SM8135A

SEIKO NPC CORPORATION —9

Boost Mode Auto-switching

The SM8135A switches between

×1.0 mode (VIN through mode), ×1.5 mode (1.5-times charge pump boost),

and

×2.0 mode (2.0-times charge pump boost) automatically in response to the operating state to reduce current

consumption. The switching transition flow chart is shown in the following figure.

This section describes the boost mode switching in response to insufficient LED drive current level detection.

The next section describes the mode reset functions. Initially, when the LED drive current begins to flow, oper-

ation begins in

×1.0 mode at the set current value if possible, without boosting the input voltage VIN. If the set

current value cannot be attained in

×1.0 mode due to a VIN voltage drop or an increase in the set current value,

a low-current detector circuit connected to each LED drive circuit is activated that switches the VOUT output

to

×1.5 mode automatically, and the charge pump circuit starts boosting the voltage by 1.5 times. If the set cur-

rent value still cannot be attained, the LED drive circuits are switched to

×2.0 mode, and the charge pump starts

boosting the voltage by 2 times.

Boost mode switching flow chart

LED ON

1.0 mode

1.5 mode

1.5 mode

1.0sec

sufficient

insufficient

insufficient

1.0sec

2.0 mode

sufficient

insufficient

sufficient

insufficient

Mode reset

1.5

1.0

Mode reset

2.0

1.5

sufficient