SP6687 Datasheet, PDF (9/12 Pages) Sipex Corporation – 4 Channel Charge Pump White LED Driver

LED Current Setting with NMOS
LED current setting control can also be achieved by using
an external NMOS transistor to change the equivalent
resistor of the I_SET pin. Figure 3 illustrates this application
circuit which has 3 bit signals and can set 8 different levels
of LED current. Table 3 shows the relation between the
equivalent resistor of the I_SET pin and the respective
control signal.
®
SP6687 _ISET
R1
S1
R4
R2
S2
R3
S3
Figure 3. Typical application circuit for setting LED
current using an NMOS transistor to set R_SET
S1
S2
S3
Equivalent Resister of I_SET pin
(R_SET)
0
0
0
R_SET=R₄
0
0
1
R_SET=R₃//R₄
0
1
0
R_SET=R₂//R₄
0
1
1
R_SET=R₂//R₃//R₄
1
0
0
R_SET=R₁//R₄
1
0
1
R_SET=R₁//R₃//R₄
1
1
0
R_SET=R₁//R₂//R₄
1
1
1
R_SET=R₁//R₂//R₃//R₄
Table 3. Control signal and equivalent resistor of the
I_SET pin.
APPLICATION INFORMATION
LED Dimming Control Methods
The SP6687 uses two methods to achieve LED dim-
ming control. These methods are detailed below.
PWM Dimming
The first dimming method utilizes a PWM control signal
into CTRL0, CTRL1, and CTRL2. Table 1 shows the
relation between CTRLx and the 4 LED current states.
For example, when CTRL1 and CTRL2 are at logic high
and CTRL0 receives a PWM signal then 4 LEDs will be
dimmed simultaneously. The average LED current can
be derived by using a known PWM signal value. When
the PWM signal logic is low the current can be set at a
fixed value with the R_SET resistor. The following equation
will give the approximate value of the LED current:
I_LED(AVG) =
T_OFF X I_LED(ON)
T_PWM
Where T_PWM is the period of the PWM dimming signal.
T
OFF
is
the
time
of
the
PWM
signal
at
low.
I_LED(ON)
is
LED
ON state current.
VIN
®
SP6687
CTRL2
CTRL1
CTRL0
EN
PWM
LED
LED ON
OFF
Figure 4. Typical application circuit for PWM dimming
when driving 4 LEDs_.
VIN
®
SP6687
CTRL2
CTRL1
CTRL0
EN
PWM
LED
LED ON
OFF
Date: 11/15/05
Figure 5. Typical application circuit for PWM dimming
when driving 3 LEDs_.
SP6687 4Channel Charge Pump White LED Driver
© Copyright 2005 Sipex Corporation
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