPONENTS SIZING
The following procedure defines the value of
essential parameters for the transformer and the
sensing resistance in a typical application. The
user can adapt by himself the final design,
according to specific needs, if any.
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12/16
S
1.
V
operate in constant current mode.
2. Check that the ratio between the minimum
operating output voltage V
lower than 2.5. This ratio is limited by the
overvoltage protection value (Typically 29 V)
and V
tolerances.
3. Compute the transformer turn ratio n from
primary to secondary with the formula :
n
4. Compute the sense resistance value with the
formula :
5. Compute the transformer turn ratio n
from auxiliary to secondary with the formula :
n
6. The current control function requires the
converter to work in discontinuous mode. The
primary inductance value L
can be computed by respecting this constraint
in all conditions, or by using the following
formula : L
V
from the mains.
AUX
MIN
MAX
OUT
IN
V
n
Define
100
MAX
OUT
is the minimum input rectified DC voltage
x
V
DDreg
for which the converter has still to
0.175
25
MAX
OUT
I
OUT
P
the maximum output voltage
n
n
p
s
10
(Typically 10
n
n
n
a
x
s
V
MIN
IN
I
OUT
x
OUT
P
MIN
T
of the transformer
SW
V) and
and V
where :
MAX
OUT
their
AUX
is
START UP SEQUENCE
An integrated high voltage current source
provides a bias current from the DRAIN pin during
the start-up phase. This current is partially
absorbed by internal control circuits which are
placed into a standby mode with reduced
consumption and also provided to the external
capacitors connected to the V
soon as the voltage on this pin reaches the high
voltage threshold V
device turns into active mode and starts
switching. The start up current generator is
switched off, and the converter should normally
provide the needed current on the VDD pin
through the auxiliary winding of the transformer,
as shown on figure 13.
The sum of the external capacitors C
V
time needed by the converter to start up, when
the device starts switching. This time t
on many parameters, among which transformer
design,
implemented on the CREF pin (See soft start
consideration here after). The following formula
can be used for defining the minimum capacitor
needed :
C
I
when switching. Refer to specified I
values.
t
device begins to switch. Worst case is generally
at full load.
V
logic. Refer to the minimum specified value.
C
capacitors on V
allot a standard 4.7 F / 16 V on the V
the rest on the V
insures a correct decoupling of the internal serial
regulator between V
Soft start feature is implemented through the
CREF capacitor which is also filtering the CREF
voltage. The minimum value of this capacitor has
to be set according to the switching frequency, in
order to filter the charging and discharging current
issued from the CREF pin (Refer to the current
control description part). It can be increased from
DD
SS
DD
DDhyst
START
START
T
SW
is the start up time of the converter when the
is the consumption current on the V
and V
is the switching period.
= C
is the voltage hysteresis of the UVLO
output
I
CC
DD
V
DDhyst
VDD
pins must be sized according to the
x
DD
t
SS
+ C
and V
capacitors,
CC
DDon
CC
VCC
and V
CC
pin. The V
of the UVLO logic, the
pins. Once is defined,
is the sum of both
DD
DD
.
capacitor
and V
where :
DD
DD1
START
SS
DD
CC
capacitor
depends
and I
pins. As
pin, and
DD
on the
value
DD2
pin