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LT3498
APPLICATIONS INFORMATION — LED DRIVER
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 6 is approximately 40μs for a
3V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
f = 100Hz, t SETTLE = 40 μ s
The dimming range can be further extended by changing
the amplitude of the PWM signal. The height of the PWM
signal sets the commanded sense voltage across the sense
resistor through the CTRL1 pin. In this manner both analog
dimming and direct PWM dimming extend the dimming
range for a given application. The color of the LEDs no
longer remains constant because the forward current of
the LED changes with the height of the CTRL1 signal. For
the four LED application described above, the LEDs can
be dimmed ?rst, modulating the duty cycle of the PWM
signal. Once the minimum duty cycle is reached, the height
Dim Range = PERIOD =
1 1
t PERIOD = = = 0.01s
f 100
t SETTLE 40 μ s
t 0.01s
= 250 : 1
of the PWM signal can be decreased below 1.5V down to
125mV. The use of both techniques together allows the
average LED current for the four LED application to be
varied from 20mA down to less than 20μA. Figure 9 shows
the application for dimming using both analog dimming
Min Duty Cycle =
t SETTLE 40 μ s
? 100 = ? 100 = 0.4%
t PERIOD 0.01s
Duty Cycle Range = 100% → 0.4% at 100Hz
and PWM dimming. A potentiometer must be added to
ensure that the gate of the NMOS receives a logic-level
signal, while the CTRL1 signal can be adjusted to lower
amplitudes.
V IN 3V TO 5V
The calculations show that for a 100Hz signal the dimming
range is 250:1. In addition, the minimum PWM duty cycle
R SENSE1
10Ω
L1
15μH
C IN
1μH
of 0.4% ensures that the LED current has enough time to
settle to its ?nal value. Figure 8 shows the dimming range
achievable for three different frequencies with a settling
C OUT1
CAP1 SW1
V IN SW2 CAP2 V OUT2
LT3498
time of 40μs.
1μF
LED1
10000
CTRL1 GND1
5V
GND2
CTRL2
FB2
1000
PULSING MAY BE VISIBLE
PWM
FREQ
0V
3498 F09
Q1
Si2304BDS
100
10
100k
Figure 9. Li-Ion to Four White LEDs with
Both PWM Dimming and Analog Dimming
1
10
100 1000
PWM FREQUENCY (Hz)
10000
3498 F08
Figure 8. Dimming Ratio vs Freqeuncy
3498fa
13
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