LT3480
10
3480fb
APPLICATIONS INFORMATION
VVV
ft VV
IN MAX OUT D
SW ON MIN DSW() ()
–=++
where VIN(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VDis the catch diode drop
(~0.5V), VSW is the internal switch drop (~0.5V at max
load), fSW is the switching frequency (set by RT), and
tON(MIN) isthe minimumswitch ontime (~150ns).Note that
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:
VVV
ft VV
IN MIN OUT D
SW OFF MIN DSW() ()
––=++
1
where VIN(MIN) is the minimum input voltage, and tOFF(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 ΔILincreases with higher VIN or VOUT
anddecreaseswith higherinductance and fasterswitching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔIL= 0.4(IOUT(MAX))
where IOUT(MAX) is the maximum output load current. To
guarantee sufficient output current, peak inductor current
mustbe lower thanthe LT3480’s switchcurrent limit(ILIM).
The peak inductor current is:
I
L(PEAK) = IOUT(MAX) + ΔIL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ΔILis the inductor
ripple current. The LT3480’s switch current limit (ILIM) is
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
OUT(MAX) = ILIM – ΔIL/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
LVV
fI
VV
V
OUT D
SW L
OUT D
IN MAX
=+
⎛
⎝
⎜⎞
⎠
⎟+
⎛
⎝
⎜⎞
⎠
Δ1–
()
⎟⎟
where VDis the voltage drop of the catch diode (~0.4V),
VIN(MAX) is the maximum input voltage, VOUT is the output
voltage, fSW is the switching frequency (set by RT), and L
is in the inductor value.
Theinductor’s RMScurrentrating mustbe greater thanthe
maximumload current andits saturationcurrent shouldbe
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
efficiency 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
and suitable types.
