Datasheet LT8300 (Analog Devices) - 8

link to page 6 link to page 7 LT8300
OPERATION
reducing the effective quiescent current to improve light switching frequency determines how often the output volt- load efficiency. In this condition, the LT8300 operates age is sampled and also the minimum load requirement. in low ripple Burst Mode. The typical 7.5kHz minimum
APPLICATIONS INFORMATION Output Voltage
bandgap reference voltage VBG. The resulting relationship The R between VFLBK and VBG can be expressed as: FB resistor as depicted in the Block Diagram is the only external resistor used to program the output voltage. ⎛ V ⎞ The LT8300 operates similar to traditional current mode FLBK ⎜ ⎟ •RREF = VBG ⎝ R switchers, except in the use of a unique flyback pulse FB ⎠ sense circuit and a sample-and-hold error amplifier, which or sample and therefore regulate the isolated output voltage from the flyback pulse. ⎛ V ⎞ V BG ⎜ ⎟ •R Operation is as follows: when the power switch M1 turns FLBK = FB = IRFB • RFB ⎝RREF ⎠ off, the SW pin voltage rises above the VIN supply. The amplitude of the flyback pulse, i.e., the difference between VBG = Bandgap reference voltage the SW pin voltage and VIN supply, is given as: IRFB = RFB regulation current = 100µA VFLBK = (VOUT + VF + ISEC • ESR) • NPS Combination with the previous VFLBK equation yields an VF = Output diode forward voltage equation for VOUT, in terms of the RFB resistor, trans- former turns ratio, and diode forward voltage: ISEC = Transformer secondary current ESR = Total impedance of secondary circuit ⎛ R ⎞ V FB OUT = 100µA • ⎜ ⎟− VF ⎝N N PS ⎠ PS = Transformer effective primary-to-secondary turns ratio
Output Temperature Coefficient
The flyback voltage is then converted to a current IRFB by the flyback pulse sense circuit (M2 and M3). This current The first term in the VOUT equation does not have tem- I perature dependence, but the output diode forward volt- RFB also flows through the internal trimmed 12.23k RREF resistor to generate a ground-referred voltage. The result- age VF has a significant negative temperature coefficient ing voltage feeds to the inverting input of the sample- (–1mV/°C to –2mV/°C). Such a negative temperature coef- and-hold error amplifier. Since the sample-and-hold error ficient produces approximately 200mV to 300mV voltage amplifier samples the voltage when the secondary current variation on the output voltage across temperature. is zero, the (ISEC • ESR) term in the VFLBK equation can be For higher voltage outputs, such as 12V and 24V, the output assumed to be zero. diode temperature coefficient has a negligible effect on the The bandgap reference voltage V output voltage regulation. For lower voltage outputs, such BG, 1.223V, feeds to the non-inverting input of the sample-and-hold error ampli- as 3.3V and 5V, however, the output diode temperature fier. The relatively high gain in the overall loop causes coefficient does count for an extra 2% to 5% output voltage the voltage across R regulation. For customers requiring tight output voltage REF resistor to be nearly equal to the regulation across temperature, please refer to other LTC parts with integrated temperature compensation features. Rev. A 8 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application Related Parts