SOT-89 Step-up Dual-Mode Switcher with Shutdown

Impala Linear Corporation

5

(408) 574-3939

www.impalalinear.com

Sept 1999

ILC6380/1 1.4

0.5mA. The only difference 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.

Start-up Mode

The ILC6380 has an internal soft-start mode which suppresses

ringing or overshoot on the output during start-up. The following

diagram illustrates this start-up condition’s typical performance:

External Components and Layout Consideration

The ILC6380 is designed to provide a complete DC-DC convertor

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 1

Ω

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 ILC6381, using an external transistor, the use of a 47mH

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 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 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 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 effi-

ciencies.] The base components should remain at 1k

Ω 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.

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.

V

V

OUT MIN

T

SOFT-START

(~10msec)

t = 0