SOT-89 Step up Switching Regulator with Shutdown
For load currents higher than 10mA, use an inductor from 47µH to
100µH. [The
100µH inductor shown in the data sheet is the most
typical used for this application.]
For load currents of around 5mA, such as pagers, use an indcutor
in the range of 100µH to 330µH. 220µH 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 ILC6371, using an external transistor, the use of a 47µH
inductor is recommended based on our experience with the part.
Note that these values are recommended for both 50kHz and
100kHz operation. If using the ILC6370 or ILC6371 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 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 particu-
larly sensitive to overvoltage conditions.
In general, this capacitor should always be 47µF, 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 level of high frequency switching noise into the output
waveform, so trying out several sources may make the most
sense for your system.
For the ILC6371, 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
shoud 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
3300kΩ; 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 elliminate ground
bounce problems.
It is also critical that the ground pin of C
L
and V
SS
pin of the
device be the same pin 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
performance, but following these simple guidlines can simplify
the task by avoiding some of the more common mistakes made
in these cases. Once actual performance is completed, be
sure to double check the design on an actual manufacturing
prototype prodcut to verfy that nothing has changed which can
affect the performance.
Impala Linear Corporation
ILC6370/1 1.3
(408) 574-3939
www.impalalinear.com
July 1999
6
IMPALA [ Impala Linear Corporation ]
