LT1939
13
1939f
APPLICATIONS INFORMATION
Discontinuous operation occurs when I
OUT
is less than
I
L
/2 as calculated above.
Figure 4 illustrates the inductance value needed for a 3.3V
output with a maximum load capability of 2A. Referring
to Figure 4, an inductor value between 3.3µH and 4.7µH
will be suffi cient for a 15V input voltage and a switch
frequency of 750kHz. There are several graphs in the
Typical Performance Characteristics section of this data
sheet that show inductor selection as a function of input
voltage and switch frequency for several popular output
voltages and output ripple currents. Also, low inductance
may result in discontinuous mode operation, which is
okay, but further reduces maximum load current. For
details of maximum output current and discontinuous
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 for
more information.
capacitor is required to reduce the resulting voltage
ripple at the LT1939 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
The input capacitor must have low impedance at the
switching frequency to do this effectively, and it must
have an adequate ripple current rating.
A conservative value is the RMS input current is given
by:
I
CIN(RMS)
=
I
OUT1
V
OUT1
•V
IN
V
OUT1
()
0.5
V
IN
<
I
OUT1
2
and is largest when V
IN
= 2V
OUT1
(50% duty cycle).
The frequency, V
IN
to V
OUT
ratio, and maximum load
current requirement of the LT1939 along with the input
supply source impedance, determine the energy storage
requirements of the input capacitor. Determine the worst-
case condition for input ripple current and then size the
input capacitor such that it reduces input voltage ripple to
an acceptable level. Typical values for input capacitors run
from 10µF at low frequencies to 2.2µF at higher frequencies.
The combination of small size and low impedance (low
equivalent series resistance or ESR) of ceramic capacitors
make them the preferred choice. The low ESR results in
very low voltage ripple and the capacitors can handle plenty
of ripple current. They are also comparatively robust and
can be used in this application at their rated voltage. X5R
and X7R types are stable over temperature and applied
voltage, and give dependable service. Other types (Y5V and
Z5U) have very large temperature and voltage coeffi cients
of capacitance, so they may have only a small fraction of
their nominal capacitance in your application. While they
will still handle the RMS ripple current, the input voltage
ripple may become fairly large, and the ripple current may
end up fl owing from your input supply or from other by-
pass capacitors in your system, as opposed to being fully
sourced from the local input capacitor. An alternative to a
high value ceramic capacitor is a lower value along with
a larger electrolytic capacitor, for example a 1µF ceramic
capacitor in parallel with a low ESR tantalum capacitor.
For the electrolytic capacitor, a value larger than 10µF will
be required to meet the ESR and ripple current require-
ments. Because the input capacitor is likely to see high
Figure 4. Inductor Values for 2A Maximum Load Current
(V
OUT1
= 3.3V, I
RIPPLE
= 1A)
Input Capacitor Selection
Bypass the input of the LT1939 circuit with a 4.7µF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
INPUT VOLTAGE (V)
5
250
FREQUENCY (kHz)
500
1000
1250
1500
2500
1939 F04
750
15
10
20 25
1750
2000
2250
L = 1µH
L = 1.5µH
L = 2.2µH
L = 3.3µH
L = 4.7µH
L = 6.8µH
