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LT3466 Fiches technique(PDF) 10 Page - Linear Technology

No de pièce LT3466
Description  Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

LT3466 Fiches technique(HTML) 10 Page - Linear Technology

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LT3466
3466f
DUTY CYCLE
The duty cycle for a step-up converter is given by:
D
VV
V
VV
V
OUT
D
IN
OUT
D
CESAT
=
+
+
where:
VOUT = Output voltage
VD = Schottky forward voltage drop
VCESAT = Saturation voltage of the switch
VIN = 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 RT pin to ground.
The nominal voltage on the RT 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 RT for a given operating
frequency is shown in the 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 (RT). RT is chosen so as to run the
converters at 800kHz (RT = 63.4k), 1.25MHz (RT = 39.1k)
and 2MHz (RT = 20.5k). The efficiency comparison for
different RT values is shown in Figure 7.
APPLICATIO S I FOR ATIO
Figure 6. Timing Resistor (RT) Value
Figure 7. Efficiency Comparison for Different RT Resistors
OSCILLATOR FREQUENCY (kHz)
1000
3466 F06
10
100
600
1800
1400
1000
200
LED CURRENT (mA)
0
70
75
85
15
3466 F07
65
60
510
20
55
50
80
RT = 63.4k
RT = 39.1k
RT = 20.5k
CIRCUIT OF FIGURE 1
VIN = 3.6V
8/8 LEDs


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