CM9130 Datasheet, PDF (10/12 Pages) California Micro Devices Corp – Three Output Driver for White LEDs

PRELIMINARY
CM9130
Application Information (cont’d)
CM9130
ISET
82.5k
R
55k
R_SET
Open Drain
Controller
Output
Figure 6. Logic Signal Dimming
For example, to reduce the luminosity intensity by half,
using the LED curve from Figure 3, the current setting
needs to be changed from 20-mA to about 8-mA. The
values in Figure 6 will accomplish this, are where
obtained using the following equations;
rent. Only the time averaged current changes. Above a
minimum frequency, the human eye will perceive the
change in duty cycle as a change in brightness.
V_BATT
on
PWM
off
R_SET
VIN
EN
ISET
VOUT
LED1
LED2
LED3
CM9130
GND
Display
Figure 7. PWM applied to EN
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 CM9130 allows the output to
lower the LED brightness by applying a pulsing (PWM)
signal to EN, as shown in Figure 7. 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.
The recommended frequency is between 100 Hz and
200 Hz, with a duty cycle greater than 20%. If a fre-
quency of less then 100 Hz is used, flicker might be
seen in the LEDs. The frequency should also be
greater than the refresh rate of the TFT display. Higher
frequencies will cause a loss of brightness control lin-
earity. In addition, higher frequency can cause chroma-
ticity shifts because the fixed rise and fall times of the
PWM signal will shift the forward current.
The PWM signal will cause the average LED current to
be reduced. The average current is determined by the
PWM duty cycle, which can vary from 0% to 100%.
Decreased Duty Cycle will linearly lower LED bright-
ness, 0% Duty Cycle will turn off the display LEDs.
EN
VOUT
I_{LED (1,2,3)}
PWM signal
I_SET
Using a PWM signal allows the LEDs to be dimmed
without substantially shifting their color balance due to
Figure 8. PWM Signal Dimming
chromaticity shifts related to changing white LED for-
ward current. The PWM signal causes the LEDs to
o^© p²⁰e⁰⁶ra^Cate^lifore^niait^Mhⁱe^crr^{o D}a^et^vict^eh^s e^Corf^pu^. l^Al^ll I^riS^ghE^ts T^resec^rvu^er^dr^. ent, or at zero cur-
04/26/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
10