LT3474/LT3474-1
APPLICATIONS INFORMATION
Setting the Switching Frequency
V
+ V
F
(
)
OUT
DC =
The LT3474 uses a constant frequency architecture that
can be programmed over a 200kHz to 2MHz range with a
V – V + V
F
(
)
IN
SW
single external timing resistor from the R pin to ground.
T
where V is the forward voltage drop of the catch diode
The current that flows into the timing resistor is used
F
(~0.4V) and V is the voltage drop of the internal switch
to charge an internal oscillator capacitor. A graph for
SW
(~0.4V at maximum load). This leads to a minimum input
selecting the value of R for a given operating frequency
T
voltage of:
is shown in the Typical Performance Characteristics
section. Table 1 shows suggested R selections for a
T
V
OUT + V
F
V
=
– V + VSW
F
variety of switching frequencies.
IN MIN
(
)
DCMAX
= 1–t
Table 1. Switching Frequencies
SWITCHING FREQUENCY (MHz)
with DC
where t
frequency.
• f
OFF(MIN)
MAX
R (kΩ)
T
is equal to 200ns and f is the switching
2
10
0FF(MIN)
1.5
1
18.7
33.2
52.3
80.6
147
232
Example: f = 500kHz, V
= 4V
OUT
0.7
0.5
0.3
0.2
DCMAX =1− 200ns •500kHz = 0.90
4V + 0.4V
V
=
– 0.4V + 0.4V = 4.9V
IN MIN
(
)
0.9
The maximum operating voltage is determined by the
Operating Frequency Selection
absolute maximum ratings of the V and BOOST pins,
IN
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
useofsmallerinductorsatthecostofincreasedswitching
losses and decreased efficiency.
and by the minimum duty cycle.
V
OUT + V
DCMIN
F
V
=
– V + VSW
F
IN MAX
(
)
with DC
where t
= t
• f
MIN
ON(MIN)
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 fixed oscillator off-time and
a variable on-time. As a result, the maximum duty cycle
increases as the switching frequency is decreased.
is equal to 160ns and f is the switching
ON(MIN)
frequency.
Example: f = 500kHz, V
= 2.5V
OUT
DCMIN =160ns •500kHz = 0.08
2.5V + 0.4V
V
=
– 0.4V + 0.4V = 36V
IN MAX
(
)
Input Voltage Range
0.08
Theminimumoperatingvoltageisdeterminedeitherbythe
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:
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.
3474fd
10
Linear [ Linear ]
