SC604A
POWER MANAGEMENT
Methods for Setting LED Current (Cont.)
Method 2. The example circuit in Figure 1 uses a 16.5kΩ resistor Method 3. The circuit in Figure 2 uses open drain NMOS transistors
and an analog input DC voltage, VADJ , which varies from 1.2V to 0V to set an equivalent resistance for RSET. Parallel combinations are
to control LED current from 1mA to 30mA. Table 3 shows the switched on and off for R1, R2 and R3. R4 is always connected,
resulting output. If necessary, the analog VADJ voltage can be so that a minimum value of LED current can be maintained at
sourced from a voltage higher than 1.2V, but the source must be 1.5mA.
divided down so that the VADJ mode will not exceed 1.2V. For lower
current applications and for higher resolution, a larger resistor
may be substituted in this circuit. PWM applications are also
possible with this circuit by application of RC filtering. (Consult
with Semtech for detailed application support).
Figure 1 - Analog Voltage for LED Current Control
Figure 2 - 3 Bit LED Current Control with Open Drain
Table 3 - Analog Voltage for LED Current Control
VADJ [V]
ILED [mA]
VADJ [V]
ILED [mA]
0.000
0.100
0.200
0.300
0.400
0.500
30.2
27.7
25.1
22.5
20.0
17.3
0.600
0.700
0.800
0.900
1.000
1.150
14.8
12.3
9.7
7.1
2.1
1.0
2005 Semtech Corp.
www.semtech.com
10
SEMTECH [ SEMTECH CORPORATION ]
