LP3942YQ NSC [National Semiconductor], LP3942YQ Datasheet - Page 9

• Manufacturer Part Number: LP3942YQ
• Description: The Dual RGB LED Controller with 1.5x/2x Charge Pump and SPI Interface
• Manufacturer: NSC [National Semiconductor]

I
I
R
f
Leakage
MAX(RGB)
RGB
Symbol
Addr.
RGB
RGB Driver Electrical Characteristics
Limits in standard typeface are for T
(−30˚C ≤ T
V
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
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.
Enable1 and Enable2 bits are used to enable RGB1 and
RGB2 outputs.
>
00’h
01’h
02’h
03’h
VDD
Vsat. In the LP3942, the nominal value for Rds is 10 kΩ,
= 2.78V, V
default
default
default
default
RGB1
RGB2
CTRL
R1/2, G1/2, B1/2 Pin Leakage
Current
Maximum Sink Current
Accuracy
Current Mirror Ratio
RGB1 and RGB2 Mismatch
RGB Switching Frequency
Reg.
J
OVL
≤ +105˚C). Unless otherwise noted, specifications apply to the LP3942 Typical Application Circuit (pg. 1) with:
IN
= 3.6V, C
@
A
Overlap2
Color[3]
Color[3]
NSTBY
20 mA
Ir + (Vr / Rds). This is true only, if Vr
Parameter
[7]
0
0
0
0
IN
= 2.2 µF, C
Color[2]
Color[2]
CP_ON
J
R2SW
= +25˚C. Limits in boldface type apply over the full operating junction temperature range
[6]
0
0
0
0
1
= 1.0 µF, C
Color[1]
Color[1]
CC/SW2
G2SW
V
Constant Current Mode: Limited
with external Rr, Rg, Rb
resistors by user 3.20V
Constant Current Mode: 3.0
V
Switch Mode: Limited with
ballast resistors by user
V
20 mA LED Current
Switch mode
Accuracy proportional to
internal clock freq.
<
[5]
RGB
IN
DD
0
0
0
0
5.0V
<
= 2.78V V
2
= 5V
3.2V
= 1.0 µF, C
Condition
Color[0]
Color[0]
CC/SW1
9
B2SW
[4]
RGB
0
0
0
0
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)
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.
OVL register can be used, if overlapping mode is needed,
default mode is non-overlapping. If overlapping mode is
OUT
= 0.2V
= 3.0 µF, R
<
Overlap1
Bright[2]
Bright[2]
1.5x/2x
V
IN
[3]
<
0
0
0
0
BIAS
Min
−10
15
Bright[1]
Bright[1]
reserved
= 27k. (Note 9).
R1SW
[2]
0
0
0
0
1:100
Typ
±
±
20
2
3
5
Bright[0]
Bright[0]
reserved
G1SW
[1]
0
0
0
0
Max
A
3.5
40
30
50
10
26
1
9.9 mA. Note
www.national.com
reserved
Enable1
Enable2
B1SW
Units
[0]
0
0
0
0
kHz
mA
mA
mA
µA
%
%
Ω