ZXLD1366

ZXLD1366

Document number: DS31992 Rev. 5 - 2

5 of 32

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June 2010

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Device Description

converter.

Operation can be best understood by assuming that the ADJ pin of the device is unconnected and the voltage on this pin

circuit. Under this condition, the (-) input to the comparator is at ground and its output is high. This turns MN on and switches

resistor R1 by the current sense circuit and produces a proportional current in internal resistors R2 and R3. This produces a

comparator output switches low and MN turns off. The comparator output also drives another NMOS switch, which

bypasses internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be nominally 15%

determined by the LED(s) and diode forward voltages to produce a falling voltage at the input of the comparator. When this

Switching thresholds

Adjusting output current

The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting

resistor (50k

Ω nom) between ADJ and the internal reference voltage. This allows the ADJ pin to be overdriven with either

Details of the different modes of adjusting output current are given in the applications section.

Output shutdown

The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit falls below the threshold

(0.2V nom.), the internal regulator and the output switch are turned off. The voltage reference remains powered during

shutdown to provide the bias current for the shutdown circuit. Quiescent supply current during shutdown is nominally 60

μA

and switch leakage is below 5

μA.