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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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X-11181-002 데이터시트(PDF) 10 Page - Linear Technology |
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X-11181-002 데이터시트(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3798 10 3798f fundamental frequency of the supply voltage is 120Hz so the control loop unity gain frequency needs to be set less than approximately 12Hz. Without a large amount of energy storage on the secondary side, the output current will be affected by the supply voltage changes, but the DC com- ponent of the output current will be accurate. For DC input or non-PFC AC input applications, connect a 25k resistor from VIN_SENSE to INTVCC instead of the AC line voltage. Startup The LT3798 uses a hysteretic start-up to operate from high offline voltages. A resistor connected to the supply voltage protects the part from high voltages. This resistor is connected to the VIN pin on the part and bypassed with a capacitor. When the resistor charges the VIN pin to a turn-on voltage set with the EN/UVLO resistor divider and the INTVCC pin is at its regulation point, the part begins to switch. The resistor cannot provide power for the part in steady state, but relies on the capacitor to start-up the part, then the third winding begins to provide power to the VIN pin along with the resistor. An internal voltage clamp is attached to the VIN pin to prevent the resistor current from allowing VIN to go above the absolute maximum voltage of the pin. The internal clamp is set at 40V and is capable of 8mA(typical) of current at room temperature. Setting the VIN Turn-On and Turn-Off Voltages A large voltage difference between the VIN turn-on voltage and the VIN turn-off voltage is preferred to allow time for the third winding to power the part. The EN/UVLO sets these two voltages. The pin has a 10μA current sink when the pins voltage is below 1.25V and 0μA when above 1.25V. The VIN pin connects to a resistor divider as shown in Figure 2. The UVLO threshold for VIN rising is: VIN(UVLO,RISING) = 1.25V • R1+ R2 () R2 + 10μA •R1 The UVLO Threshold for VIN Falling is : VIN(UVLO,FALLING) = 1.25V • R1+ R2 () R2 Programming Output Voltage The output voltage is set using a resistor divider from the third winding to the FB pin. From the Block Diagram, the resistors R4 and R5 form a resistor divider from the third winding. The FB also has an internal current source that compensates for the diode drop. This current source causes an offset in the output voltage that needs to be ac- counted for when setting the output voltage. The output voltage equation is: VOUT = VBG (R4+R5)/(NST • R5)–(VF + (R4 • ITC)/NST) where VBG is the internal reference voltage, NST is the winding ratio between the secondary winding and the third winding, VF is the forward drop of the output rectifying diode, and ITC is the internal current source for the FB pin. The temperature coefficient of the diode's forward drop needs to be the opposite of the term, (R4 • ITC)/NST. By taking the partial derivative with respect to temperature, the value of R4 is found to be the following: R4 = NST(1/(δITC/δT)(δVF/δT)) δITC/δT = 12.4nA/°C ITC = 4.25μA where δITC/δT is the partial derivative of the ITC current source, and δVF/δT is the partial derivative of the forward drop of the output rectifying diode. With R4 set with the above equation, the resistor value for R5 is found using the following: R5 = (VBG • R4)/(NST(VOUT+VF)+R4 • ITC-VBG) OPERATION LT3798 EN/UVLO GND R2 R1 VIN 3798 F02 Figure 2. Undervoltage Lockout (UVLO) |
유사한 부품 번호 - X-11181-002 |
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유사한 설명 - X-11181-002 |
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