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| 型号： | A8740EEETR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 13/18页 |
| 文件大小： | 0K |
| 描述： | IC PHOTOFLASH CAP CHARGER 8DFN |
| 标准包装： | 24,000 |
| 拓扑： | 交直流离线开关，反激式，PFC |
| 输出数： | 1 |
| 内部驱动器： | 无 |
| 类型 - 主要： | 闪灯/白光 |
| 电源电压： | 1.5 V ~ 5.5 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-UFDFN 裸露焊盘 |
| 供应商设备封装： | 8-DFN-EP（2x2） |
| 包装： | 带卷 (TR) |
| 工作温度： | -40°C ~ 85°C |
A8740
Ultra Small Mobile Phone Xenon Photoflash
Capacitor Charger with IGBT Driver
Applications Information
Transformer Design
1. The transformer turns ratio, N, determines the output voltage:
N = N S / N P
V OUT = 31.5 × N – V d ,
where 31.5 is the typical value of V OUTTRIP , and V d is the for-
ward drop of the output diode.
2. The primary inductance, L PRIMARY , determines the on-time of
the switch:
t on = (– L PRIMARY / R ) × ln (1 – I SWlim × R / V IN ) ,
where R is the total resistance in the primary current path (includ-
ing R SWDS(on) and the DC resistance of the transformer).
If V IN is much larger than I SWlim × R, then t on can be approxi-
mated by:
t on = I SWlim × L PRIMARY / V IN .
3. The secondary inductance, L SECONDARY , determines the off-
time of the switch. Given:
L SECONDARY / L PRIMARY = N × N , then
t off = ( I SWlim / N ) × L SECONDARY / V OUT
= ( I SWlim × L PRIMARY × N ) / V OUT .
t on
V SW
The minimum pulse width for t off determines what is the mini-
mum L PRIMARY required for the transformer. For example, if
ILIM = 1.5 A, N = 10, and V OUT = 315 V, then L PRIMARY must
be at least 4.2 μ H in order to keep t off at 200 ns or longer. These
relationships are illustrated in figure 6.
In general, choosing a transformer with a larger L PRIMARY results
in higher efficiency (because a larger L PRIMARY corresponds to
a lower switch frequency and hence lower switching loss). But
transformers with a larger L PRIMARY also require more windings
and larger magnetic cores. Therefore, a trade-off must be made
between transformer size and efficiency.
Leakage Inductance and Secondary Capacitance
The transformer design should minimize the leakage induc-
tance to ensure the turn-off voltage spike at the SW node does
not exceed the absolute maximum specification on the SW pin
(refer to the Absolute Maximum Ratings table). An achievable
minimum leakage inductance for this application, however, is
usually compromised by an increase in parasitic capacitance.
Furthermore, the transformer secondary capacitance should be
minimized. Any secondary capacitance is multiplied by N 2 when
reflected to the primary, leading to high initial current swings
when the switch turns on, and to reduced efficiency.
t off
I SW
V IN
I SW
V r
t f
V IN
V SW
t neg
Figure 6. Transformer Selection Relationships
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12
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