LT1374CS8 datasheet(10/32 Pages) LINER

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LT1374

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APPLICATIONS INFORMATION

MAXIMUM OUTPUT LOAD CURRENT

Maximum load current for a buck converter is limited by

the maximum switch current rating (IP) of the LT1374.

This current rating is 4.5A up to 50% duty cycle (DC),

decreasing to 3.7A at 80% duty cycle. This is shown

graphically in Typical Performance Characteristics and as

shown in the formula below:

IP = 4.5A for DC ≤ 50%

IP = 3.21 + 5.95(DC) – 6.75(DC)2 for 50% < DC < 90%

DC = Duty cycle = VOUT/VIN

Example: with VOUT = 5V, VIN = 8V; DC = 5/8 = 0.625, and;

ISW(MAX) = 3.21 + 5.95(0.625) – 6.75(0.625)2 = 4.3A

Current rating decreases with duty cycle because the

LT1374 has internal slope compensation to prevent current

mode subharmonic switching. For more details, read Ap-

plication Note 19. The LT1374 is a little unusual in this regard

because it has nonlinear slope compensation which gives

better compensation with less reduction in current limit.

Maximum load current would be equal to maximum

switch current

for an infinitely large inductor, but with

finite inductor size, maximum load current is reduced by

one-half peak-to-peak inductor current. The following

formula assumes continuous mode operation, implying

that the term on the right is less than one-half of IP.

IOUT(MAX) =

Continuous Mode

For the conditions above and L = 3.3

µH,

OUT MAX

( )

−

=−

() −

()















 ()

=−

58 5

2 3 3 10

500 10

057

3 73

.•

•

At VIN = 15V, duty cycle is 33%, so IP is just equal to a fixed

4.5A, and IOUT(MAX) is equal to:

515 5

2 3 3 10

500 10

45 1 3 5

.•

•

−

() −

()















 ()

=− =

−

Note that there is less load current available at the higher

input voltage because inductor ripple current increases.

This is not always the case. Certain combinations of

inductor value and input voltage range may yield lower

available load current at the lowest input voltage due to

reduced peak switch current at high duty cycles. If load

current is close to the maximum available, please check

maximum available current at both input voltage

extremes. To calculate actual peak switch current with a

given set of conditions, use:

Lf V

SW PEAK

OUT

( ) =+

−

()

()( )( )

For lighter loads where discontinuous operation can be

used, maximum load current is equal to:

IOUT(MAX) =

Discontinuous mode

Example: with L = 1.2

µH, VOUT = 5V, and VIN(MAX) = 15V,

OUT MAX

( )

−

() 











()

() −

()

4 5

500 10

1 2 10

2 5 15 5

182

.•

. •

The main reason for using such a tiny inductor is that it is

physically very small, but keep in mind that peak-to-peak

inductor current will be very high. This will increase output

ripple voltage. If the output capacitor has to be made larger

to reduce ripple voltage, the overall circuit could actually

wind up larger.

CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR

For most applications the output inductor will fall in the

range of 3

µH to 20µH. Lower values are chosen to reduce

physical size of the inductor. Higher values allow more

output current because they reduce peak current seen by

the LT1374 switch, which has a 4.5A limit. Higher values

also reduce output ripple voltage, and reduce core loss.

Graphs in the Typical Performance Characteristics section

show maximum output load current versus inductor size

and input voltage. A second graph shows core loss versus

inductor size for various core materials.

IP − ()

−

()

()( )( )

Lf V

OUT

If L V

OUT

() ( )( )( )

() −

()

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