efficiencies of 85% at 50mA output (5V product), 87% maxi-
mum duty cycles for high load conditions, while maintaining
very low shutdown current levels of 0.5mA. The only differ-
ence between the ILC6380 and ILC6381 parts is that the
6381 is configured to drive an external transistor as the
switch element. Since larger transistors can be selected for
this element, higher effective loads can be regulated.
with the part. Note that these values are recommended for
both 50kHz and 100kHz operation. If using the ILC6380 or
ILC6381 at 180kHz, the inductor size can be reduced to
approximately half of these stated values.
The capacitor should, in general, always be tantalum type,
as tantalum has much better ESR and temperature stability
than other capacitor types. NEVER use electrolytics or
chemical caps, as the C-value changes below 0°C so
much as to make the overall design unstable.
Start-up Mode
The ILC6380 has an internal soft-start mode which sup-
presses ringing or overshoot on the output during start-up.
The following diagram illustrates this start-up condition’s typ-
ical performance
Different C-values will directly impact the ripple seen on the
output at a given load current, due to the direct charge-to-
voltage relationship of this element. Different C-values will
also indirectly affect system reliability, as the lifetime of the
capacitor can be degraded by constant high current influx
and outflux. Running a capacitor near its maximum rated
voltage can deteriorate lifetime as well; this is especially
true for tantalum caps which are particularly sensitive to
overvoltage conditions.
V
OUT MIN
V
- V
IN
f
In general, then, this capacitor should always be 47mF,
Tantalum, 16V rating.
T
SOFT-START (~10msec)
The diode must be of shottkey type for fast recovery and
minimal loss. A diode rated at greater than 200mA and
maximum voltage greater than 30V is recommended for
the fastest switching time and best reliability over time. Dif-
ferent diodes may introduce different levels of high fre-
quency switching noise into the output waveform, so trying
out several sources may make the most sense for your
system.
t = 0
External Components and Layout Consideration
The ILC6380 is designed to provide a complete DC-DC con-
vertor solution with a minimum of external components. Ide-
ally, only three externals are required: the inductor, a pass
diode, and an output capacitor.
For the IL6381, much of the component selection is as
described above, with the addition of the external NPN
transistor and the base drive network. The transistor needs
to be of NPN type, and should be rated for currents of 2A
or more. [This translates to lower effective on resistance
and, therefore, higher overall efficiencies.] The base com-
ponents should remain at 1kW a nd 3300pF ; any changes
need to be verified prior to implementation.
The inductor needs to be of low DC Resistance type, typi-
cally 1W value. Toroidal wound inductors have better field
containment (less high frequency noise radiated out) but
tend to be more expensive. Some manufacturers like Coil-
craft have new bobbin-wound inductors with shielding
included, which may be an ideal fit for these applications.
Contact the manufacturer for more information.
As for actual physical component layout, in general, the
more compact the layout is, the better the overall perfor-
mance will be. It is important to remember that everything
in the circuit depends on a common and solid ground refer-
ence. Ground bounce can directly affect the output regula-
tion and presents difficult behavior to predict. Keeping all
ground traces wide will eliminate ground bounce problems.
The inductor size needs to be in the range of 47mH to 1mH.
In general, larger inductor sizes deliver less current, so the
load current will determine the inductor size used.
For load currents higher than 10mA, use an inductor from
47mH to 100mH. [The 100mH inductor shown in the
datasheet is the most typical used for this application.]
It is also critical that the ground pin of C and the V pin of
L
SS
the device be the same point on the board, as this capaci-
tor serves two functions: that of the output load capacitor,
and that of the input supply bypass capacitor.
For load currents of around 5mA, such as pagers, use an
inductor in the range of 100mH to 330mH. 220mH is the most
typical value used here.
Layouts for DC-DC converter designs are critical for overall
performance, but following these simple guidelines can
simplify the task by avoiding some of the more common
mistakes made in these cases. Once actual performance
is completed, though, be sure to double-check the design
on actual manufacturing prototype product to verify that
nothing has changed which can affect the performance.
For lighter loads, an inductor of up to 1mH can be used. The
use of a larger inductor will increase overall conversion effi-
ciency, due to the reduction in switching currents through
the device.
For the ILC6381, using an external transistor, the use of a
47mH inductor is recommended based on our experience
Impala Linear Corporation
6
IMPALA [ Impala Linear Corporation ]
