Datasheet

LT3474/LT3474-1
10
3474fd
APPLICATIONS INFORMATION
Setting the Switching Frequency
The LT3474 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 current that fl ows into the timing resistor is used
to charge an internal oscillator capacitor. A graph for
selecting the value of R
T
for a given operating frequency
is shown in the Typical Performance Characteristics
section. Table 1 shows suggested R
T
selections for a
variety of switching frequencies.
Table 1. Switching Frequencies
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
210
1.5 18.7
1 33.2
0.7 52.3
0.5 80.6
0.3 147
0.2 232
Operating Frequency Selection
The choice of operating frequency is determined by sev-
eral factors. There is a tradeoff between effi ciency and
component size. 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 LT3474 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
LT3474’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
VV
VV V
OUT F
IN SW 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
VV
DC
VV
IN MIN
OUT F
MAX
FSW
()
=
+
+–
with DC
MAX
= 1–t
OFF(MIN)
• f
where t
0FF(MIN)
is equal to 200ns and f is the switching
frequency.
Example: f = 500kHz, V
OUT
= 4V
DC ns kHz
V
VV
MAX
IN MIN
= − =
=
+
()
1 200 500 0 90
404
0
•.
.
.
99
04 04 49–. . .VVV+=
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
VV
DC
VV
IN MAX
OUT F
MIN
FSW
()
=
+
+–
with DC
MIN
= t
ON(MIN)
• f
where t
ON(MIN)
is equal to 160ns and f is the switching
frequency.
Example: f = 500kHz, V
OUT
= 2.5V
DC ns kHz
V
VV
MIN
IN MAX
==
=
+
()
160 500 0 08
25 04
0
•.
..
.
008
04 04 36–. .VVV+=
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
Rating.