TE

CH

tm

T8308

TM Technology, Inc. reserves the right

P. 5

Publication Date: Oct. 2014

to change products or specifications without notice.

Revision:C

Functional Description

The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a selfoscillating

continuous-mode buck converter.

Device operation

Operation can be best understood by assuming that the EN pin of the device is unconnected and the

output from the current sense circuit. Under this condition, the (-) input to the comparator is at

ground and its output is high. This turns MN on and switches the LX pin low, causing current to

forced across internal resistor R1 by the current sense circuit and produces a proportional current in

internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of the

comparator.When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to

falling voltage at the input of the comparator.

Switching thresholds

Define an average VSENSE switching threshold of 100mV (measured on the ISENSE pin with

respect to VIN). The average output current IOUTnom is then defined by this voltage and RS

according to:

IOUTnom = 100mV/RS

Nominal ripple current is ±15mV/RS

Output shutdown

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

falls below the threshold, 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 35uA and switch leakage is below 5uA.