RGB Driver Electrical Characteristics

Limits in standard typeface are for T

(−30˚C

≤ T

J

≤ +105˚C). Unless otherwise noted, specifications apply to the LP3942 Typical Application Circuit (pg. 1) with:

V

Symbol

Parameter

Condition

Min

Typ

Max

Units

I

Leakage

R1/2, G1/2, B1/2 Pin Leakage

Current

V

1

µA

I

MAX(RGB)

Maximum Sink Current

Constant Current Mode: Limited

with external Rr, Rg, Rb

IN

40

mA

V

30

mA

Switch Mode: Limited with

ballast resistors by user

50

mA

V

−10

%

Current Mirror Ratio

1:100

RGB1 and RGB2 Mismatch

20 mA LED Current

R

RGB

Switch mode

2

3.5

Ω

f

RGB

RGB Switching Frequency

Accuracy proportional to

internal clock freq.

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

Ω,

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 k

Ω) = 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 2

Ω.

Using now the case from previous example and setting the

LED current with an external ballast resistor of 150

Ω. 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.

Addr.

Reg.

[7]

[6]

[5]

[4]

[3]

[2]

[1]

[0]

00’h

RGB1

Color[3]

Color[2]

Color[1]

Color[0]

Bright[2]

Bright[1]

Bright[0]

Enable1

default

0

0

0

0

0

0

0

0

01’h

RGB2

Color[3]

Color[2]

Color[1]

Color[0]

Bright[2]

Bright[1]

Bright[0]

Enable2

default

0

0

0

0

0

0

0

0

02’h

CTRL

NSTBY

CP_ON

CC/SW2

CC/SW1

1.5x/2x

reserved

reserved

reserved

default

0

0

0

0

0

0

0

0

03’h

OVL

Overlap2

R2SW

G2SW

B2SW

Overlap1

R1SW

G1SW

B1SW

default

0

0

0

0

0

0

0

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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