lt3475efe-trpbf Linear Technology Corporation, lt3475efe-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt3475efe-trpbf
• Description: Dual Step-down 1.5a Led Driver
• Manufacturer: Linear Technology Corporation

Table 1 shows suggested R
switching frequencies.
Operating Frequency Selection
The choice of operating frequency is determined by
several factors. There is a tradeoff between effi ciency and
component size. A higher switching frequency allows the
use of smaller inductors at the cost of increased switching
losses and decreased effi ciency.
Another consideration is the maximum duty cycle. In certain
applications, the converter needs to operate at a high duty
cycle in order to work at the lowest input voltage possible.
The LT3475 has a fi xed oscillator off time and a variable
on time. As a result, the maximum duty cycle increases
as the switching frequency is decreased.
Input Voltage Range
The minimum operating voltage is determined either by the
LT3475’s undervoltage lockout of 4V, or by its maximum
duty cycle. The duty cycle is the fraction of time that the
internal switch is on and is determined by the input and
output voltages:
where V
(~0.4V) and V
(~0.4V at maximum load). This leads to a minimum input
voltage of:
with DC
where t
frequency.
Example: f = 600kHz, V
APPLICATIONS INFORMATION
DC =
DC
V
V
IN MIN
IN MIN
MAX
(
(
0FF(MIN)
MAX
F
(
is the forward voltage drop of the catch diode
V
) =
) =
= 1− 167ns • 600kHz = 0.90
IN
(
= 1–t
V
SW
V
4V + 0.4V
OUT
– V
OUT
DC
is equal to 167ns and f is the switching
is the voltage drop of the internal switch
0.9
SW
OFF(MIN)
MAX
+ V
+ V
+ V
F
F
OUT
)
– V
– 0.4V + 0.4V = 4.9V
F
)
• f
F
= 4V
T
+ V
selections for a variety of
SW
The maximum operating voltage is determined by the
absolute maximum ratings of the V
and by the minimum duty cycle.
with DC
where t
frequency.
Example: f = 750kHz, V
The minimum duty cycle depends on the switching fre-
quency. Running at a lower switching frequency might
allow a higher maximum operating voltage. Note that
this is a restriction on the operating input voltage; the
circuit will tolerate transient inputs up to the Absolute
Maximum Ratings of the V
voltage should be limited to the V
during overload conditions (short circuit or start up).
Minimum On Time
The LT3475 will regulate the output current at input volt-
ages greater than V
with an output voltage of 3V and switching frequency of
1.2MHz has a V
4 shows operation at 35V. Output ripple and peak inductor
DC
V
V
IN MAX
IN MAX
MIN
(AC COUPLED)
(
(
ON(MIN)
MIN
500mV/DIV
20V/DIV
= 140ns • 750kHz = 0.105
1A/DIV
) =
) =
V
V
= t
OUT
SW
I L
3.4V + 0.4V
V
Figure 3. Operation at V
V
ON(MIN)
IN(MAX)
OUT
is equal to 140ns and f is the switching
OUT
DC
0.105
MIN
= 3V and f
+ V
IN(MAX)
• f
of 20V, as shown in Figure 3. Figure
F
OUT
LT3475/LT3475-1
– V
IN
. For example, an application
– 0.4V + 0.4V = 36V
= 3.4V
SW
F
and BOOST pins. The input
+ V
= 1.2MHHz
IN
SW
operating range (36V)
IN(MAX)
IN
and BOOST pins,
= 20V.
3475 F03
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