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LT3497
APPLICATIONS INFORMATION
time to settle to its ?nal value. Figure 9 shows the avail-
3V TO 5V
able dimming range for different PWM frequencies with
a settling time of 40μs.
L1
15 μ H
L2
15 μ H
1 μ F
10000
SW1
CAP1
V IN
SW2
CAP2
1000
PULSING MAY BE VISIBLE
1 μ F
R SENSE1
10 ?
LT3497
R SENSE2
10 ?
1 μ F
LED1 LED2
CTRL1 GND CTRL2
100
5V
5V
10
0V
PWM
PWM
0V
FREQ
FREQ
1
10
100 1000
10000
Q1
Si2318DS
100k
100k
Q2
Si2318DS
PWM FREQUENCY (Hz)
3497 F10
3497 F09
Figure 9. Dimming Ratio vs Frequency
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 CTRL 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 CTRL signal. For
the 4-LED application described above, the LEDs can be
dimmed ?rst, modulating the duty cycle of the PWM signal.
Figure 10. Li-Ion to 4/4 White LEDs with Both PWM Dimming
and Analog Dimming
lower battery voltage. This technique allows the LEDs to
be powered off two alkaline cells. Most portable devices
have a 3.3V supply voltage which can be used to power
the LT3497. The LEDs can be driven straight from the
battery, resulting in higher ef?ciency.
Figure 11 shows 3/3 LEDs powered by two AA cells.
The battery is connected to the inductors and the chip is
powered off a 3.3V logic supply voltage.
3.3V
Once the minimum duty cycle is reached, the height of the
PWM signal can be decreased below 1.5V down to 100mV.
The use of both techniques together allows the average LED
current for the 4-LED application to be varied from 20mA
2 AA CELLS
2V TO 3.2V
C1
1 μ F
L1
15 μ H
SW1
V IN
L2
15 μ H
SW2
C2
1 μ F
down to less than 20μA. Figure 10 shows the application
CAP1
CAP2
for dimming using both analog dimming and PWM dim-
ming. A potentiometer must be added to ensure that the
R SENSE1
10 ?
LT3497
LED1 LED2
R SENSE2
10 ?
gate of the NMOS receives a logic-level signal, while the
CTRL signal can be adjusted to lower amplitudes.
C3
1 μ F
CTRL1 GND CTRL2
OFF ON
OFF ON
C4
1 μ F
SHUTDOWN
AND DIMMING
SHUTDOWN
AND DIMMING
LOW INPUT VOLTAGE APPLICATIONS
The LT3497 can be used in low input voltage applica-
tions. The input supply voltage to the LT3497 must be
CONTROL 1 CONTROL 2
C1, C2: TAIYO YUDEN LMK212BJ105MG
C3, C4: TAIYO YUDEN GMK212BJ105KG
L1, L2: MURATA LQH32CN150K53
3497 F11
2.5V or higher. However, the inductors can be run off a
Figure 11. 2 AA Cells to 3/3 White LEDs
3497f
13
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