ILC6381 Просмотр технического описания (PDF) - Fairchild Semiconductor

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ILC6381

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

Fairchild Semiconductor 

ILC6380/81

requirements, though, will vary by application and by actual

system design and layout, so generalizations in this area are

difficult, at best.

For load currents of around 5mA, such as pagers, use an

inductor in the range of 100µH to 330µH. 220µH is the most

typical value used here.

However, PWM control for boost DC-DC conversion is

widely used, especially in audio-noise sensitive applications

or applications requiring strict filtering of the high frequency

components. Impala’s products give very good efficiencies

of 85% at 50µA output (5V product), 87% maximum duty

cycles for high load conditions, while maintaining very low

shutdown current levels of 0.5µA. 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.

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

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 ILC6380 or

ILC6381 at 180kHz, the inductor size can be reduced to

approximately half of these stated values.

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

typical performance:

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.

V IN - V f

VOUT MIN

T SOFT-START (~10msec)

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 over-

voltage conditions.

t=0

In general, then, this capacitor should always be 4 7 µF, Tan-

talum, 16V rating.

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.

The inductor needs to be of low DC Resistance type, typi-

cally 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 inductors 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 47µH to 1µH.

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 100µH. [The 100µH inductor shown in the

datasheet is the most typical used for this application.]

The diode must be of shottkey type for fast recovery and

minimal loss. A diode rated at greater than 200mA and max-

imum voltage greater than 30V is recommended for the fast-

est switching time and best reliability over time. Different

diodes may introduce different levels of high frequency

switching noise into the output waveform, so trying out sev-

eral 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 tran-

sistor 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, there-

fore, higher overall efficiencies.] 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 perfor-

mance will be. It is important to remember that everything in

the circuit depends on a common and solid ground reference.

Ground bounce can directly affect the output regulation and

presents difficult behavior to predict. Keeping all ground

©2001 Fairchild Semiconductor Corporation

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