LT3990 Datasheet, PDF (10/20 Pages) Linear Technology – 60V, 350mA Step-Down Regulator with 2.5μA Quiescent Current and Integrated Diodes

LT3990
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 ^{VOUT + VD}
f_SW
Table 2. Inductor Vendors
VENDOR
Coilcraft
Sumida
Toko
Würth Elektronik
Coiltronics
Murata
URL
www.coilcraft.com
www.sumida.com
www.tokoam.com
www.we-online.com
www.cooperet.com
www.murata.com
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 applications. 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 LT3990 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 LT3990 and will easily handle
3990p
10