Device Operation; Main-Channel Led Output Current; Sub-Channel Led Current; Uv Threshold Configuration - ISSI IS31LT3135-V1 Manual

IS31LT3135­V1, IS31LT3135­V2
Dual Ch. (Main/Sub) LED Driver Evaluation Board Guide
Device Operation
Main­Channel LED Output Current
The main‐channel LED current is adjusted by using a resistor connected to Iset pin. The voltage of Iset pin will remain at 1.2V,
independent of external factors including transient temperature and source voltage fluctuations. The through the
main‐channel LED is calculated by using the following equation:
The main‐channel current will continue through the LED to the GND pin. 0.1V dropout voltage occurs when the
main‐channel current is ≤ 200mA.
Sub‐channel LED current originates from the SUB terminal to ground, and likewise its corresponding LED current can be set
via an external resistor.
V is the power supply voltage. V
CC
UV Threshold Configuration
The UV pin detects any voltage swings and compensates by utilizing 0.8v hysteresis. This form of error correction
mechanism will ensure reliable device operation, even when the power supply experiences an unwanted ripple effect.
Sweeping power supply from a high to low voltage, results in main channel switch to sub‐channel while UV pin is below the
UV threshold of 1.17V (IS31LT3135‐V1 typical) or 1.23V (IS31LT3135‐V2 typical.) Sweeping power supply from a low to high
voltage, will cause the sub‐channel mode to switch back to main‐channel operation, under conditions where UV pin is 80mV
higher than the UV threshold. U
 
R
   
2
V
When

CC
 
R R
1 2
occurs. Lithium‐ion batteries typically maintain a voltage level of 3.7V during discharge, so the recommended UV threshold
is 3.4V. Typically resistors R1 and R2 are set to 10K magnitude（i.e.51k）Small resistance values for R1 and R2 will
translate to losses in heat or efficiency. To minimize these losses, it is recommended to derive precise values for the
resistors so that
Short­Circuit Protection Mechanism
When the main‐channel LED is shorted, IS31LT3135 will automatically switch to sub‐channel mode.
PCB layout reference
The switch should be placed in an area with low voltages, to avoid an instant spark while switching. The traces between
GND pin and I pin to real ground should be as short and wide as possible, with the purpose of improving the main LED
SET
current accuracy and under voltage threshold accuracy. Adding a 0.1uF, capacitor may also help with this purpose.
Integrated Silicon Solution, Inc. –
V2, 4/23/2012
I
MAIN
I
SUB
is the sub‐LED voltage. V
F2
is set by the ratio of resistor R1 over resistor R2.
uv

1.17 V (V1) or 1.23V(V2)
the end device
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V
 
SET
8000
R
SET
 
V V V

CC F2 D2

R R
4 SUB
denotes the diode voltage and its value should be minimized.
D2
; an immediate switch from main to sub‐channel operation
operates with the highest efficiency rate achievable.
3