IS31LT3135-V1GRLS2 Datasheet, PDF (5/6 Pages) Integrated Silicon Solution, Inc – MAIN/SUB DUAL-CHANNEL LED DRIVER

IS31LT3135
APPLICATION NOTES
MAIN LED CURRENT SET
Current of the main LED is set by resistor connected to
Iset pin. Voltage of Iset pin is 1.2V. It is constant
even if the environment temperature and power supply
voltage changes.(Refer to figure2, figure3). The
current of main channel is 8000 times to the current of
Iset pin. It is obtained by:
I _main
 8000 * ^Vset
R_set
Main LED current flows through the MAIN pin to
ground. There is only 0.1V voltage dropout when the
main channel current is 200mA (Note 1).
SUB LED CURRENT SET
Sub-LED current flows through SUB to ground. This
current can be set by the resistor of SUB pin and is
given by:
I _sub

V_CC
 V_F2  V_d2
R _SUB
V_CC is power supply voltage, V_F2 is the sub-LED
voltage. V_d2 is the diode voltage（Refer to Figure 1），
its value should be as small as possible.
UNDERVOLTAGE THRESHOLD SET
UV pin has undervoltage detect function and the
undervoltage threshold gets a 0.8V hysteresis to avoid
error operation caused by power supply ripple.
Sweeping power supply from high voltage to low
voltage, the main channel switches to sub-channel
while UV pin is below undervoltage threshold, 1.17V
(IS31LT3135-V1 typical) or 1.23V (IS31LT3135-V2
typical). Sweeping power supply from low voltage to
high voltage, the sub-channel switches to the main
channel while UV pin is 80mV (typical) higher than the
undervoltage threshold. Uuv is set by the ratio of
resistor of R1 and R2 (Refer to Figure1).
When
R₂
R₁  R₂
V_CC
=1.17V
or
1.23V,
chip
switches
from the main channel to the sub-channel.
Typically，Li battery voltage maintains at 3.7V during
discharge. Therefore, it is recommended to set the
undervoltage threshold at 3.4V.
Typically, R1 and R2 are set to 10K Ohm magnitude
（e.g. 51K Ohm）, the current flows through R1 and R2
is large if the value of R1 and R2 are small, which will
reduce system efficiency. We can specify the value of
R2, and then calculate the value of R1 based on the
ratio.
MAIN LED SHORT PROTECTION
When the main LED is short circuit, chip will switch to
sub-channel automatically.
OVER-TEMPERATURE PROTECTION
When the chip temperature reaches 160℃, the chip
turns off automatically. When temperature drops back
to 140℃, the chip turns on again.
PCB LAYOUT REFERENCE
The switch should be placed at the low voltage point
(Refer to Figure 1) to avoid an instant spark while
switching. The traces between GND pin and Iset pin to
real ground should be as short as possible and as wide
as possible to improve the main LED current accuracy
and undervoltage threshold accuracy. Adding a 0.1uF
capacitor connected to UV pin can reduce the noise
pick-up at this pin.
Note 1: When power supply voltage is larger than LED voltage, the
extra voltage would be added on the Main pin, which will reduce the
efficiency of the whole system.
Integrated Silicon Solution, Inc. — www.issi.com
5
Rev.A, 11/08/2011