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參數(shù)資料
| 型號(hào)： | A8439EEJTR-T |
| 廠商： | Allegro MicroSystems, Inc. |
| 英文描述： | Photoflash Capacitor Charger with IGBT Driver and Refresh |
| 中文描述： | 照相閃光燈電容充電器與IGBT驅(qū)動(dòng)器和刷新 |
| 文件頁數(shù)： | 11/12頁 |
| 文件大小： | 537K |
| 代理商： | A8439EEJTR-T |
A8439-DS, Rev. 1
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Photoflash Capacitor Charger with IGBT Driver
A8439
11
Adjusting Output Voltage
The A8439 senses output voltage during switch off-time. This
allows the voltage divider network, R1 through R3 (see figure
8), to be connected at the anode of the high voltage output diode,
D1, eliminating power loss due to the feedback network when
charging is complete. The output voltage can be adjusted by
selecting proper values of the voltage divider resistors. Use the
following equation to calculate values for Rx (
Ω
):
3
2
1
=
+
FB
V
V
OUT
R
R
R
1 .
(3)
R1 and R2 together need to have a breakdown voltage of at
least 300 V. A typical 1206 surface mount resistor has a 150 V
breakdown voltage rating. It is recommended that R1 and R2
have similar values to ensure an even voltage stress between
them. Recommended values are:
R1 = R2 = 4.99 M
Ω
(1206)
R3 = 39 k
Ω
(0603)
which together yield a stop voltage of 303 V.
Using higher resistance ratings for R1, R2, and R3 does not
offer significant efficiency improvement, because the power loss
of the feedback network occurs mainly during switch off-time,
and because the off-time is only a small fraction of each charging
cycle.
Output Diode Selection
Choose the rectifying diode(s), D1, to have small parasitic
capacitance (short reverse recovery time) while satisfying the
reverse voltage and forward current requirements.
The peak reverse voltage of the diode, V
D_Peak
, occurs when the
internal MOSFET switch is closed, and the primary-side current
starts to ramp-up. It can be calculated as:
BATT
.
V
N
V
OUT
D_Peak
V
×
+
=
(4)
The peak current of the rectifying diode, ID_Peak, is calculated
as :
.
I
Primary_Peak
D_Peak
I
=
N
/
(5)
Input Capacitor Selection
Ceramic capacitors with X5R or X7R dielectrics are recom-
mended for the input capacitor, C2. It should be rated at least
4.7
μ
F / 6.3 V to decouple the battery input, V
BATT
, at the primary
of the transformer. When using a separate bias, V
BIAS
, for the
A8439 VIN supply, connect at least a 0.1
μ
F / 6.3 V bypass
capacitor to the VIN pin.
Layout Guidelines
Key to a good layout for the photoflash capacitor charger circuit
is to keep the parasitics minimized on the power switch loop
(transformer primary side) and the rectifier loop (secondary side).
Use short, thick traces for connections to the transformer primary
and SW pin.
Output voltage sensing circuit elements must be kept away from
switching nodes such as SW pin. It is important that the
ˉDˉOˉNˉ
signal trace and other signal traces be routed away from the
transformer and other switching traces, in order to minimize noise
pickup. In addition, high voltage isolation rules must be followed
carefully to avoid breakdown failure of the circuit board.
Recommended Components Table
Component
Rating
Part Number
Source
C1, Input Capacitor
0.1
μ
F, ±10%, 16 V X7R ceramic
capacitor (0603)
4.7
μ
F, ±10%, 10 V, X5R ceramic
capacitor (0805)
20 to 180
μ
F, 330 V
GRM188R71C104KA01D
Murata
C2, Input Capacitor
LMK212BJ475KG
Taiyo Yuden
COUT, Photoflash Capacitor
Chemi-Con
Philips Semiconductor,
Fairchild Semiconductor
Yageo
Yageo
TDK
Tokyo Coil Engineering
Asatech
D1, Output Diode
2 x 250 V, 225 mA, 5 pF
BAV23S
R1, R2, FB Resistors
R3, FB Resistor
4.99 M
Ω
,
1
/
4
W ±1% (1206)
39.0 k
Ω
1
/
10
W ±1% (0603)
1:10.2, L
Primary
= 14.5
μ
H
1:10.2, L
Primary
= 12
μ
H
1:10, L
Primary
= 10.8
μ
H
9C12063A4994FKHFT
9C06031A3902FKHFT
LDT565630T-002
T-16-024A
ST-532517A
T1, Transformer
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