SM8136A Datasheet, PDF (8/15 Pages) Nippon Precision Circuits Inc – White LED Driver IC

SM8136A
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.
1.0 mode
1.5 mode
(high efficiency) (low efficiency)
High VF LED
1.0 mode
(high efficiency)
Low VF LED
1.5 mode
(low efficiency)
High
VIN
Low
V_OUT mode switching interval comparison due to drive LED “VF” variation
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.
VIN voltage
ILED detector
1.0 1.5 threshold
(1)
Lack of LED current detect
(2)
VOUT mode
1.0 mode
VOUT voltage
1.5 mode
(3)
5.0V
(4)
V_IN voltage drop and V_OUT output voltage
(1) The VIN voltage drops below the ×1.5 mode threshold value.
(2) When the voltage drops below the ×1.5 mode threshold, the I_LED detection circuit raises an I_LED undercur-
rent signal.
(3) The VOUT output switches to ×1.5 mode in response to the I_LED undercurrent signal.
(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.
SEIKO NPC CORPORATION —8