schematic knowledge

The present reference is a special application of current source IC Type LM334. It has a tiny temperature coefficient and draws only a minute current: at room temperature, only 10 µA, which increases with large rises in temperature by only a few µA. The circuit is basically a bandgap reference, because the positive temperature coefficient of the LM334 is combined with the negative temperature coefficient of the base-emitter junction of a transistor (which ensures good thermal coupling). To obtain a temperature coefficient of zero, or very nearly so, the output voltage of the circuit is adjusted to exactly 1.253 V with P1. It is, therefore, advisable to measure the set value of P1 accurately after it has been adjusted and to replace the combination of R1+P1 by a fixed resistor of the precise value.

Use a 1% metal-oxide film type from the E96 series. Since current source IC1 is tapped at the control input, a reference source with a negative output resistance of about 3.8 kΩ ensues. Resistor R3 ensures that the ultimate output resistance is about 400 Ω. The load current is then limited to not more than 5 µA. The performance of the reference is good: when the input voltage is increased from 5 V to 30 V, the output voltage varies by only 0.6 µV (from 1,2530 V to 1.2536 V). The temperature coefficient stays below 50 ppm °C–1, and, with careful adjustment, may even come down to 5 ppm °C–1. The current drawn by the prototype is 9.8 µA at an ambient temperature of 22 °C.

The LM3570 evaluation board has a chip enable pin (active high logic) as well as a PWM (active high logic) pin which allows current sources to be turned on and off without completely disabling the part..The LM3570 is capable of supplying up to 80mA of current split between the regulated current sources and VOUT. The LM3570 comes in National’s LLP-14 package.