T8305 datasheet(8/10 Pages) TMT

8 / 10 page

T8305

TM Technology, Inc. reserves the right

P. 7

Publication Date: Mar. 2013

to change products or specifications without notice.

Revision: A

Inductor selection

Recommended inductor values for the T8305 are in the range 33uH to 100uH. Higher values of

inductance are recommended at higher supply voltages in order to minimize errors due to switching

delays, which result in increased ripple and lower efficiency. Higher values of inductance also result

in a smaller change in output current over the supply voltage range. The inductor should be mounted

as close to the device as possible with low resistance connections to the LX and VIN pins. The

chosen coil should have a saturation current higher than the peak output current and a continuous

current rating above the required mean output current.

The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times

within the specified limits over the supply voltage and load current range.

LX switch on time : ton = L∆I / (VIN – VLED - Iavg (RS+rL+RLX)) ,

LX switch off time : toff = L∆I / (VLED + VD + Iavg (RS+rL)) ,

Where:

“L” is the coil inductance (H)

“∆I” is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}

“rL” is the coil resistance (ohm)

“RS” is the current sense resistance

“Iavg” is the required LED current (A)

“VIN” is the supply voltage (V)

“VLED” is the total LED forward voltage (V)

“RLX” is the switch resistance (ohm) {=0.2 ohm nominal}

“VD” is the diode forward voltage at the required load current (V)

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