LTC1872 datasheet(29/36 Pages) LINER

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LTC1871

1871fe

APPLICATIONS INFORMATION

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 •

+ 0.5

= 4.5A

The inductor ripple current is:

IL = •IO(MAX) •

DMAX

1– DMAX

= 0.4 •1.5 •

0.714

1– 0.714

= 1.5A

And so the inductor value is:

VIN(MIN)

2• IL • 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)

•

• T

•

VO + VD

VIN(MIN)

+ 1

0.12

1.5

•

1.2 • 1.5

•

12.5

+ 1

= 12.7m

For a SEPIC converter, the switch BVDSS rating must be

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

an IRF7811W, which is rated to 30V, and has a maximum

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 ESR

ceramic. Based on a maximum output ripple voltage 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

To satisfy this high RMS current demand, two 47μF

Kemet capacitors (T495X476K020AS) are required. As a

result, the output ripple voltage is a low 50mV to 60mV.

In parallel with these tantalums, two 10μF, low ESR (X5R)

Taiyo Yuden ceramic capacitors (TMK432BJ106MM) are

added for HF noise reduction. Check the output ripple

with a single oscilloscope probe connected directly

across the output capacitor terminals, where the HF

switching currents ﬂow.

8. The choice of an input capacitor for a SEPIC converter

depends on the impedance of the source supply and the

amount of input ripple the converter will safely toler-

ate. For this particular design and lab setup, a single

47μF Kemet tantalum capacitor (T495X476K020AS) is

adequate. As with the output node, check the input ripple

with a single oscilloscope probe connected across the

input capacitor terminals. If any HF switching noise is

observed it is a good idea to decouple the input with

a low ESR, X5R ceramic capacitor as close to the VIN

and GND pins as possible.

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