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LTC3872

3872fa

APPLICATIO S I FOR ATIO

3. Assuming a MOSFET junction temperature of 125°C,

the room temperature MOSFET RDS(ON) should be less

than:

DS ON

SENSE MAX

MAX

OMAX

()

(

)

(

•

–

•

≤

⎛

⎝⎜

⎞

⎠⎟

)) •

.•

–.

•• .

ρT

⎛

⎝⎜

⎞

⎠⎟

≈Ω

0 175

10 39

21 5

The MOSFET used was the Si3460, which has a maximum

RDS(ON) of 27mΩ at 4.5V VGS, a BVDSS of greater than

30V, and a gate charge of 13.5nC at 4.5V VGS.

4. The diode for this design must handle a maximum DC

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

voltage of VOUT, or 5V. A 25A, 15V diode from On Semi-

conductor (MBRB2515L) was chosen for its high power

dissipation capability.

5. The output capacitor usually consists of a lower valued,

low ESR ceramic.

6. The choice of an input capacitor for a boost converter

depends on the impedance of the source supply and the

amount of input ripple the converter will safely tolerate.

For this particular design two 22μF Taiyo Yuden ceramic

capacitors (JMK325BJ226MM) is required (the input

and return lead lengths are kept to a few inches. As with

the output node, check the input ripple with a single

oscilloscope probe connected across the input capacitor

terminals.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3872. These items are illustrated graphically in

the layout diagram in Figure 7. Check the following in

your layout:

1. The Schottky diode should be closely connected be-

tween the output capacitor and the drain of the external

MOSFET.

2. The input decoupling capacitor (0.1μF) should be con-

nected closely between VIN and GND.

3. The trace from SW to the switch point should be kept

short.

4. Keep the switching node NGATE away from sensitive

small signal nodes.

5. The VFB pin should connect directly to the feedback

resistors. The resistive divider R1 and R2 must be con-

nected between the (+) plate of COUT and signal ground.

Figure 7. LTC3872 Layout Diagram (See PC Board Layout Checklist)

IPRG

ITH

VFB

GND

RUN/SS

VIN

NGATE

LTC3872

3872 F07

RITH

CIN

COUT

VOUT

VIN

CITH

BOLD LINES INDICATE HIGH CURRENT PATHS

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