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TNY255 参数 Datasheet PDF下载

TNY255图片预览
型号: TNY255
PDF下载: 下载PDF文件 查看货源
内容描述: 的TinySwitch系列高效节能,低功耗离线式切换器 [TinySwitch Family Energy Efficient, Low Power Off-line Switchers]
分类和应用:
文件页数/大小: 20 页 / 498 K
品牌: POWERINT [ Power Integrations ]
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TNY253/254/255  
circuit is sampled at the rising edge of the oscillator Clock  
signal (at the beginning of each cycle). If it is high, then the  
powerMOSFETisturnedon(enabled)forthatcycle,otherwise  
the power MOSFET remains in the off state (cycle skipped).  
Since the sampling is done only once at the beginning of each  
cycle, any subsequent changes at the ENABLE pin during the  
cycle are ignored.  
device are constant, the power delivered is proportional to the  
primary inductance of the transformer and is relatively  
independent of the input voltage. Therefore, the design of the  
power supply involves calculating the primary inductance of  
the transformer for the maximum power required. As long as  
the TinySwitch device chosen is rated for the power level at the  
lowest input voltage, the calculated inductance will ramp up the  
current to the current limit before the DCMAX limit is reached.  
5.8 V Regulator  
The 5.8 V regulator charges the bypass capacitor connected to  
theBYPASSpinto5.8Vbydrawingacurrentfromthevoltage  
on the DRAIN, whenever the MOSFET is off. The BYPASS  
pin is the internal supply voltage node for the TinySwitch.  
When the MOSFET is on, the TinySwitch runs off of the energy  
stored in the bypass capacitor. Extremely low power  
consumption of the internal circuitry allows the TinySwitch to  
operate continuously from the current drawn from the DRAIN  
pin. A bypass capacitor value of 0.1 µF is sufficient for both  
high frequency de-coupling and energy storage.  
Enable Function  
The TinySwitch senses the ENABLE pin to determine whether  
or not to proceed with the next switch cycle as described earlier.  
Once a cycle is started TinySwitch always completes the cycle  
(even when the ENABLE pin changes state half way through  
the cycle). This operation results in a power supply whose  
output voltage ripple is determined by the output capacitor,  
amountofenergyperswitchcycleandthedelayoftheENABLE  
feedback.  
TheENABLEsignalisgeneratedonthesecondarybycomparing  
the power supply output voltage with a reference voltage. The  
ENABLE signal is high when the power supply output voltage  
is less than the reference voltage.  
Under Voltage  
Theunder-voltagecircuitrydisablesthepowerMOSFETwhen  
theBYPASSpinvoltagedropsbelow5.1V. OncetheBYPASS  
pin voltage drops below 5.1 V, it has to rise back to 5.8V to  
enable (turn-on) the power MOSFET.  
In a typical implementation, the ENABLE pin is driven by an  
optocoupler. The collector of the optocoupler transistor is  
connected to the ENABLE pin and the emitter is connected to  
the SOURCE pin. The optocoupler LED is connected in series  
with a Zener across the DC output voltage to be regulated.  
When the output voltage exceeds the target regulation voltage  
level (optocoupler diode voltage drop plus Zener voltage), the  
optocoupler diode will start to conduct, pulling the ENABLE  
pin low. The Zener could be replaced by a TL431 device for  
improved accuracy.  
Hysteretic Over Temperature Protection  
The thermal shutdown circuitry senses the die junction  
temperature. Thethresholdissetat135 °Cwith70 °Chysteresis.  
When the junction temperature rises above this threshold  
(135 °C) the power MOSFET is disabled and remains disabled  
until the die junction temperature falls by 70 °C, at which point  
it is re-enabled.  
Current Limit  
The current limit circuit senses the current in the power  
MOSFET. When this current exceeds the internal threshold  
(ILIMIT), the power MOSFET is turned off for the remainder of  
that cycle.  
The ENABLE pin pull-down current threshold is nominally  
50 µA, but is set to 40 µA the instant the threshold is exceeded.  
This is reset to 50 µA when the ENABLE pull-down current  
drops below the current threshold of 40 µA.  
The leading edge blanking circuit inhibits the current limit  
comparator for a short time (tLEB) after the power MOSFET is  
turned on. This leading edge blanking time has been set so that  
current spikes caused by primary-side capacitance and  
secondary-side rectifier reverse recovery time will not cause  
premature termination of the switching pulse.  
ON/OFF Control  
The internal clock of the TinySwitch runs all the time. At the  
beginning of each clock cycle the TinySwitch samples the  
ENABLE pin to decide whether or not to implement a switch  
cycle. If the ENABLE pin is high (< 40 µA), then a switching  
cycle takes place. If the ENABLE pin is low (greater than  
50 µA) then no switching cycle occurs, and the ENABLE pin  
statusissampledagainatthestartofthesubsequentclockcycle.  
TinySwitch Operation  
TinySwitch is intended to operate in the current limit mode.  
When enabled, the oscillator turns the power MOSFET on at  
the beginning of each cycle. The MOSFET is turned off when  
the current ramps up to the current limit. The maximum on-  
time of the MOSFET is limited to DCMAX by the oscillator.  
Since the current limit and frequency of a given TinySwitch  
At full load TinySwitch will conduct during the majority of its  
clock cycles (Figure 4). At loads less than full load, the  
TinySwitch will skipmore cycles in order to maintain voltage  
regulation at the secondary output (Figure 5). At light load or  
no load, almost all cycles will be skipped (Figure 6). A small  
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