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参数资料
| 型号： | A8740EEETR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 11/18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
Functional Description
General Operation Overview
The charging operation is started by a low-to-high signal on the
CHARGE pin, provided that V IN is above the V UVLO level. It is
strongly recommended to keep the CHARGE pin at logic low
during power-up. After V IN exceeds the UVLO level, a low-
to-high transition on the CHARGE pin is required to start the
charging. The ˉDˉˉ ˉOˉ ˉˉNˉ ˉEˉ open-drain indicator is pulled low when
CHARGE is high and target output voltage is reached.
When a charging cycle is initiated, the transformer primary side
current, I PRIMARY , ramps-up linearly at a rate determined by the
combined effect of the battery voltage, V BAT , and the primary
side inductance, L PRIMARY . When I PRIMARY reaches the current
limit, I SWLIM , the internal MOSFET is turned off immediately,
allowing the energy to be pushed into the photoflash capacitor,
C OUT , from the secondary winding. The secondary side current
drops linearly as C OUT charges. The switching cycle starts again,
either after the transformer flux is reset, or after a predetermined
time period, t OFF (max) (18 μ s), whichever occurs first.
The A8740 senses output voltage indirectly on primary side. This
eliminates the need for high voltage feedback resistors required
for secondary sensing. Flyback converter stops switching when
output voltage reaches:
V OUT = K × N – V d ,
Where:
K = 31.5 V typically,
V d is the forward drop of the output diode (approximately 2 V),
and
N is transformer turns ratio.
Timer Mode and Fast Charging Mode
The A8740 achieves fast charging times and high efficiency by
operating in discontinuous conduction mode (DCM) through
most of the charging process. The relationship of Timer mode and
Fast Charging mode is shown in figure 2.
The IC operates in Timer mode when beginning to charge a com-
pletely discharged photoflash capacitor, usually when the output
voltage, V OUT , is less than approximately 30 V (depending on
transformer used). Timer mode is a fixed period, 18 μ s, off-time
control. One advantage of having Timer mode is that it limits the
initial battery current surge and thus acts as a “soft-start.” A time-
expanded view of a Timer mode interval is shown in figure 3.
V OUT
I IN
Figure 2. Timer mode and Fast Charging mode: t = 1 s/div;
V OUT = 50 V/div; I IN = 150 mA/div., V IN = V BAT = 3.6 V;
C OUT = 100 μ F / 330 V; and ILIM = 1.0 A.
Switch On-Time and Off-Time Control
The A8740 implements an adaptive on-time/off-time control. On-
time duration, t on , is approximately equal to
C4
I SW
t on = I SWlim × L PRIMARY / V BAT .
Off-time duration, t off , depends on the operating conditions
during switch off-time. The A8740 applies two charging modes:
C2,C3
V SW
V BAT
Fast Charging mode and Timer mode, according to the conditions
described in the next section.
C1
Figure 3. Expanded view of Timer mode: V OUT ≤ 10 V, V BAT = 5.5 V,
Ch1: V OUT = 20 V / div., Ch2: V BAT = 5 V / div., Ch3: V SW = 5 V / div.,
Ch4: I SW = 750 mA / div., t = 5 μ s / div.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
V OUT
10
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