Datasheet

LT3497
13
3497f
time to settle to its fi nal value. Figure 9 shows the avail-
able dimming range for different PWM frequencies with
a settling time of 40μs.
Figure 9. Dimming Ratio vs Frequency
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 effi 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.
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 fi rst, modulating the duty cycle of the PWM signal.
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
down to less than 20µA. Figure 10 shows the application
for dimming using both analog dimming and PWM dim-
ming. A potentiometer must be added to ensure that the
gate of the NMOS receives a logic-level signal, while the
CTRL signal can be adjusted to lower amplitudes.
LOW INPUT VOLTAGE APPLICATIONS
The LT3497 can be used in low input voltage applica-
tions. The input supply voltage to the LT3497 must be
2.5V or higher. However, the inductors can be run off a
Figure 11. 2 AA Cells to 3/3 White LEDs
APPLICATIONS INFORMATION
PWM FREQUENCY (Hz)
10
PWM DIMMING RANGE
100
1000
10000
100 1000 10000
3497 F09
1
10
PULSING MAY BE VISIBLE
SW1 V
IN
LT3497
GND
SW2
L2
15µH
R
SENSE1
10Ω
Q1
Si2318DS
Q2
Si2318DS
R
SENSE2
10Ω
1µF
1µF
3497 F10
3V TO 5V
L1
15µH
CAP1 CAP2
LED1 LED2
CTRL1 CTRL2
1µF
100k 100k
0V
PWM
FREQ
5V
PWM
FREQ
0V
5V
Figure 10. Li-Ion to 4/4 White LEDs with Both PWM Dimming
and Analog Dimming
SW1 V
IN
LT3497
GND
SW2
L2
15µH
R
SENSE2
10Ω
R
SENSE1
10Ω
C2
1µF
C1
1µF
C4
1µF
C1, C2: TAIYO YUDEN LMK212BJ105MG
C3, C4: TAIYO YUDEN GMK212BJ105KG
L1, L2: MURATA LQH32CN150K53
C3
1µF
3497 F11
3.3V
L1
15µH
2 AA CELLS
2V TO 3.2V
CAP1 CAP2
LED1 LED2
CTRL1
SHUTDOWN
AND DIMMING
CONTROL 1
SHUTDOWN
AND DIMMING
CONTROL 2
CTRL2
OFF ON
OFF ON