LT1370 데이터 시트보기 (PDF) - Linear Technology

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LT1370

500kHz High Efficiency 6A Switching Regulator

Linear Technology 

LT1370

UU W U

APPLICATIO S I FOR ATIO

Shutdown and Synchronization

The device has a dual function S/S pin which is used for

both shutdown and synchronization. This pin is logic level

compatible and can be pulled high, tied to VIN or left

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

activates shutdown, reducing the part’s supply current to

12µA. 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 12µs resetable

shutdown delay network guarantees the part will not go

into shutdown while receiving a synchronization signal.

Caution should be used when synchronizing above 700kHz

because at higher sync frequencies the amplitude of the

internal slope compensation used to prevent subhar-

monic 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 elimi-

nate this problem.

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 T7 package

but these numbers will vary depending on the mounting

techniques (copper area, airflow, etc.). Heat is transferred

from the package via the tab.

Average supply current (including driver current) is:

IIN = 4.5mA + DC(ISW/45)

ISW = Switch current

DC = Switch duty cycle

Switch power dissipation is given by:

PSW = (ISW)2(RSW)(DC)

RSW = Output switch ON resistance

Total power dissipation of the die is the sum of supply

current times supply voltage, plus switch power:

PD(TOTAL) = (IIN)(VIN) + PSW

Surface mount heat sinks are available which can lower

package thermal resistance by two or three times. One

manufacturer, Wakefield Engineering, offers surface mount

heat sinks for the R package and can be reached at (617)

245-5900 or at www.wakefield.com.

Choosing the Inductor

For most applications the inductor will fall in the range of

2.2µH 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 6A limit. Higher values also

reduce input ripple voltage and reduce core loss.

When choosing an inductor you need to consider maxi-

mum 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

converter is equal to load current times VOUT/ VIN and

decide whether or not the inductor must withstand

continuous overload conditions. If average inductor

current at maximum load current is 3A, for instance, a

3A inductor may not survive a continuous 6A 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

saturate abruptly and other core materials fall in

between. The following formula assumes continuous

mode operation but it errs only slightly on the high side

for discontinuous mode, so it can be used for all

conditions.

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