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SM8135B

White LED Driver IC

Nippon Precision Circuits  

SM8135B

VIN input voltage

VOUT mode

VOUT output voltage

1.0 1.5 threshold

1.5

Lack of LED

current detect

2.0 threshold

2.0 mode

1.0 mode

1.5 mode

4.9V

VIN voltage drop and VOUT output voltage

In ×1.0 mode, VIN = VOUT. However, the VOUT output does not always reach the electrical characteristics typi-

cal rating of 4.9V. However, as long as an insufficient current condition is not detected, ×1.0 mode operation

continues. In other words, VOUT may drop below 4.9V if sufficient LED drive current is flowing to counter the

LED forward-bias voltage drop VF. The longer the device can operate in the high output efficiency ×1.0 mode,

the lower the total current consumption and the longer the battery drive time can be extended. Furthermore, the

more that low VF LEDs and lower LED drive current are used, the longer the device can operate automatically

in ×1.0 mode.

1.0 mode

(high efficiency)

High VF LED

1.5 mode

2.0 mode

(low efficiency)

1.0 mode

(high efficiency)

Low VF LED

1.5 mode

2.0 mode

(low efficiency)

High

VIN input

Low

VOUT mode switching time comparison due to drive LED “VF” variation

The LED low-current detector circuit switches the output boost mode whenever a single insufficient current

condition is detected among the 4 channel LED drive currents, hence drive LEDs with small VF variation

should be used to increase overall efficiency. If, after startup, a LED connection to the DIN pins is switched

ON/OFF, the LED current detection circuit will operate incorrectly and normal boost mode switching does not

occur. Also, if LED drive current control using a DIM pin is not used, the low-current detector circuit does not

operate and the boost mode does not switch to either ×1.5 or ×2.0 mode.

SEIKO NPC CORPORATION —10

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