sp6887 Exar Corporation, sp6887 Datasheet - Page 7

• Manufacturer Part Number: sp6887
• Description: 4 Channel Regulated Charge Pump Wled Driver
• Manufacturer: Exar Corporation

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
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
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
matched forward voltage will produce the
best possible matched currents. For best
Rev. C – 10/31/2007
1X
from
logic
mode,
the
control
saving
LEDs.
over
power
LEDs
SP6887 4 Channel WLED Driver
the
13. Output Ripple
with
LED
and
7/10
matching performance it is recommended
that the Vf between LEDs be under
250mV.
The unused LED channels can be turned
off
connecting the respective LED pins to
VOUT
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
reference current needed for a constant
LED current. Values of RSET for a fixed
LED current are given in Table 2, “RSET
by
ELECTRICAL SPECIFICATIONS
resistor
pin,
CTR0,
THEORY OF OPERATION
in
RSET
CTR1
© 2007 Exar Corporation
which
and
establishes
case,
CTR2,
and
the
the