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LEDD16_24 – Rev. 2018-1.2

Specifications subject to change without notice.

LEDD16_24 Series

Constant current power LED driver - Wide Input - Non-Isolated & Regulated

Output current adjustment by external DC control volta-

Thermal feedback circuit

In this application, the LED modules of

each LED drivers should be independent

from electrical connection to the others

and input power - this is to prevent the

damaging to LED drivers also to avoid the

un-necessaried interference among each

LED module driven by the LED driver.

The selection of components for the thermal feedback circuit is not only dependent on the

choice of R2 and R3, but also on the amount of heat sink area required to extract heat from the

LEDs. To maximize the light output at high ambient or operating temperature conditions, the

LEDs must have a sufficient thermal extraction path, otherwise the thermal control circuit will

effect current drive reduction in non-optimal conditions. The thermal control threshold point is

set by adjusting R2. For this design, three values (33k, 22k and 10k) were evaluated. These values

were chosen to give break points at approximately 25°C, 40°C and 60°C.

Note that the light output will not continually dim to zero - the thermal control is applying DC

control to the ADJ pin and therefore has a dimming ratio from maximum Current of approxi-

mately 5:1. Once the reduced DC level goes below the shutdown threshold of around 200mV, the

LED drive current will fall to zero and the LEDs will be extinguished. The slope of the current re-

duction is determined by the beta value of the thermistor. The larger the beta value, the sharper

will be the resultant current control response. The slope of the current reduction is also affected

by Q1‘s base emitter voltage (VBE) variation with temperature.