LP3942 Datasheet, PDF (9/20 Pages) National Semiconductor (TI) – The Dual RGB LED Controller with 1.5x/2x Charge Pump and SPI Interface

RGB Driver Electrical Characteristics
Limits in standard typeface are for T_J = +25˚C. Limits in boldface type apply over the full operating junction temperature range
(−30˚C ≤ T_J ≤ +105˚C). Unless otherwise noted, specifications apply to the LP3942 Typical Application Circuit (pg. 1) with:
V_VDD = 2.78V, V_IN = 3.6V, C_IN = 2.2 µF, C₁ = 1.0 µF, C₂ = 1.0 µF, C_OUT = 3.0 µF, R_BIAS = 27k. (Note 9).
Symbol
Parameter
Condition
Min
Typ
Max
Units
I_Leakage
R1/2, G1/2, B1/2 Pin Leakage
Current
V_RGB = 5V
1
µA
I_MAX(RGB) Maximum Sink Current
Constant Current Mode: Limited
with external Rr, Rg, Rb
resistors by user 3.20V < V_IN
< 5.0V
Constant Current Mode: 3.0 <
V_IN < 3.2V
Switch Mode: Limited with
40
mA
30
mA
50
mA
ballast resistors by user
Accuracy @ 20 mA
Current Mirror Ratio
V_DD = 2.78V V_RGB = 0.2V
−10
±3
10
%
1:100
RGB1 and RGB2 Mismatch
20 mA LED Current
±5
%
R_RGB
f_RGB
RGB Switching Frequency
Switch mode
Accuracy proportional to
internal clock freq.
2
3.5
Ω
15
20
26
kHz
RGB1 and RGB2 Output
Description
RGB1 and RGB2 outputs can be used in two modes: con-
stant current and switch modes. There are few basic param-
eters in the RGB1 and RGB2 outputs, which can cause
lowered output current values and/or extra current mis-
match, if not considered carefully in application. This chapter
helps to do this analysis.
CONSTANT CURRENT MODE
Outputs can be understood as Ideal current sources with
certain output resistance Rds. Additionally, outputs have a
minimum voltage Vsat, which must be exceeded at the
output. If voltage limit is not reached, the ideal current
source’s current will be less than expected. Taking as an
example the R1 output and defining the voltage between R1
pin and GND3 pin to Vr, and defining the ideal output current
as Ir, one can estimate the true output current with following
simple formula: Ireal A Ir + (Vr / Rds). This is true only, if Vr
> Vsat. In the LP3942, the nominal value for Rds is 10 kΩ,
and for Vsat is 200 mV.
For example, if we have a LED that has 3.0V forward bias
voltage at 10 mA and we have set the LED current to 10 mA.
Then we can estimate the true LED current to be appr.
10 mA + ((4.5V-3.0V) / 10 kΩ) = 10.15 mA. And voltage on
RGB output pin is 1.5V, which is high enough for the con-
stant current generator. Note that this example does not
consider the worst case tolerance of output current which
results from tolerance of external bias resistor and current
mismatch in the LP3942.
SWITCH MODE
In switch mode the function is more straightforward. De-
signer needs to only consider the output resistance of the
switch, which is typically 2Ω.
Using now the case from previous example and setting the
LED current with an external ballast resistor of 150Ω. Now,
we get true output current of 1.5V/(150 + 2)A 9.9 mA. Note
that this example does not consider the worst case tolerance
of output current which results from tolerance of POUT
voltage and LED forward voltage.
RGB Functionality
Both RGB outputs RGB1 and RGB2 have separate control
for mode, brightness and color. The RGB LEDs are con-
trolled through the SPI interface. The control register table is
shown below.
Addr.
00’h
01’h
02’h
03’h
Reg.
RGB1
default
RGB2
default
CTRL
default
OVL
default
[7]
Color[3]
0
Color[3]
0
NSTBY
0
Overlap2
0
[6]
Color[2]
0
Color[2]
0
CP_ON
0
R2SW
0
[5]
Color[1]
0
Color[1]
0
CC/SW2
0
G2SW
0
[4]
Color[0]
0
Color[0]
0
CC/SW1
0
B2SW
0
[3]
Bright[2]
0
Bright[2]
0
1.5x/2x
0
Overlap1
0
[2]
Bright[1]
0
Bright[1]
0
reserved
0
R1SW
0
[1]
Bright[0]
0
Bright[0]
0
reserved
0
G1SW
0
[0]
Enable1
0
Enable2
0
reserved
0
B1SW
0
Enable1 and Enable2 bits are used to enable RGB1 and
RGB2 outputs.
OVL register can be used, if overlapping mode is needed,
default mode is non-overlapping. If overlapping mode is
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