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LT3599
APPLICATIONS INFORMATION
LED Current Dimming
Two different types of dimming control can be used with
the LT3599. The LED brightness can be set either by analog
dimming (CTRL pin voltage adjustment between 0V and 1V)
or PWM dimming (PWM pin duty cycle adjustment).
For some applications, the preferred method of brightness
control is to use a variable DC input voltage. The CTRL
pin voltage can be adjusted to set the dimming of the LED
string (see Figures 4 and 5). As the voltage on the CTRL
pin increases from 0V to 1V, the LED current increases
from 0 to the programmed LED current level. Once the
CTRL pin voltage increases beyond 1V, it has no effect on
the LED current.
For True Color PWM dimming, the LT3599 provides
up to a 3000:1 PWM dimming range by allowing the duty
cycle of the PWM pin to be reduced from 100% to as low as
0.033% at a PWM frequency of 100Hz (Figure 6). Dimming
by PWM duty cycle, allows for constant LED color to be
maintained over the entire dimming range.
For LT3599 PWM dimming control during startup and
normal operation, observe the following guidelines:
(1) STARTUP
LT3599 V OUT start-up requires the SHDN /UVLO and PWM
pins to be asserted from off to on and the PWM on-time
to be above a minimum value. The lowest PWM on-time
allowed for fault detection is ≈4μs. The lowest PWM
on-time allowed for reaching V OUT regulation is typically
4μs but might be greater depending on external circuit
parameters. Once LED current is in regulation, PWM on-
time can be reduced as low as 3μs depending on external
component selection.
(2) V OUT Collapse
120
100
80
60
40
I SET = 13.3k
If during normal operation V OUT collapses due to a fault
or because PWM on-time is too low, a re-start is required
(see STARTUP in item (1)).
V REF
R2
LT3599
CTRL
20
R1
0
0
0.2
0.4
0.6
0.8
1.0
1.2
CTRL PIN VOLTAGE (V)
3599 F04
3599 F05
Figure 4. LED Current vs CTRL Voltage
PWM
INDUCTOR
CURRENT
T PWM
TON PWM
(= 1/f PWM )
Figure 5. LED Current vs CTRL
LED
MAX I LED
CURRENT
3599 F06
Figure 6. LED Current Using PWM Dimming
3599fe
12
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