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LT3466-1
APPLICATIO S I FOR ATIO
DUTY CYCLE
The duty cycle for a step-up converter is given by:
OPERATING FREQUENCY SELECTION
The choice of operating frequency is determined by sev-
D =
V OUT + V D – V IN
V OUT + V D – V CESAT
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
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 LT3466-1 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 or OLED at a
given switching frequency.
SETTING THE SWITCHING FREQUENCY
The LT3466-1 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 the Figure 6.
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 LT3466-1 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 = 38.3k)
and 2MHz (R T = 20.5k). The efficiency comparison for
different R T values is shown in Figure 7.
INDUCTOR SELECTION
The choice of the inductor will depend on the selection of
switching frequency of LT3466-1. The switching fre-
quency can be programmed from 200kHz to 2MHz. Higher
switching frequency allows the use of smaller inductors
albeit at the cost of increased switching losses.
1000
90
CIRCUIT OF FIGURE 1
100
80
70
60
50
V IN = 3.6V
6 LEDs
R T = 63.4k
R T = 20.5k
R T = 38.3k
10
200
600
1000
1400
1800
40
0
5
10
15
20
OSCILLATOR FREQUENCY (kHz)
34661 F06
Figure 6. Timing Resistor (R T ) Value
LED CURRENT (mA)
34661 F07
Figure 7. Efficiency Comparison for Different R T Resistors
34661f
10
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