SOT-89 Step-Up PFM Switcher with Auto-Load Sense
External Components and Layout Consideration
The ILC6390 is designed to provide a complete DC-DC converter
solution with a minimum of external components. Ideally, only
three externals are required: the inductor, a pass diode, and an
output capacitor.
The inductor needs to be of low DC Resistance type, typically 1Ω
value. Toroidal wound inductors have better field containment (less
high frequency noise radiated out) but tend to be more expensive.
Some manufacturers like Coilcraft have new bobbin-wound induc-
tors with shielding included, which may be an ideal fit for these
applications. Contact the manufacturer for more information.
The inductor size needs to be in the range of 47mH to 1mH. In
general, larger inductor sizes deliver less current, so the load cur-
rent 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.]
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.
For lighter loads, an inductor of up to 1mH can be used. The use
of a larger inductor will increase overall conversion efficiency, due
to the reduction in switching currents through the device.
For the ILC6391, using an external transistor, the use of a 47mH
inductor is recommended based on our experience with the part.
The capacitor should, in general, always be tantalum type, as tan-
talum 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.
Different C-values will directly impact the ripple seen on the output
at a given load current, due to the direct charge-to-voltage rela-
tionship 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 sen-
sitive to overvoltage conditions.
In general, then, this capacitor should always be 47mF, Tantalum,
16V rating.
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. Different diodes may introduce dif-
ferent levels of high frequency switching noise into the output
waveform, so trying out several sources may make the most
sense for your system.
For the IL6391, 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 effi-
ciencies.]
The base components should remain at 1kΩ and
3300pF; any changes need to be verified prior to implementation.
As for actual physical component layout, in general, the more
compact the layout is, the better the overall performance will be. It
is important to remember that everything in the circuit depends on
a common and solid ground reference. Ground bounce can direct-
ly affect the output regulation and presents difficult behavior to
predict. Keeping all ground traces wide will eliminate ground
bounce problems.
It is also critical that the ground pin of C
L
and the V
SS
pin of the
device be the same point on the board, as this capacitor serves
two functions: that of the output load capacitor, and that of the
input supply bypass capacitor.
Layouts for DC-DC converter designs are critical for overall per-
formance, 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 proto-
type product to verify that nothing has changed which can affect
the performance.
Impala Linear Corporation
ILC6390 1.1
(408) 574-3939
www.impalalinear.com
Feb 2001
6
IMPALA [ Impala Linear Corporation ]
