LP3942 Datasheet, PDF (10/20 Pages) National Semiconductor (TI) – The Dual RGB LED Controller with 1.5x/2x Charge Pump and SPI Interface

RGB Functionality (Continued)
needed, then Overlap1 and/or Overlap2 bits are written to
‘1’. Switch enable bits G1SW, R1SW and B1SW bits need to
be written to ‘1’ if corresponding RGB1 output is to be
activated. The switch enable bits G2SW, R2SW and B2SW
have same effect to RGB2 outputs. Switch enable bits have
effect only in overlapping mode.
Following chapters describe RGB functionality and modes in
more details.
MODES OF OPERATION
Constant Current/Switch Mode
The LEDs are driven either by constant current sink or
switches.
By using constant current mode the LED current is limited
based on external resistors Rr, Rg and Rb connected to pins
IR, IG and IB. Only one RGB LED per output is recom-
mended in this mode.
The LED currents can be adjusted with resistors based on
the following table.
RR or RG or RB
[Ω]
5.6k
6.8k
8.2k
10k
12k
15k
18k
Typical I_LED
[mA]
22.0
18.1
15.0
12.3
10.3
8.2
6.8
The switch mode uses constant current mirrors as low ohmic
switches. Switch mode requires an external ballast resistor
to limit the LED current. This mode is used if 2 or more RGBs
are connected in parallel. To prevent the current variation
due output voltage accuracy it is recommended to use
charge pump in double (2x) mode.
The mode selection is shown on the following table.
CC/SW(1/2)
Mode
0
Constant Current Mode
1
Switch Mode
Comment
LEDs are driven with constant current sinks. Maximum current is limited by Rr,
Rg and Rb
LEDs are driven with switches. Maximum current is limited by ballast resistors.
Overlapping/Non-Overlapping Mode
This mode control defines how the RGBs are controlled by
PWM timing logic. Overlapping mode turns on and off the
RGBs at the same time. Non-overlapping mode splits the R,
G and B to phases based on selected color and thus limits
the maximum output current through the RGB LED.
The selection for this mode is as follows.
Overlap(1/2)
Mode
0
Non-overlapping Mode
1
Overlapping Mode
Comment
LEDs are driven with non-overlapping method to minimize the noise and
current consumption. Frame based color control is enabled.
LEDs are driven with overlapping method. Maximum current consumption is
6*Imax. Color control is disabled.
Overlapping Mode
Since RGB outputs are on simultaneously, the maximum
load peak current is
I_MAX = I(R1)_MAX + I(G1)_MAX + I(B1)_MAX + I(R2)_MAX +
I(G2)_MAX + I(B2)_MAX .
Overlapping Mode Timing Diagram
The brightness is controlled using PWM duty cycle based
control method as the following figure shows.
www.national.com
10