LT3003
7
3003fa
Input Capacitor Selection
The LT3003 is typically driven from the same input voltage
used for the partner LED driver IC. The LED driver and
inductor govern the requirements for the input capacitor
of the application. A ceramic input capacitor in the range of
1µF to 10µF works for most applications. In cases where
the LT3003 input voltage is derived separately from the
LED driver, a 1µF input capacitor works well.
LED Current Matching
An LED driver programs the LED current and LT3003 ac-
tively ballasts three separate strings of LEDs. The currents
in all three channels will be matched to better than ±3%.
To achieve best current matching, (V
IN
– V
EE
) should be
between 3V and 10V. A low (V
IN
– V
EE
) also minimizes
LT3003 internal power dissipation (see Thermal Calcula-
tion section for more information).
PWM Dimming
The LT3003 has a wide PWM dimming range for constant
color LED dimming. PWM dimming is superior to analog
dimming as it preserves true color quality. PWM dimming
control with the LT3003 is achieved using a simple ground
referenced PWM signal with a 0.5V on/off threshold. The
LEDs operate at either programmed or zero current but
their brightness changes with the PWM signal duty cycle.
When PWM is low, LED strings are completely discon-
nected. In addition, the LT3003 switches to low power
standby mode ~10µs after PWM low edge, resulting in
higher system power effi ciency.
For the widest dimming range, the PWM signal should be
100Hz. The human eye is typically sensitive to fl ickering
below ~80Hz. Operating the PWM higher than 100Hz results
in a reduced PWM dimming ratio.
Achieving high PWM dimming ratios require attention to
circuit leakages, such as reverse bias leakage currents
through the external Schottky. Hence, for high PWM dim-
ming ratios, components should be chosen to minimize
leakage currents.
If an application does not require PWM dimming, the PWM
pin can be left open (unconnected) and an internal 10µA
source current pulls PWM high.
Boost PWM Dimming
The LT3003 supports up to a 3000:1 PWM dimming ratio
with a 100Hz PWM dimming frequency. To achieve such
high PWM ratios, leakages of the LED driver and other
external components should be minimal.
Buck Mode: PWM Dimming
The LT3003 supports up to a 3000:1 PWM dimming ratio in
buck mode. The PWM dimming in buck mode is achieved
by an architecture that allows the LT3003 power ground
(V
EE
) to move with output capacitor voltage. PWM dim-
ming control is achieved by a simple ground referenced
PWM signal, eliminating the need for any external level-
shift component.
High PWM dimming ratios require very low V
EE
and
V
MAX
pin currents during the PWM off state. The LT3003
V
IN
– V
EE
(V)
3
I
LED1,3
vs I
LED2
(%)
8
12
3003 F02
2
69
0
10
6
4
3633302724211815
I
LED2
= 350mA
∆V
LED
= 1.4V
Figure 2. I
LED
Matching vs (V
IN
– V
EE
)
LED Pin Current Range
The steady-state operational current range for each LED pin
is between 100mA and 350mA. Internal protection circuitry
limits absolute maximum pin current to 550mA.
LED Open-Circuit Protection
If any LED string is open, then currents in all three chan-
nels reduce to zero. The driver chip, which supplies LED
current, should have an overvoltage clamp to protect the
LT3003 from high LED pin voltages.
APPLICATIONS INFORMATION
