CM9142 Datasheet, PDF (10/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
Ratio = ------0---.-6---6----V--------
Vc–0.66V
The resistors can be determined from the equations
below.
R = ⁽---R-----^×-----R-----a---t--i--o---⁾----+----R-----p-
Ratio
Rset = Ratio × R
CM9142
ISET(1,2)
82.5k
R
55k
R_SET
Open Drain
Controller
Output
Figure 6. Logic Signal Dimming
For example, a V_C max of 2.5V and a maximum current
setting of 20mA, R=125k, R_SET=44.8k. Figure 5 shows
the control curve.
LED Current vs. Vc
25
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
Control Voltage, Vc
Figure 5. LED Current Control Curve
The circuit in Figure 6 is an example of logic dimming
control, which changes the LED forward current in dis-
crete steps. The NMOS source is an open drain (or
open collector if bipolar) device, either the output of a
host controller, or a discrete device. Open drain, or
open collector devices sink current in their active, low
voltage state (logic 0), and are high impedance in their
high voltage, non-active state (logic 1). The open drain
must not be pulled high with an external resistor, but
instead connected only to the current setting resistors.
The parallel combination of R and R_SET determine the
full intensity current. When the drain goes high, R_SET
determines the lower intensity current.
For example, to reduce the luminosity intensity by half,
using the LED curve from Figure 3, the current setting
needs to be changed from 20mA to about 8mA. The
values in Figure 6 will accomplish this, are where
obtained using the following equations;
Rp = ^{.66V * 1000}
I_LED (max)
Rset = ^{.66V * 1000}
I_LED (min)
R=
1
1_− 1
Rp Rset
Additional parallel resistors can be added in the same
way.
PWM Control of Display Intensity
Typically, portable devices control the backlight display
intensity in response to ambient light conditions, or
lower the intensity after a short standby interval to con-
verse battery charge. The CM9142 allows the output to
lower the LED brightness by applying a pulsing (PWM)
signal to EN, as shown in Figure 7 for group 2. The
waveforms are shown in Figure 8.
The white in white LEDs is typically bichromatic, pro-
duced by a blue or UV LED that excites yellow phos-
phors. The two colors combine and the human eye
sees these them as white light. The forward current of
the LED influences the chromaticity, with higher LED
current increasing the blue content of the color.
Using a PWM signal allows the LEDs to be dimmed
without substantially shifting their color balance due to
chromaticity shifts related to changing white LED for-
ward current. The PWM signal causes the LEDs to
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