a8837 Allegro MicroSystems, Inc., a8837 Datasheet - Page 9

• Manufacturer Part Number: a8837
• Description: Photoflash Capacitor Charger With Igbt Driver
• Manufacturer: Allegro MicroSystems, Inc.

A8837
Transformer Design
Turns Ratio.
ary : Primary) should be chosen based on the following formula:
where:
For example, if V
be the case when two high voltage diodes were in series), and the
desired V
In a worst case, when V
at their maximum tolerance limit, N will be higher. Taking V
= 5.5 V, V
the worst case condition, N can be determined to be 9.5.
Recommended Components
C1, Input Capacitor
C2, Input Capacitor
COUT, Photoflash
Capacitor
D1, Output Diode
R1, R2, FB Resistors
R3, FB Resistor
T1, Transformer
V
V
V
40 (V) is the rated voltage for the internal MOSFET switch,
representing the maximum allowable reflected voltage from the
output to the SW pin.
OUT
D_Drop
BATT
Component
(V) is the required output voltage level,
(V) is the transformer battery supply, and
OUT
D_Drop
(V) is the forward voltage drop of the output diode(s),
The minimum transformer turns ratio, N, (Second-
is 320 V, then the turns ratio should be at least 8.9.
= 2 V, and V
BATT
N
is 3.5 V and V
0.1 μF, ±10%, 16 V X7R
ceramic capacitor (0603)
4.7 μF, ±10%, 10 V, X5R
ceramic capacitor (0805)
330 V 100 μF (or 19 to 180 μF) EPH-331ELL101B131S
2 x 250 V, 225 mA, 5 pF
150 kΩ each
1206, 0805, or 0603 resistors
rated for 150 V
1.2 kΩ
0402)
BATT
L
N = 10
L
N = 10
L
N=10
V
P
P
P
OUT
= 14.2 μH, I
= 7.4 μH, I
= 14 μH, I
40
is highest and V
OUT
1
/
10
V
V
= 320 V × 102 % = 326.4 V as
BATT
D _ Drop
W ±1% (0603 or
Rating
1
D_Drop
/
P
4
P
= 1.5 A,
W, ± 1%;
P
= 2 A,
= 2 A,
is 1.7 V (which could
D_Drop
Applications Information
and V
Photoflash Capacitor Charger with IGBT Driver
GRM188R71C104KA01D Murata
LMK212BJ475KG
BAV23S
T-15-154M
T-16-103A
ST-652956A
OUT
(1)
BATT
are
Part Number
In practice, always choose a turns ratio that is higher than the
calculated value to give some safety margin. In the worst case
example, a minimum turns ratio of N = 10 is recommended.
Primary Inductance
primary inductance, L
Ideally, the charging time is not affected by transformer pri-
mary inductance. In practice, however, it is recommended that a
primary inductance be chosen between 10 μH and 20 μH. When
L
quency, which increases switching loss proportionally. This leads
to lower efficiency and longer charging time. When L
greater than 20 μH, the rating of the transformer must be dra-
matically increased to handle the required power density, and the
series resistances are usually higher. A design that is optimized to
achieve a small footprint solution would have an L
14 μH, with minimized leakage inductance and secondary capaci-
tance, and minimized primary and secondary series resistance.
See the table Recommended Components for more information.
Primary
is lower than 10 μH, the converter operates at higher fre-
Taiyo Yuden
Chemi-Con
Philips Semiconductor,
Fairchild Semiconductor
Tokyo Coil
Tokyo Coil
Asatech
L
Primary
. As a loose guideline when choosing the
Primary
Source
300
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
N
(μH), use the following formula:
10
I
SWLIM
9
V
OUT
10 V minimum rating can
be used
Instead of two resistors,
a single 300 kΩ resistor
with 350 V rating can be
used
Suitable for I
0.7 to 2.0 A
Suitable for I
1.2 to 2.0 A only
Suitable for I
0.7 to 1.4 A only
.
Remarks
Primary
Primary
LIM
LIM
LIM
of 12 to
from
from
from
(2)
is
9