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LT3466
APPLICATIO S I FOR ATIO
D =
DUTY CYCLE
The duty cycle for a step-up converter is given by:
V OUT + V D – V IN
V OUT + V D – V CESAT
where:
V OUT = Output voltage
V D = Schottky forward voltage drop
V CESAT = Saturation voltage of the switch
V IN = Input battery voltage
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 T pin to ground.
The nominal voltage on the R T pin is 0.54V, and the
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 T for a given operating
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 T ). R T is chosen so as to run the
converters at 800kHz (R T = 63.4k), 1.25MHz (R T = 39.1k)
and 2MHz (R T = 20.5k). The efficiency comparison for
different R T values is shown in Figure 7.
1000
85
CIRCUIT OF FIGURE 1
V IN = 3.6V
80 8/8 LEDs
R T = 63.4k
R T = 39.1k
75
70
100
65
60
55
R T = 20.5k
10
200
600
1000
1400
1800
50
0
5
10
15
20
OSCILLATOR FREQUENCY (kHz)
3466 F06
Figure 6. Timing Resistor (R T ) Value
LED CURRENT (mA)
3466 F07
Figure 7. Efficiency Comparison for Different R T Resistors
3466fa
10
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