ON Semiconductor recently introduced the NCP100, a sub-1-V programmable precision reference. The NCP100 is a similar device to the TL431 and TLV431 with the advantage of a lower operating voltage range, important because isolated power supply output voltages are dropping below 2.0 volts.

The NCP100 is a 0.7-V programmable precision reference with a 1% tolerance. It requires 0.9 V from cathode to anode to operate, and a 1.0 µF capacitor is required for stability.

In a traditional opto-coupler isolated feedback network, the TL431 can be used in output voltages ranging from 3.75 to 36 V. The TLV431 lowers the range from 2.5 to 16 V. With a 0.9 V minimum, the NCP100 lowers the output voltage range from 2.15 to 6 V. The traditional isolated feedback network is illustrated in Fig. 1 where the NCP100 does lower the minimum output voltage level, but does not break the 2.0 V level.

By modifying the traditional opto-coupler isolated feedback network with a pnp transistor and a resistor (see Fig.2) the output voltage range can be reduced. The reduction in voltage is the opto-isolator diode forward voltage drop, 1.25 V, minus the emitter-to-base voltage drop, 0.7 V. This 0.55 V reduction results in a minimum power supply output voltage of 3.2 V, 1.95 V and 1.6 V for the TL431, TLV431 and the NCP100, respectively

For multiple-output power supplies, the NCP100 lowers the allowable voltage range to be regulated. As a rule of thumb the reference level of the programmable precision reference should be less than 90% of the output voltage. This rule equates to 2.75 V, 1.38 V and 0.77 V for the TL431, TLV431 and the NCP100 respectively. Refer to Fig. 3 for the opto-coupler isolated feedback schematic of a multiple output power supply.

The NCP100 lowers the operating voltage range of isolated power supplies. Three feedback schemes were discussed on the extension of the voltage range. Table 1 outlines the operating voltage differences between the TL431, the TLV431 and the NCP100.

Fig. 1. Traditional Isolated Feedback Network

Fig. 2. 1.6-V Isolated Feedback Network

Fig. 3. Multiple Output Voltage Feedback Schematic

Part Number	NCP100	TLV431	TL431
Reference Level	0.7 V	1.25 V	2.5 V
Operating Voltage	0.9 V	1.25 V	2.5 V
Operating Range of Traditional Isolated Feedback	2.15 to 6 V	2.5 to 16 V	3.75 to 36 V
Extended Operating Range with a pnp transistor	1.6 V	1.95 V	3.2 V
Multiple Output Power Supply Operating Range Minimum	0.77 V	1.38 V	2.75 V

Table 1. Programmable Precision Reference Voltage Comparison.

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