Linear Technology LT3480 Manual page 10

LT3480
APPLICATIONS INFORMATION
+
V V
OUT D
=
V
IN MAX ( )
f t
SW ON MIN (
where V is the maximum operating input voltage,
IN(MAX)
V is the output voltage, V
OUT
(~0.5V), V is the internal switch drop (~0.5V at max
SW
load), f is the switching frequency (set by R
SW
t is the minimum switch on time (~150ns). Note that
ON(MIN)
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 60V are acceptable regardless of the
switching frequency. In this mode, the LT3480 may enter
pulse skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation. Above 38V switching
will stop.
The minimum input voltage is determined by either the
LT3480's minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
+
V V
OUT
=
V
IN MIN ( )
1 – f t
SW OFF MIN (
where V is the minimum input voltage, and t
IN(MIN)
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI
increases with higher V
L
and decreases with higher inductance and faster switching
10
+
– V V
D SW
)
is the catch diode drop
D
T
D
+
– V V
D SW
)
OFF(MIN)
or V
IN
frequency. A reasonable starting point for selecting the
ripple current is:
ΔI
= 0.4(I
L OUT(MAX)
where I
OUT(MAX)
guarantee suffi cient output current, peak inductor current
must be lower than the LT3480's switch current limit (I
), and
The peak inductor current is:
I = I
L(PEAK) OUT(MAX)
where I
L(PEAK)
the maximum output load current, and ΔI
ripple current. The LT3480's switch current limit (I
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
I = I
OUT(MAX)
Be sure to pick an inductor ripple current that provides
suffi cient maximum output current (I
The largest inductor ripple current occurs at the highest
V . To guarantee that the ripple current stays below the
IN
specifi ed maximum, the inductor value should be chosen
according to the following equation:
⎛
+
V
OUT
=
L
⎝ ⎜
Δ
f
SW
where V is the voltage drop of the catch diode (~0.4V),
D
V is the maximum input voltage, V
IN(MAX)
voltage, f is the switching frequency (set by RT), and L
SW
is in the inductor value.
The inductor's RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3.5A. To keep the
effi ciency high, the series resistance (DCR) should be less
than 0.1 , and the core material should be intended for
high frequency applications. Table 1 lists several vendors
OUT
and suitable types.
)
is the maximum output load current. To
+ ΔI
/2
L
is the peak inductor current, I
– ΔI
/2
LIM L
OUT(MAX)
⎛ ⎞
⎞
+
V V V
D OUT D
⎜ 1– ⎟ ⎟
⎠ ⎟
I V
⎝ ⎠
L IN MAX ( )
).
LIM
is
OUT(MAX)
is the inductor
L
) is
LIM
).
is the output
OUT
3480fb