CM9142 Datasheet, PDF (9/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
LED Current Set (ISET)
An external resistor programs a reference current, set-
ting the maximum driver current. This resistor must be
tied to a good analog ground. If it is pulled to ground
through a switch, for example, from the host controller
output, the voltage drop across that switch should not
exceed 10mV.
The voltage at the ISET1 and ISET2 pins is provided
by a .66V bandgap reference. The LED current is
approximately 1000x the current set by the R_SET resis-
tor, according to the following formula:
R_SET
=
0---.--6---6---V------–-----(_{--L----o----g---i--c---L----o---w-----}) _{× 1000}
I_LED
Logic Low is the voltage on device driving this pin to
ground. If the resistor is tied to ground directly, Logic
low = 0. For 20mA LED current, R_SET≅33 k. When this
pin is driven high or open, the device will enter a sleep
mode with V_OUT = 4.5V and, with no load,
I_QUIESCENT≅ 500μA.
Analog 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 luminous intensity of white
LEDs is proportional to the amount of forward current
through them, but the color wavelength emitted is also
dependent upon the forward current. In applications
where color shift is not critical, brightness can be con-
trolled by adjusting the diode’s current. A typical white
LED Intensity vs. forward current curve is shown in
Figure 3.
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 CM9142 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.
CM9142
ISET(1,2)
V_C
R
R_SET
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 than determine the resistor ratio.
© 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
9