CM9130 Datasheet, PDF (9/12 Pages) California Micro Devices Corp – Three Output Driver for White LEDs

PRELIMINARY
CM9130
Application Information (cont’d)
Relative Luminous Intensity
1.5
1.0
0.5
0.0
0.0
5.0 10.0 15.0 20.0 25.0 30.0
Forward Current (mA)
Figure 3. Typical Luminous Intensity vs.
LED Current
The Iset pins of the CM9130 can be used to connect
an analog DC signal for analog dimming of the white
LEDs, as shown in Figure 4 This requires an additional
resistor, R, and a DC source voltage, Vc.
CM9130
ISET
V_C
R
R_SET
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
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
Figure 4. Analog LED current adjust
A control voltage, V_C, applied to the resistor divider will
decrease the current for all LEDs. The maximum LED
current occurs with 0V on V_C, which is set by R_P is the
parallel combination of R and R_SET.
R_P
=
-----0---.-6---6---V-------
I_LEDmax
×
1000
Choose the maximum control voltage, V_C, which sets
zero LED current, and then determine the resistor ratio.
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.
© 2006 California Micro Devices Corp. All rights reserved.
04/26/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
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