 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
LT3748IMSPBF Datasheet(PDF) 9 Page - Linear Technology |
|
||||||||||||||||||||||||||||||
LT3748IMSPBF Datasheet(HTML) 9 Page - Linear Technology |
|
|
9 / 30 page  LT3748 9 3748fa Pseudo-DC Theory of Operation The RREFandRFBresistorsasdepictedintheBlockDiagram are external resistors used to program the output voltage. The LT3748 operates much the same way as traditional current mode switchers with the exception of the unique error amplifier which derives its feedback information from the flyback pulse. Operation is as follows: when the NMOS output switch turns off, its drain voltage rises above VIN. The amplitude of this flyback pulse (i.e., the difference between it and VIN) is given as: VFLBK = (VOUT + VF + ISEC • ESR) • NPS VF = DOUT forward voltage ISEC = Transformer secondary current ESR = Total impedance of secondary circuit NPS = Transformer effective primary-to-secondary turns ratio The flyback voltage is converted to a current by RFB and Q2. Nearly all of this current flows through resistor RREF to form a ground-referred voltage. This voltage is fed into the flyback error amplifier. The flyback error amplifier samples this output voltage information when the secondary-side winding current reaches zero. The error amplifier uses a bandgap voltage, 1.223V, as the reference voltage. The relatively high gain in the overall loop will then cause the voltage at the RREF resistor to be nearly equal to the bandgap reference voltage, VBG. The relationship between VFLBK and VBG may then be expressed as: VFLBK RFB ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ = VBG RREF or VFLBK = VBG RFB RREF ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ VBG = Internal bandgap reference Combining with the previous VFLBK expression yields an expression for VOUT, in terms of the internal reference, programming resistors, transformer turns ratio and diode forward voltage drop: VOUT = VBG RFB RREF ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ 1 NPS ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ − VF −ISEC (ESR) Additionally, it includes the effect of nonzero secondary output impedance (ESR). This term can be assumed to be zero in boundary control mode. Temperature Compensation The first term in the VOUT equation does not have a tem- perature dependence, but the diode forward drop, VF, has a significant negative temperature coefficient. To compensate for this, a positive temperature coefficient current source is internally connected to the RREF pin. The current is set by resistor RTC to ground connected between the TC pin and ground. To cancel the temperature coefficient, the following equation is used: δVF δT = − RFB RTC • 1 NPS • δVTC δT or, RTC = −RFB NPS • 1 δVF / δT • δVTC δT ≈ RFB NPS (δVF/δT) = Diode’s forward voltage temperature coef- ficient (δVTC/δT) = 1.85mV/°C VTC = 0.55V The resistor value given by this equation should also be verified experimentally and adjusted, if necessary, to achieve optimal regulation over temperature. The revised output voltage is as follows: VOUT = VBG RFB RREF ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ 1 NPS ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ − VF − VTC RTC ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ • RFB NPS –ISEC (ESR) APPLICATIONS INFORMATION |
Similar Part No. - LT3748IMSPBF |
|
Similar Description - LT3748IMSPBF |
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEET.NET |
| Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
