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This is design circuit for DC coupled current monitor that is eliminates the previous circuit’s trim but pulls more current from the APD bias supply. A1 floats powered by the APD bias rail. This is the figure of the circuit.

The 15V zener diode and current source Q2 ensure A1 never is exposed to destructive voltages. The 1kW current shunt’s voltage drop sets A1’s positive input potential. A1 balances its inputs by feedback controlling its negative input via Q1. As such, Q1’s source voltage equals A1’s positive input voltage and its drain current sets the voltage across its source resistor. Q1’s drain current produces a voltage drop across the ground referred 1k resistor identical to the drop across the 1kW current shunt and, hence, APD current. This relationship holds across the 20V to 90V APD bias voltage range. The 5.6V zener assures A1’s inputs are always within their common mode operating range and the 10M resistor maintains adequate zener current when APD current is at very low levels.

Two output options are shown. A2, a chopper stabilized amplifier, provides an analog output. Its output is able to swing to (and below) zero because its V– pin is supplied with a negative voltage. This potential is generated by using A2’s internal clock to activate a charge pump which, in turn, biases A2’s V– pin. A second output option substitutes an A-to-D converter, providing a serial format digital output. No V– supply is required, as the LTC2400 A-to-D will convert inputs to (and slightly below) zero volts. [Schematic’s circuit source: Linear Technology Notes].

A very simple LED torch can be designed using the NSI45090JDT4G adjustable constant current regulator (CCR) designed by ON Semiconductor ,using extreme few external components. NSI45090JDT4G device is designed to provide a cost effective solution for regulating current in LEDs. This Constant current regulator is based on patent-pending Self-Biased Transistor (SBT) technology and regulates current over a wide voltage range. It is designed with a negative temperature coefficient to protect LEDs from thermal runaway at extreme voltages and currents.

LED Torch Circuit Diagram




The Radj pin allows Ireg(SS) to be adjusted to higher currents by attaching a resistor between Radj (Pin 3) and the Cathode (Pin 4). The Radj pin can also be left open (No Connect) if no adjustment is required.The maximum current that can be adjusted using this chip is around 160 mA , and the maximum input voltage is around 45 volts The D1 from the circuit shown here is used for reverse battery protection .

Bellow you can see how simple is to design a circuit using this chip ( all data shown bellow are for this schematic ) .
LED’s = ((Vin − QX VF − D1 VF)/LED VF)
Example: Vin = 12 Vdc, QX VF = 3.5 Vdc, D1VF = 0.7 V
LED VF = 2.2 Vdc @ 30 mA
(12 Vdc − 4.2 Vdc)/2.2 Vdc = 3 LEDs in series.