Non-synchronous flyback topologies are widely used in isolated power supplies, ranging from below 1W to tens of watts. Linear Technology's family of isolated flyback converters without optocouplers greatly simplifies isolated power supply design by utilizing proprietary master-side sensing that eliminates the need for optocouplers or transformer third windings for output regulation . The new LT8300 is the first low-power device in the family that significantly improves light load efficiency and reduces the no-load input standby current to approximately 200μA.

The LT8300 operates from an input voltage range of 6V to 100V and provides up to 2W of isolated output power. The 150V integrated DMOS power switch eliminates the need to add a damper to most applications. By sampling the isolated output voltage directly from the main-ended flyback waveform, the LT8300 eliminates the need for an optocoupler or transformer third winding for regulation. The output voltage is set with a single external resistor.

Internal loop compensation and soft start functions further reduce the number of external components. Since the boundary mode operation is performed under heavy load, it is allowed to use small magnetic components and achieve excellent load regulation performance. Low ripple Burst Mode® operation maintains high efficiency under light load conditions while minimizing output voltage ripple. All of these features are integrated into a 5-pin TSOT-23 package (Figure 1) with a high voltage pin pitch that meets IPC-2221 requirements.

Figure 1: The LT8300 is available in a 5-lead TSOT-23 package (this package provides high voltage pin spacing between pins 4 and 5).

As shown in Figure 2, a complete isolated flyback solution has a footprint of less than 1 x 1/2 inches. Figure 3 shows a typical application of the LT8300, which uses a 36V to 72V input to produce a 5V isolated output. The solution requires only five external components (input capacitor, output capacitor, transformer, feedback resistor, and output diode) and two optional undervoltage lockout resistors.

Figure 2: The LT8300 Isolated Flyback Converter Solution Sizes Less Than 1" x 1/2" in the Standard Demo Board DC1825A

Figure 3: 5V/300mA Micropower Isolated Flyback Converter with a 36V to 72V Input

While the LT8300 simplifies the isolated flyback converter design, it still delivers outstanding performance. Figure 4 shows the power efficiency (peak of 85%) for the 5V application circuit in Figure 3. Figure 5 shows the load and voltage regulation performance (±0.5%) of the 5V application circuit shown in Figure 3. Figure 6 and Figure 7 show their 50mA to 250mA load step transients and 1mA resistive load start waveforms, respectively.

Figure 4: Power efficiency of the 5V application circuit shown in Figure 3.

Figure 5: Output load and voltage regulation performance of the 5V application circuit shown in Figure 3.

Figure 6: 50mA to 250mA load step transient waveform for the 5V application circuit shown in Figure 3.

Figure 7: 1mA resistive load start-up waveform for the 5V application circuit shown in Figure 3.

The output voltage of a typical LT8300 application can be expressed as:

The first term of the VOUT calculation formula is temperature independent, but the output diode forward voltage VF has a large negative temperature coefficient (-1mV/°C to -2mV/°C). Such a negative temperature coefficient will produce a voltage change of approximately 200 mV to 300 mV at the output over the entire temperature range.

For relatively high voltage outputs (say 12V and 24V), the effect of the output diode temperature coefficient on output voltage regulation is negligible. However, for lower voltage outputs (such as 3.3V and 5V), the effect of the output diode temperature coefficient on the output voltage regulation will increase by 2% to 5%.

For designs that require tight output voltage regulation over temperature, a micropower low dropout linear regulator can be added to post-regulate the LT8300 output. The LT8300 should be set slightly above the “regulation voltage + LDO drop voltage”.

Figure 8 shows the combination of the LT8300 with an LT3009-3.3 post regulator to produce a 3.3V/20mA isolated output from an 18V to 32V input. As shown in Figure 9, the no-load input standby current is less than 250μA, which is consistent with the requirements of the DEF-STAN61-5 standard.

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