CM9130 Datasheet, PDF (8/12 Pages) California Micro Devices Corp – Three Output Driver for White LEDs

PRELIMINARY
CM9130
Application Information (cont’d)
The typical conversion efficiency plots for these modes,
with some losses, are shown in Figure 2.
current. If a LED opens, the other LEDs will still be reg-
ulated at the programmed current.
VLED=3.5V
90
1X
75
1X-1.5X
60
dual mode
45
1.5X
2X
30
3.0 3.5 4.0 4.5 5.0 5.5 6.0
Input Voltage (V)
Figure 2. Ideal charge pump efficiency
As can be seen, the CM9130, with 1x and 1.5x modes,
has better efficiency in this application than a fixed 2x
charge pump. At low battery voltages, the higher effi-
ciency of the CM9130 charge pump’s 1,5x gain
reduces the battery drain. At higher input voltages, typ-
ically seen when the system is running off an AC
adapter, the CM9130, operating the 1x mode, has bet-
ter efficiency than single mode 1.5x or 2x charge
pumps, lowering the power dissipation for cooler circuit
operation and long life.
While the charge pump efficiency is easily determined,
the system efficiency is more difficult due to the current
source outputs, which complicate measuring the output
power. The forward voltage of the white LEDs will vary,
and the constant current sources will adjust to maintain
the current. When comparing systems, it is best to
compare the input current for a specified LED drive
current.
The 1x mode has better efficiency than the 1.5x mode.
Selecting LEDs with low forward voltage (V_LED)
increases the time spent in the 1x mode as the battery
discharges, extending the operating time.
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 10 mV.
The voltage at the ISET pin is provided by a .66V band-
gap reference. The LED current is approximately
1000x the current set by the R_SET resistor, 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 = 33k. 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 =
500 μA.
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.
Failed LED Detection
If a LED is shorted, the CM9130 will continue to oper-
ate and drive the remaining LEDs at the programmed
© 2006 California Micro Devices Corp. All rights reserved.
8 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
04/26/06