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SM8135B

White LED Driver IC

Nippon Precision Circuits  

SM8135B

Mode Reset Function

VF increases immediately after the LED current starts to flow, and then decreases as the LED temperature

increases due to the heating effect of the current flow. It can take about 10 seconds for the LED temperature to

stabilize and for VF to reach equilibrium, and VF may fluctuate more than 200mV. The VF fluctuation is

affected by the ambient temperature and LED current setting, and has a large affect on the automatic mode

switching voltage tolerances. To counter the effects of VF fluctuation, the SM8135B outputs a mode reset sig-

nal once every 1.0 seconds which automatically switches the output mode to ×1.0, and then a determination is

made whether to make the ×1.0 → ×1.5 mode switch.

Mode reset

1.0sec/cycle

(1)

VOUT mode

Mode hold

L: hold

VOUT output voltage

1.5 mode

1.0 mode

min 80µs

max 200µs

ILED detector

ignore

1.5 mode

(2)

(3)

4.9V

ILED detector

(4)

Switching from ×1.5 mode to ×1.0 mode due to mode reset signal

(1) Mode reset signal is output once every 1.0 seconds.

(2) Mode switches from ×1.5 to ×1.0 mode on the rising edge of the mode reset signal.

(3) A 200µs (max) mode hold time begins after switching to ×1.0 mode. LED drive current insufficient state is

detected but the mode is not switched during this interval.

(4) LED drive current insufficient signal is ignored during the mode hold time, but normal mode switching

operation resumes when the mode hold time elapses.

SEIKO NPC CORPORATION —11

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