8

Date: 11/15/05

SP6687 4Channel Charge Pump White LED Driver

© Copyright 2005 Sipex Corporation

Selecting Capacitors

APPLICATION INFORMATION

The flying capacitors C

current capability of the charge pump and influence the

overall efficiency of the system. Lower values will im-

prove efficiency, but will limit the current to the LEDs at

low input voltages. For 4 X 20mA load over the entire

input range of 2.7 to 5.5V, a capacitor of 1µF is optimal.

Figure 1. C-R-C filter used to reduce input ripple.

Setting the LED Current

The SP6687 can be set to a fixed LED current by a

resistor R

lishes the reference current and mirrors the current into

LED1, LED2, LED3, and LED4. The current into each

LED is about 440 times the current that flows through

R

The approximate setting formula is given as

follows:

I

Figure 2 shows the typical value of R

LED current and Table 2 shows the values of R

fixed LED current.

average current.

SP6687

®

IN

V

2.2µF

2.2µF

IN

V

1.0Ω

484(V)

0

50

100

150

200

250

300

05

10

15

20

25

30

LED Current (mA)

I

D

E

L

)

A

m

(

R

T

E

S

)

k

(

d

r

a

d

n

a

t

S

t

s

e

r

a

e

N

)

k

(

T

E

S

R

r

o

f

e

u

l

a

V

50

.

1

90

.

1

9

0

19

.

7

45

.

7

4

5

17

.

2

34

.

2

3

0

20

.

4

20

.

4

2

5

26

.

9

16

.

9

1

0

34

.

6

15

.

6

1

If maximum accuracy is required, a precision resistor is

needed. The following equation shows how to calculate

the error: I

I

LED(SET)

X 100%

Where I

I

Ω

Ω

To get better performance from the SP6687, the selec-

tion of appropriate capacitors is very important. These

capacitors determine some parameters such as input

and output ripple, power efficiency, maximum supply

current by the charge pump and startup time. To reduce

the input and output ripple effectively, low ESR ceramic

capacitors are recommended.

To reduce output ripple, increasing the output capaci-

tance C

increase the startup time of the output voltage.

For LED driver applications, the input voltage ripple is

more important than output ripple. Input ripple is con-

trolled by the input capacitor C

of input capacitance can further reduce the ripple. Prac-

tically, the input voltage ripple depends on the imped-

ance of the power supply. If a single input capacitor C

IN

cannot satisfy the requirement of the application, it is

necessary to add a low-pass filter. Figure 1 shows a C-

R-C filter used on the SP6687. The input ripple can be

reduced to less than 30mVp-p when driving 80mA of

output current.