Datasheet

LT3475/LT3475-1
9
3475fb
Table 1 shows suggested R
T
selections for a variety of
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:
DC =
V
OUT
+ V
F
()
V
IN
–V
SW
+ V
F
()
where V
F
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V
IN MIN
()
=
V
OUT
+ V
F
DC
MAX
–V
F
+ V
SW
with DC
MAX
= 1–t
OFF(MIN)
• f
where t
0FF(MIN)
is equal to 167ns and f is the switching
frequency.
Example: f = 600kHz, V
OUT
= 4V
DC
MAX
= 1− 167ns • 600kHz = 0.90
V
IN MIN
()
=
4V + 0.4V
0.9
– 0.4V + 0.4V = 4.9V
The maximum operating voltage is determined by the
absolute maximum ratings of the V
IN
and BOOST pins,
and by the minimum duty cycle.
V
IN MAX
()
=
V
OUT
+ V
F
DC
MIN
–V
F
+ V
SW
with DC
MIN
= t
ON(MIN)
• f
where t
ON(MIN)
is equal to 140ns and f is the switching
frequency.
Example: f = 750kHz, V
OUT
= 3.4V
DC
MIN
= 140ns • 750kHz = 0.105
V
IN MAX
()
=
3.4V + 0.4V
0.105
– 0.4V + 0.4V = 36V
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
IN
and BOOST pins. The input
voltage should be limited to the V
IN
operating range (36V)
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
IN(MAX)
. For example, an application
with an output voltage of 3V and switching frequency of
1.2MHz has a V
IN(MAX)
of 20V, as shown in Figure 3. Figure
4 shows operation at 35V. Output ripple and peak inductor
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
500mV/DIV
(AC COUPLED)
3475 F03
Figure 3. Operation at V
IN(MAX)
= 20V.
V
OUT
= 3V and f
SW
= 1.2MHHz
APPLICATIONS INFORMATION