IS31LT3918 T8 Lighting Evaluation Board Guide
Integrated Silicon Solution, Inc. – www.issi.com 2
R1.0, 18/12/2012
Detailed Description
Switch Dimming
The action of the main power switch can be divided into
two types. The first is “normal switch operation”
wherein the switch is toggled from ON to OFF,
remaining OFF for longer than 2 seconds (typical). The
other is “switch dimming action” wherein the switch is
toggled from ON to OFF and back ON within 2 seconds
(typical). When the device experiences normal switch
operation, it merely powers on in the first state when
the power switch is toggled to ON, and the device turns
off when the main power switch is changed to OFF.
Switch dimming output current levels are configured by
connecting the DIM1 and DIM2 pins as indicated in the
table below：
Setting Dimming Current-Level
Pin Name / Setting DESCRIPTION
DIM 1 DIM 2 Functionality Dimming Levels
Floating Floating No Dimming 100%
Floating GND 3 (Three) - levels of dimming 100% -- 30% -- 100%
GND Floating 3 (Three) - levels of dimming 100% -- 50% -- 100%
GND GND 2 (Two) - levels of dimming 100% -- 50% -- 20% -- 100%
When operating the power switch normally the device
will always power up at 100% output current. The
operation of the power switch and the configuration of
the DIM1 and DIM2 pins control the dimming process
as follows：
1. When DIM1 and DIM2 pins are both floating, there
is no switch dimming, and the output current is
100% of the programmed value when the power is
on.
2. When DIM1 is floating and DIM2 is GND, the
output current is：
a) 100% at power on.
b) The first switch dimming action causes the
current to change to 30%.
c) A second switch dimming action causes the
current to return to 100%.
d) A fourth switch dimming action has the same
effect as the first switch dimming action.
e) Subsequent switch dimming actions causes
the cycle to continue.
3. When DIM1 is GND and DIM2 is floating, the
dimming sequence is as described in (2) above,
except that the current sequence is
100%-50%-100%.
4. When both DIM1 and DIM2 are connected to GND,
the dimming sequence is as described in (2) above,
except that the current sequence is
100%-50%-20%-100%.
If the switch is operated normally, that is, switched ON
once after being in the OFF position for a long time, or if
both the DIM1 and DIM2 pins are floating, then the
output current always starts up at the initial value of
100%.
Note: Because the main power switch is used to initiate
the switch dimming function, the device must have a
large enough external capacitor on VIN to maintain
device operation for 2 seconds. Please refer to the
schematic for specific values.
