参数资料
| 型号： | ZXSC440X8TA |
| 厂商： | Diodes Inc |
| 文件页数： | 6/11页 |
| 文件大小： | 0K |
| 描述： | IC PHOTOFLASH CHARGER 8MSOP |
| 产品变化通告： | End of Life 16/Sept/2011 |
| 标准包装： | 1 |
| 应用： | 照相机闪光灯电容器充电器 |
| 电流 - 电源： | 220µA |
| 电源电压： | 1.8 V ~ 8 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-TSSOP，8-MSOP（0.118"，3.00mm 宽） |
| 供应商设备封装： | 8-MSOP |
| 包装： | 标准包装 |
| 产品目录页面： | 1478 (CN2011-ZH PDF) |
| 其它名称： | ZXSC440X8DKR |
ZXSC440
PHOTOFLASH CHARGER
Application Information (cont.)
Output Power Calculation
This is approximately the power stored in the coil times the
frequency of operation times the efficiency. Assuming a
current of 1.2 amps in a 30 μ H primary, the stored energy
will be 21.6 μ J. The frequency is set by the time it takes the
primary to reach 1.2 amps plus the 1.7 μ s time allowed to
discharge the energy into the reservoir capacitor. Using
3 volts, the ramp time is 12 μ s, so the frequency will be
73kHz, giving an input power of about 1.6 watts. With an
efficiency of 75% the output power will be 1.2 watts. An
80 μ F capacitor charged to 300 volts stores 3.6J, so
1.2 watts will take 3 seconds to charge it. Higher input
voltages reduce the ramp time, the frequency therefore
goes up and the output power is increased, resulting in
shorter charging times.
Output Voltage Adjustment
The ZXSC440 are adjustable output converters allowing
the end user the maximum flexibility. For adjustable
operation a potential divider network is connected as
follows:
The output voltage is determined by the equation:
V OUT = V FB (1 + RA / RB),
where V FB =300mV
In a circuit giving 300 volts, the "1" in the above equation
becomes negligible compared to the ratio which is around
1000. It will not be exactly 1000 because of the negative
input current in the feedback pin. The resistor values,
RA and RB, should be maximized to improve efficiency
and decrease battery drain. Optimization can be achieved
by providing a minimum current of I FB(MAX) =200nA to the
V FB pin. Output is adjustable from V FB to the (BR)V CEO of
the switching transistor, Q1.
In practice, there will be some stray capacitance across
RA and this will cause a lead in the feedback which can
affect hysteresis (it makes the device shut down too early)
and it is best to swamp this with a capacitor CA and then
use a capacitor CB across RB where CB/CA = RA/RB.
This is similar to the method used for compensating
oscilloscope probes.
Layout Issues
Layout is critical for the circuit to function in the most
efficient manner in terms of electrical efficiency, thermal
considerations and noise.
For 'step-up converters' there are four main current loops,
the input loop, power-switch loop, rectifier loop and output
loop. The supply charging the input capacitor forms the
input loop. The power-switch loop is defined when Q1 is
'on', current flows from the input through the transformer
primary, Q1, R SENSE and to ground. When Q1 is 'off', the
energy stored in the transformer is transferred from the
secondary to the output capacitor and load via D1, forming
the rectifier loop. The output loop is formed by the output
capacitor supplying the load when Q1 is switched back off.
To optimize for best performance each of these loops kept
separate from each other and interconnected with short,
thick traces thus minimizing parasitic inductance,
capacitance and resistance. Also the R SENSE resistor
should be connected, with minimum trace length, between
emitter lead of Q1 and ground, again minimizing stray
parasitics.
ZXSC440
Document number: DS33619 Rev. 3 - 2
6 of 11
June 2012
? Diodes Incorporated
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