Datasheet

LT3970 Series
11
3970fc
applicaTions inFormaTion
The highest allowed V
IN
during normal operation
(V
IN(OP-MAX)
) is limited by minimum duty cycle and can
be calculated by the following equation:
V
IN(OP-MAX)
=
V
OUT
+ V
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on-time (~150ns).
However, the circuit will tolerate inputs up to the absolute
maximum ratings of the V
IN
and BOOST pins, regardless of
chosen switching frequency. During such transients where
V
IN
is higher than V
IN(OP-MAX)
, the switching frequency will
be reduced below the programmed frequency to prevent
damage to the part. The output voltage ripple and inductor
current ripple may also be higher than in typical operation,
however the output will still be in regulation.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V
IN
or V
OUT
and
decreases with higher inductance and faster switching
frequency. A good starting point for selecting the induc
-
tor value is:
L = 3
V
OUT
+ V
D
f
SW
where V
D
is the voltage drop of the catch diode (~0.7V),
L is in µH and f
SW
is in MHz. 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 500mA. To keep the efficiency high, the
series resistance (DCR) should be less than 0.1Ω, and the
core material should be intended for high frequency ap
-
plications. Table 2 lists several vendors and suitable types.
This simple design guide will not always result in the
optimum
inductor
selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the ap
-
plication requires less than 350mA load current, then a
lesser inductor value may be acceptable. This allows use
of a physically smaller inductor
, or one with a lower DCR
resulting in higher efficiency
. There are several graphs in
the Typical Performance Characteristics section of this data
sheet that show the maximum load current as a function
of input voltage for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu
-
ous mode operation, see Linear Technology Application
Note
44. Finally, for duty cycles greater than 50% (V
OUT
/
V
IN
> 0.5), there is a minimum inductance required to
avoid subharmonic oscillations. See Application Note 19.
Input Capacitor
Bypass the input of the LT3970 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 1µF to 4.7µF ceramic capacitor is
adequate to bypass the LT3970 and will easily handle the
ripple current. Note that larger input capacitance is required
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com