Datasheet LT8303 (Analog Devices) - 8

LT8303
operaTion
tive quiescent current to improve light load efficiency. In determines how often the output voltage is sampled and this condition, the LT8303 operates in low ripple Burst also the minimum load requirement. Mode. The typical 7kHz minimum switching frequency
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 LT8303 operates similar to traditional current mode FLBK ⎝⎜ R switchers, except in the use of a unique flyback pulse FB ⎠⎟ • RREF = VBG sense circuit and a sample-and-hold error amplifier, which or sample and therefore regulate the isolated output voltage ⎛ V ⎞ from the flyback pulse. V BG FLBK = ⎝⎜ RREF ⎠⎟ •RFB = IRFB •RFB Operation is as follows: when the power switch M1 turns off, the SW pin voltage rises above the VIN supply. The VBG = Bandgap reference voltage amplitude of the flyback pulse, i.e., the difference between I the SW pin voltage and V RFB = RFB regulation current = 100µA IN supply, is given as: Combination with the previous V V FLBK equation yields an FLBK = (VOUT + VF + ISEC • ESR) • NPS equation for VOUT, in terms of the RFB resistor, transformer VF = Output diode forward voltage turns ratio, and diode forward voltage: ISEC = Transformer secondary current ⎛ R ⎞ V FB ESR = Total impedance of secondary circuit OUT = 100µA • ⎝⎜ NPS ⎠⎟ − VF NPS = Transformer effective primary-to-secondary turns ratio
Output Temperature Coefficient
The flyback voltage is then converted to a current I The first term in the VOUT equation does not have tempera- RFB by the flyback pulse sense circuit (M2 and M3). This cur- ture dependence, but the output diode forward voltage VF rent I has a significant negative temperature coefficient (–1mV/°C RFB also flows through the internal trimmed 12.23k R to –2mV/°C). Such a negative temperature coefficient pro- REF resistor to generate a ground-referred voltage. The resulting voltage feeds to the inverting input of the sample- duces approximately 200mV to 300mV voltage variation and-hold error amplifier. Since the sample-and-hold error on the output voltage across temperature. amplifier samples the voltage when the secondary current For higher voltage outputs, such as 12V and 24V, the output is zero, the (ISEC • ESR) term in the VFLBK equation can be diode temperature coefficient has a negligible effect on the assumed to be zero. output voltage regulation. For lower voltage outputs, such The bandgap reference voltage V as 3.3V and 5V, however, the output diode temperature BG, 1.223V, feeds to the non-inverting input of the sample-and-hold error ampli- coefficient does count for an extra 2% to 5% output voltage fier. The relatively high gain in the overall loop causes regulation. For customers requiring tight output voltage the voltage across R regulation across temperature, please refer to other LTC REF resistor to be nearly equal to the parts with integrated temperature compensation features. 8303fa 8 For more information www.linear.com/LT8303 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 Typical Applications Package Description Typical Application Related Parts