Datasheet

LT3973/LT3973-3.3/LT3973-5
13
3973fa
applicaTions inFormaTion
about 97.5%. This leads to a minimum input voltage of
approximately:
V
IN(MIN1)
=
V
OUT
+ V
D
DC
MAX
– V
D
+ V
SW
where V
OUT
is the output voltage, V
D
is the catch diode
drop (~0.7V), V
SW
is the internal switch drop (~0.5V at
max load), and DC
MAX
is the maximum duty cycle.
The final factor affecting the minimum input voltage is the
minimum dropout voltage. When the OUT pin is tied to
V
OUT
, the LT3973 regulates the output such that it stays
more than 530mV below V
IN
. This enforced minimum
dropout voltage is due to reasons that are covered in a
later section. This places a limitation on the minimum
input voltage as follows:
V
IN(MIN2)
= V
OUT
+ V
DROPOUT(MIN)
where V
OUT
is the output voltage and V
DROPOUT(MIN)
is
the minimum dropout voltage (530mV).
Combining these factors leads to the overall minimum
input voltage:
V
IN(MIN)
= max(V
IN(MIN1)
, V
IN(MIN2)
, 4.2V)
Note that the LT3973 will begin switching at a lower input
voltage (typically 3V) but will regulate to a lower FB voltage
in this region of operation (see the Typical Performance
Characteristics section).
Maximum Input Voltage Range
The highest allowed V
IN
during normal operation (V
IN(OP-
MAX)
) is limited by minimum duty cycle and can be calcu-
lated 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.
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 = 1.5
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 cur-
rent should be above 1.5A. 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
applications. Table 2 lists several inductor vendors.
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
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 750mA 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