8

LT1371

Shutdown and Synchronization

The 7-pin R and T7 package devices have a dual function

S/S pin which is used for both shutdown and synchroni-

zation. The SW package device has both a Shutdown

(SHDN) pin and a Synchronization (SYNC) pin which can

be used separately or tied together. These pins are logic

floating for normal operation. A logic low on the S/S pin or

SHDN pin activates shutdown, reducing the part’s supply

current to 12

µA. Typical synchronization range is from

1.05 to 1.8 times the part’s natural switching frequency,

but is only guaranteed between 600kHz and 800kHz. A

12

µs resetable shutdown delay network guarantees the

part will not go into shutdown while receiving a synchro-

nization signal when the functions are combined.

Caution should be used when synchronizing above 700kHz

because at higher sync frequencies the amplitude of the

internal slope compensation used to prevent subharmonic

switching is reduced. This type of subharmonic switching

only occurs when the duty cycle of the switch is above 50%.

Higher inductor values will tend to eliminate problems.

Thermal Considerations

Care should be taken to ensure that the worst-case input

voltage and load current conditions do not cause exces-

sive die temperatures. Typical thermal resistance is

30

°C/W for the R package and 50°C/W for the SW and T7

packages but these numbers will vary depending on the

mounting techniques (copper area, air flow, etc.). Heat is

transferred from the R and T7 packages via the tab and

from the SW package via pins 4 to 7 and 14 to 17.

Average supply current (including driver current) is:

DC = switch duty cycle

Switch power dissipation is given by:

Total power dissipation of the die is the sum of supply

current times supply voltage, plus switch power:

Surface mount heat sinks are also becoming available

which can lower package thermal resistance by 2 or 3

times. One manufacturer is Wakefield Engineering who

offers surface mount heat sinks for both the R package

(DD) and SW package (SW20) and can be reached at (617)

245-5900.

Choosing the Inductor

For most applications the inductor will fall in the range of

2.2

µH to 22µH. Lower values are chosen to reduce physi-

cal size of the inductor. Higher values allow more output

current because they reduce peak current seen by the

power switch, which has a 3A limit. Higher values also

reduce input ripple voltage and reduce core loss.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allowable

component height, output voltage ripple, EMI, fault

current in the inductor, saturation and, of course, cost.

The following procedure is suggested as a way of handling

these somewhat complicated and conflicting requirements.

1. Assume that the average inductor current for a boost

decide whether or not the inductor must withstand

continuous overload conditions. If average inductor

current at maximum load current is 1A, for instance, a

1A inductor may not survive a continuous 3A overload

condition. Also be aware that boost converters are not

short-circuit protected and that, under output short

conditions, inductor current is limited only by the

available current of the input supply.

2. Calculate peak inductor current at full load current to

ensure that the inductor will not saturate. Peak current

can be significantly higher than output current, espe-

cially with smaller inductors and lighter loads, so don’t

omit this step. Powdered iron cores are forgiving

because they saturate softly, whereas ferrite cores

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