LT1930 datasheet(29/32 Pages) LINER

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LTC1871X

1871xf

For more information www.linear.com/LTC1871X

APPLICATIONS INFORMATION

1. The duty cycle range is:

VO + VD

VIN + VO + VD

⎛

⎝⎜

⎞

⎠⎟

= 45.5% to 71.4%

2. The operating mode chosen is pulse skipping, so the

MODE/SYNC pin is shorted to INTVCC.

3. The operating frequency is chosen to be 300kHz to

reduce the size of the inductors; the resistor from the

FREQ pin to ground is 80k.

4. An inductor ripple current of 40% is chosen, so the peak

input current (which is also the minimum saturation

current) is:

IL1(PEAK) = 1+

⎛

⎝⎜

⎞

⎠⎟

•IO(MAX) •

VO + VD

VIN(MIN)

= 1+

0.4

⎛

⎝⎜

⎞

⎠⎟

• 1.5 •

12 + 0.5

= 4.5A

The inductor ripple current is:

IL = χ •IO(MAX) •

DMAX

1– DMAX

= 0.4 • 1.5 •

0.714

1– 0.714

= 1.5A

And so the inductor value is:

L =

VIN(MIN)

2 • ΔIL • f

•DMAX =

2 • 1.5 • 300k

• 0.714 = 4µH

The component chosen is a BH Electronics BH510-

1007, which has a saturation current of 8A.

5. With an minimum input voltage of 5V, only logic-level

power MOSFETs should be considered. Because the

maximum duty cycle is 71.4%, the maximum SENSE

pin threshold voltage is reduced from its low duty

cycle typical value of 150mV to approximately 120mV.

Assuming a MOSFET junction temperature of 125°C,

the room temperature MOSFET RDS(ON) should be less

than:

RDS(ON) ≤

VSENSE(MAX)

IO(MAX)

•

⎛

⎝⎜

⎞

⎠⎟

• ρT

•

VO + VD

VIN(MIN)

⎛

⎝

⎜

⎞

⎠

⎟ + 1

0.12

1.5

•

1.2 • 1.5

•

12.5

⎛

⎝⎜

⎞

⎠⎟

+ 1

= 12.7mΩ

For a SEPIC converter, the switch BVDSS rating must be

greater than VIN(MAX) + VO, or 27V. This comes close to

anIRF7811W,whichisratedto30V,andhasamaximum

room temperature RDS(ON) of 12mΩ at VGS = 4.5V.

6. The diode for this design must handle a maximum

DC output current of 2A and be rated for a minimum

reverse voltage of VIN + VOUT, or 27V. A 3A, 40V diode

from International Rectifier (30BQ040) is chosen for its

small size, relatively low forward drop and acceptable

reverse leakage at high temp.

7. The output capacitor usually consists of a high valued

bulk C connected in parallel with a lower valued, low

ESRceramic.Basedonamaximumoutputripplevoltage

of 1%, or 120mV, the bulk C needs to be greater than:

COUT ≥

IOUT(MAX)

0.01• VOUT • f

1.5A

0.01• 12V • 300kHz

41µF

The RMS ripple current rating for this capacitor needs

to exceed:

IRMS(COUT) ≥IO(MAX) •

VIN(MIN)

1.5A •

12V

= 2.3A

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