SM8136A

SEIKO NPC CORPORATION —8

Switching from

×1.0 Mode to ×1.5 Mode

When the LEDs are ON and can be driven at the fixed current value set by the EN pulse input without needing

to boost the VIN input voltage, the VOUT output operates in

×1.0 mode. In other words, if sufficient current

flows to counter the LED forward-bias voltage drop VF, it means the VOUT voltage may drop below 5.0V. The

longer that the driver output operates in the more efficient

×1.0 mode, the lower the total power consumption

and the longer the battery life can be extended. Also, by using LEDs with low VF and a lower LED drive cur-

rent setting, the longer the device can operate automatically in

×1.0 mode.

If the VIN voltage drops and the LED current drops below the fixed current setting in

×1.0 mode, an undercur-

rent detection circuit connected to each LED drive circuit activates and automatically switches the VOUT out-

put mode to

×1.5 mode that uses a charge pump to boost the voltage by a factor of 1.5. The LED undercurrent

detection circuit switches the output mode when it detects an undercurrent condition on any one of the three

channels. Accordingly, there should be as low a variation as possible in drive LED VF values in order to

improve overall efficiency.

(1) The VIN voltage drops below the

×1.5 mode threshold value.

(2) When the voltage drops below the

rent signal.

(3) The VOUT output switches to

(4) The VOUT voltage drops below 5.0V but still supplies sufficient current until reaching the

×1.5 switching

threshold.

Note that if the LEDs connected to the DIN pins are switched after startup, the LED connection detection cir-

cuit will operate incorrectly and the correct mode switching functions cannot occur.

High

High VF LED

Low VF LED

VIN

Low

1.0 mode

(high efficiency)

1.5 mode

1.0 mode

(high efficiency)

(low efficiency)

1.5 mode

(low efficiency)

5.0V

1.0 mode

1.5 mode

1.0

1.5 threshold

Lack of LED current detect

(1)

(2)

(3)

(4)