LT3486
APPLICATIONS INFORMATION
Duty Cycle
The duty cycle for a step-up converter is given by:
D
=
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 LT3486 is 96%
(typ) when running at 1MHz switching frequency. It in-
creases to 98% (typ) when run at 200kHz and drops to
90% (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 LT3486 uses a constant frequency architecture that
can be programmed over a 200kHz to 2.5MHz 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 5.
1000
5V
C
IN
10µF
D2
L1
10µH
SW1
25mA
OFF ON
REF
OVP1
CTRL1
SHDN
PWM1
FB1
V
C1
2.8k
8.06
4.7nF
CIN: 10V, X7R
C
OUT1
, C
OUT2
: 35V, X5R
D1, D2: ZETEX ZHCS400
L1, L2: TOKO D53LC TYPE A
GND
R
T
R
T
LT3486
V
IN
L2
10µH
SW2
OVP2
CTRL2
REF
PWM2
FB2
V
C2
2.8k
4.7nF
8.06
3486 F06a
Operating Frequency Selection
The choice of operating frequency is determined by sev-
eral factors. There is a trade-off 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 operate
at the maximum duty cycle in order to light up the maximum
number of LEDs. The LT3486 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 6a is operated with different values
of timing resistor (R
T
). R
T
is chosen so as to run the con-
verters at 800kHz (R
T
= 63.4k), 1.25MHz (R
T
= 39.1k) and
2MHz (R
T
= 21.5k). The CTRL pins are used to provide
dimming for the respective LED strings. The efficiency
comparison for different R
T
values is shown in Figure 6b.
V
OUT
+
V
D
– V
IN
V
OUT
+
V
D
– V
CESAT
D1
C
OUT1
2.2µF
C
OUT2
2.2µF
25mA
1.25V
C
REF
0.1µF
R
T
(k )
100
Figure 6a. 5V to 8/8 White LEDs
10
0
500
1000
1500
2000
OSCILLATOR FREQUENCY (kHz)
2500
3486 G09
Figure 5. Timing Resistor (R
T
) Value
3486fe
10
LINER [ LINEAR TECHNOLOGY ]
