Datasheet LT3798 (Analog Devices) - 10

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