SP6687 Datasheet, PDF (8/12 Pages) Sipex Corporation – 4 Channel Charge Pump White LED Driver

APPLICATION INFORMATION
Selecting Capacitors
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_OUT is generally necessary. However, this will
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_IN -- increasing the value
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.
VIN
2.2µF
1.0Ω
2.2µF
®
VIN _SP6687
Figure 1. C-R-C filter used to reduce input ripple.
The flying capacitors C₁ and C₂ determine the supply
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.
Setting the LED Current
The SP6687 can be set to a fixed LED current by a
resistor R_SET connected from I_SET to GND. R_SET estab-
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_SET. The approximate setting formula is given as
follows:
Figure 2 shows the typical value of R_SET versus average
LED current and Table 2 shows the values of R_SET for a
fixed LED current.
Typical Curve for RSET vs. Avg. LED Current
300
250
200
150
100
50
0
0
5
10
15
20
25
30
LED Current (mA)
Figure 2. The typical curve of R_SET vs. LEDs
average current.
I_LED
(mA)
5
10
15
20
25
30
R_SET
(k Ω)
91.0
47.9
32.7
24.0
19.6
16.4
Nearest Standard
Value for RSET (k Ω )
91.0
47.5
32.4
24.0
19.6
16.5
Table 2. R_SET Value Selection
If maximum accuracy is required, a precision resistor is
needed. The following equation shows how to calculate
the error: I_LED(ERR) = I - I _{LED(MEAS) LED(SET) X 100%}
I_LED(SET)
Where I_LED(MEAS) is practical measured LED current and
I_LED(SET) is the LED current which is determined by R_SET.
I_LED=
484(V)
R_SET(Ω)
Date: 11/15/05
SP6687 4Channel Charge Pump White LED Driver
8
© Copyright 2005 Sipex Corporation