Datasheet LT8304, LT8304-1 (Analog Devices) - 10

LT8304/LT8304-1
APPLICATIONS INFORMATION Output Voltage
Combination with the previous VFLBK equation yields an The R equation for VOUT, in terms of the RFB and RREF resistors, FB and RREF resistors as depicted in the Block Diagram are external resistors used to program the output voltage. transformer turns ratio, and diode forward voltage: The LT8304 operates similar to traditional current mode     R   1  switchers, except in the use of a unique flyback pulse V FB OUT = VREF •  R  • N  – VF sense circuit and a sample-and-hold error amplifier, which  REF   PS  sample and therefore regulate the isolated output voltage from the flyback pulse.
Output Temperature Compensation
Operation is as follows: when the power switch M1 turns The first term in the VOUT equation does not have tempera- off, the SW pin voltage rises above the V ture dependence, but the output diode forward voltage, V IN supply. The F, amplitude of the flyback pulse, i.e., the difference between has a significant negative temperature coefficient (–1mV/°C the SW pin voltage and V to –2mV/°C). Such a negative temperature coefficient pro- IN supply, is given as: duces approximately 200mV to 300mV voltage variation VFLBK = (VOUT + VF + ISEC • ESR) • NPS on the output voltage across temperature. VF = Output diode forward voltage For higher voltage outputs, such as 12V and 24V, the ISEC = Transformer secondary current output diode temperature coefficient has a negligible ef- ESR = Total impedance of secondary circuit fect on the output voltage regulation. For lower voltage N outputs, such as 3.3V and 5V, however, the output diode PS = Transformer effective primary-to-secondary turns ratio temperature coefficient does count for an extra 2% to 5% output voltage regulation. The flyback voltage is then converted to a current, IRFB, by the R The LT8304 junction temperature usually tracks the output FB resistor and the flyback pulse sense circuit (M2 and M3). This current, I diode junction temperature to the first order. To compensate RFB, also flows through the R the negative temperature coefficient of the output diode, REF resistor to generate a ground-referred voltage. The resulting voltage feeds to the inverting input of the sample- a resistor, RTC, connected between the TC and RREF pins and-hold error amplifier. Since the sample-and-hold error generates a proportional-to-absolute-temperature (PTAT) amplifier samples the voltage when the secondary current current. The PTAT current is zero at 25°C, flows into the is zero, the (I R SEC • ESR) term in the VFLBK equation can be REF pin at hot temperature, and flows out of the RREF pin assumed to be zero. at cold temperature. With the RTC resistor in place, the output voltage equation is revised as follows: The internal reference voltage, VREF, 1.00V, feeds to the noninverting input of the sample-and-hold error ampli- R 1 V FB • – V ( )–( V )• fier. The relatively high gain in the overall loop causes the OUT = VREF • R F TO TC / T REF NPS voltage at the RREF pin to be nearly equal to the internal reference voltage V R 1 REF. The resulting relationship between (T–TO)• FB • –( VF / T)•(T–TO) VFLBK and VREF can be expressed as: RTC NPS  V  TO=Room temperature 25°C FLBK   R  •RREF = VREF or  FB  ( VF / T)=Output diode forward voltage  temperature coefficient  R  V FB  FLBK = VREF •  ( V )=3.35mV/°C R  TC / T  REF  VREF = Internal reference voltage 1.00V 8304fa 10 For more information www.linear.com/LT8304 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts