10

 2005 Semtech Corp.

www.semtech.com

SC604A

POWER MANAGEMENT

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1

Table 3 - Analog Voltage for LED Current Control

Figure 1 - Analog Voltage for LED Current Control

Figure 2 - 3 Bit LED Current Control with Open Drain

Method 2. The example circuit in Figure 1 uses a 16.5k

Ω resistor

and an analog input DC voltage, V

to control LED current from 1mA to 30mA. Table 3 shows the

resulting output. If necessary, the analog V

sourced from a voltage higher than 1.2V, but the source must be

divided down so that the V

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).

Methods for Setting LED Current (Cont.)

Method 3. The circuit in Figure 2 uses open drain NMOS transistors

to set an equivalent resistance for R

switched on and off for R1, R2 and R3. R4 is always connected,

so that a minimum value of LED current can be maintained at

1.5mA.