SP6887 Datasheet, PDF (7/10 Pages) Exar Corporation – 4 CHANNEL REGULATED CHARGE PUMP WLED DRIVER

ELECTRICAL SPECIFICATIONS
13. Output Ripple
Detailed Description
As shown in the block diagram on page 2,
the main components within the SP6887
include a fractional charge pump, mode
selection circuit, output selection logic,
LED current setting detection circuit, and
4 current sense circuits.
The fractional charge pump multiplies the
input voltage a multiple of 1X and 1.5X
times the input voltage. The charge pump
switches at a fixed 1MHz when the mode
is 1.5X. The charge pump does not switch
during 1X mode, saving power and
improving efficiency.
The mode selection circuit automatically
selects the mode as 1X or 1.5X based on
circuit conditions such as LED voltage,
input voltage and load current. 1X is the
more efficient mode than 1.5X mode.
Table 1 on page 2 shows the output
selection logic control over the LED
outputs for on and off functions with 8
different output states.
The current set and detection circuit uses
an external resistor and a 1.20V reference
to program the LED current.
4 current regulating circuits sink matched
currents from the LEDs. LEDs with
matched forward voltage will produce the
best possible matched currents. For best
THEORY OF OPERATION
matching performance it is recommended
that the Vf between LEDs be under
250mV.
The unused LED channels can be turned
off by CTR0, CTR1 and CTR2, and
connecting the respective LED pins to
VOUT pin, in which case, the
corresponding LED driver sink current is
only about 20µA.
Methods for Setting LED Current
There are 4 methods for setting and
adjusting the LED current outlined here.
The methods are:
1) RSET only
2) Analog Reference VSET
3) PWM Input at CTR0
4) PWM Input at EN
Method 1: LED Current Setting Using
the External Resistor RSET
The most basic means of setting the LED
current is with a resistor connected from
RSET to GND, as shown in the application
circuit on Page 1.
The resistor RSET establishes the
reference current needed for a constant
LED current. Values of RSET for a fixed
LED current are given in Table 2, “RSET
Rev. C – 10/31/2007
SP6887 4 Channel WLED Driver
7/10
© 2007 Exar Corporation