9

Date: 11/15/05

SP6687 4Channel Charge Pump White LED Driver

© Copyright 2005 Sipex Corporation

1

S2

S3

S

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r

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R

t

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a

v

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q

E

T

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p

R

(

T

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S

)

00

0

R

T

E

S

R

=

4

00

1

R

T

E

S

R

=

3

R

/

/

4

01

0

R

T

E

S

R

=

2

R

/

/

4

01

1

R

T

E

S

R

=

2

R

/

/

3

R

/

/

4

10

0

R

T

E

S

R

=

1

R

/

/

4

10

1

R

T

E

S

R

=

1

R

/

/

3

R

/

/

4

11

0

R

T

E

S

R

=

1

R

/

/

2

R

/

/

4

11

1

R

T

E

S

R

=

1

R

/

/

2

R

/

/

3

R

/

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4

APPLICATION INFORMATION

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

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

control signal.

SP6687

®

ISET

S3

S2

S1

R1

R2

R3

R4

Figure 3. Typical application circuit for setting LED

Table 3. Control signal and equivalent resistor of the

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

will give the approximate value of the LED current:

I

T

T

PWM

Where T

T

ON state current.

SP6687

®

CTRL2

CTRL1

CTRL0

EN

PWM

LED ON

LED

OFF

SP6687

®

CTRL2

CTRL1

CTRL0

EN

PWM

LED ON

LED

OFF

Figure 4. Typical application circuit for PWM dimming

Figure 5. Typical application circuit for PWM dimming