NCV7430
www.onsemi.com
10
The calibration factors have a value of eight bits and
fraction the programmed LED modulation value between
0% and Max ON duty cycle.
With high values chosen for the coefficients in one row,
the calculation can cross the Max ON duty cycle boundary
(clipping) for the color. As a rule: For proper design, the sum
of the calibration values should stay under Max ON duty
cycle to prevent color saturation.
If one of the calculated LED1′, LED2′, or LED3′ values
exceeds the upper practical boundaries of Max ON duty
cycle, the modulator automatically adapts the modulation
speed to the color that exceeded the Max ON duty cycle.
This method guarantees that the color integrity is
maintained.
The calibration factors a11 to a33 reside in nine dedicated
OTP registers:
(0x04 to 0x08, and 0x0A to 0x0D).:
LED modulation Calibration data a11 to a33.
These registers can be programmed in OTP and are
generally used for the compensation of LED colors which
occur due to system design changes and lot−to−lot variation
of LEDs.
LED Intensity
The overall intensity of the LEDs is programmable with
a four bit scaling factor that is applied over the LED
modulation. The register containing this value is
AMBLIGHTINTENSITY. The scaling is linear. The light
output function is described with the following formula:
LED1int
LED2int
LED3int
+
ǒ
AMBLIGHTINTENSITY
16
Ǔ
*
LED1Ȁ
LED2Ȁ
LED3Ȁ
(eq. 3
If the intensity value is set to 15 the used value for the
calculation is 16, resulting in a multiplication factor of 1 (no
intensity reduction). Changing the intensity from one to
another value can follow a linear or logarithmic transition
based on the fading time as described in “Theatre dimming
function”.
LED Modulation Frequency
The LED modulation frequency can be chosen to be 122
or 244 Hz.
Theatre Dimming Function
The NCV7430 has a fading function to give a theater
dimming effect when changing color and/or intensity
settings. The effect presents itself as a smooth transition
between colors, or increases or decreases in intensity.
Transitions from color to color, or changes in intensity
will vary in a linear fashion through the color/intensity
spectrum (optional logarithmic mode for intensity). The
fading time can be set between 0 and 6.3 seconds via a 6 bit
register giving a resolution of 0.1 second. The fading
function can be enabled and disabled by programming the
FADING ON/OFF bit in the control registers. The default
state of this bit depends on the <DEFAULT> bit that is set
in OTP memory.
Intensity − Linear or logarithmic dimming
Color − Linear dimming only
LED Update Modes
Bits <UPDATECOLOR[1:0]> are used to enable the
NCV7430 for operation in different update modes. The
following modes are implemented:
UPDATECOLOR:
00 immediate update
01 store and do not update
10 update to the already stored values
11 discard
The UPDATECOLOR bits are included in the command
Set_Color (Byte 5, Bits 6 and 7).
Short Circuit and Open Circuit Detection
The NCV7430 provides protection features for each LED
driver. The device monitors for LED Open Circuit (ANODE
to LEDxC), LED Short Circuit (ANODE to LEDxC), Short
LEDxC to GND and Open Circuit R
SENSE
(LEDxR to
GND) as shown in Figure 7. Detection of these errors will
set the appropriate error bits in the status register
(<ERRLEDx[2:0]>), and proper action will be taken
(reference Table 7).
There is a minimum detection activation time of 8 msec for
error detection (use of a 0.2% duty cycle is recommended).
This is derived from a combination of color, intensity levels,
and PWM frequency settings (122 Hz or 244 Hz). The
system design should monitor error detection at high
intensity settings to avoid missing short or open circuit
conditions at low duty cycles. LEDxC shorts to ground do
not require a minimum duty cycle.
Additionally, error detection must be sequential
(transitioning from a known good state to an error state).
Mixing of errors (i.e. transitioning from a short condition to
an open condition) could result in signal false errors in
identity.
