SM8135B
1.0
V
IN
input voltage
1.5 threshold
1.5
Lack of LED
current detect
2.0 mode
2.0 threshold
V
OUT
mode
1.5 mode
1.0 mode
4.9V
V
OUT
output voltage
V
IN
voltage drop and V
OUT
output voltage
In
×1.0
mode, V
IN
= V
OUT
. However, the V
OUT
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, V
OUT
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
High
1.5 mode
2.0 mode
(low efficiency)
VIN input
Low
V
OUT
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
NPC [ NIPPON PRECISION CIRCUITS INC ]
