LT3466EFE Linear Technology, LT3466EFE Datasheet - Page 10

• Manufacturer Part Number: LT3466EFE
• Description: IC,Laser Diode/LED Driver,TSSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT3466
DUTY CYCLE
The duty cycle for a step-up converter is given by:
where:
The maximum duty cycle achievable for LT3466 is 96%
(typ) when running at 1MHz switching frequency. It in-
creases to 99% (typ) when run at 200kHz and drops to
92% (typ) at 2MHz. Always ensure that the converter is not
duty-cycle limited when powering the LEDs at a given
switching frequency.
SETTING THE SWITCHING FREQUENCY
The LT3466 uses a constant frequency architecture that
can be programmed over a 200KHz to 2MHz range with a
single external timing resistor from the R
The nominal voltage on the R
10
V
V
V
V
D
OUT
D
CESAT
IN
=
= Schottky forward voltage drop
= Input battery voltage
V
= Output voltage
OUT
1000
V
100
= Saturation voltage of the switch
OUT
10
200
Figure 6. Timing Resistor (R
+
+
V
D
V
OSCILLATOR FREQUENCY (kHz)
D
–
600
U
–
V
CESAT
V
IN
1000
U
1400
T
pin is 0.54V, and the
W
T
1800
) Value
3466 F06
T
pin to ground.
U
current that flows into the timing resistor is used to
charge and discharge an internal oscillator capacitor. A
graph for selecting the value of R
frequency is shown in Figure 6.
OPERATING FREQUENCY SELECTION
The choice of operating frequency is determined by sev-
eral factors. There is a tradeoff between efficiency and
component size. Higher switching frequency allows the
use of smaller inductors albeit at the cost of increased
switching losses and decreased efficiency.
Another consideration is the maximum duty cycle achiev-
able. In certain applications, the converter needs to oper-
ate at the maximum duty cycle in order to light up the
maximum number of LEDs. The LT3466 has a fixed
oscillator off-time and a variable on-time. As a result, the
maximum duty cycle increases as the switching frequency
is decreased.
The circuit of Figure 1 is operated with different values of
timing resistor (R
converters at 800kHz (R
and 2MHz (R
different R
Figure 7. Efficiency Comparison for Different R
T
85
55
50
80
75
70
65
60
values is shown in Figure 7.
0
T
CIRCUIT OF FIGURE 1
V
8/8 LEDs
IN
= 20.5k). The efficiency comparison for
= 3.6V
T
). R
5
LED CURRENT (mA)
T
T
= 63.4k), 1.25MHz (R
is chosen so as to run the
10
R
T
= 39.1k
R
R
T
T
= 20.5k
T
= 63.4k
15
for a given operating
3466 F07
20
T
Resistors
T
= 39.1k)
3466fa