LT3682
13
3682f
where f
SW
is the switching frequency in MHz, V
OUT
is the
output voltage, V
D
is the catch diode drop (~0.5V) and L
is the inductor value in µH.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. 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 and suitable types.
For robust operation in fault conditions (start-up or short
circuit) and high input voltage (>30V), the saturation
current should be chosen high enough to ensure that the
inductor peak current does not exceed 3.5A. For example,
an application running from an input voltage of 36V us-
ing a 10µH inductor with a saturation current of 2.5A will
tolerate the mentioned fault conditions.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current and
reduces the output voltage ripple. If your load is lower than
the maximum load current, then you can relax the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one with
a lower DCR resulting in higher effi ciency. Be aware that
if the inductance differs from the simple rule above, then
the maximum load current will depend on input voltage.
In addition, low inductance may result in discontinuous
mode operation, which further reduces maximum load
current. For details of maximum output current and
discontinuous mode operation, see Linear Technology’s
Application Note 44. Finally, for duty cycles greater than
50% (V
OUT
/V
IN
> 0.5), a minimum inductance is required
to avoid sub-harmonic oscillations:
LVV
f
MIN OUT D
SW
=+()•
.12
The current in the inductor is a triangle wave with an av-
erage value equal to the load current. The peak inductor
and switch current is:
III
I
SW PEAK L PEAK OUT MAX
L
()() ()
== +
Δ
2
where I
L(PEAK)
is the peak inductor current, I
OUT(MAX)
is
the maximum output load current, and ΔI
L
is the induc-
tor ripple current. The LT3682 limits its switch current in
order to protect itself and the system from overload faults.
Therefore, the maximum output current that the LT3682 will
deliver depends on the switch current limit, the inductor
value and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
ΔI
DC V V
Lf
L
OUT D
SW
=
−+()•( )
•
1
where f
SW
is the switching frequency of the LT3682, DC
is the duty cycle and L is the value of the inductor.
To maintain output regulation, the inductor peak current
must be less than the LT3682’s switch current limit I
LIM
.
If SYNC pin is grounded I
LIM
is at least 1.45A at low duty
cycles and decreases to 1.1A at DC = 90%. If SYNC pin
is tied to 0.8V or more or if it is tied to a clock source for
synchronization I
LIM
is at least 1.18A at low duty cycles
and decreases to 0.85A at DC = 90%. The maximum output
current is also a function of the chosen inductor value
and can be approximated by the following expressions
depending on the SYNC pin confi guration:
For SYNC pin grounded:
II
I
ADC
I
OUT MAX LIM
LL
()
.•(.•)=−= − −
ΔΔ
2
145 1 024
2
APPLICATIONS INFORMATION
