LTC3350_15 Datasheet, PDF(31/46 Page) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

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LTC3350_15 Datasheet, PDF (31/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

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LTC3350

Applications Information

Connect the drain of the top MOSFET and cathode of

the top diode directly to the positive terminal of COUT.

Connect the source of the bottom MOSFET and anode

of the bottom diode directly to the negative terminal

of COUT. This capacitor provides the AC current to the

MOSFETs.

2. Ground is referenced to the negative terminal of the

VCAP decoupling capacitor in step-down mode and to

the negative terminal of the VOUT decoupling capacitor

in step-up mode. The negative terminal of COUT should

be as close as possible to the negative terminal of

CCAP by placing the capacitors next to each other and

away from the switching loop described above. The

combined IC SGND pin/PGND paddle and the ground

returns of CINTVCC and CDRVCC must return to the com-

bined negative terminal of COUT and CCAP.

3. Effective grounding techniques are critical for success-

ful DC/DC converter layouts. Orient power components

such that switching current paths in the ground plane

do not cross through the SGND pin and exposed pad

on the backside of the LTC3350 IC. Switching path

currents can be controlled by orienting the MOSFET

switches, Schottky diodes, the inductor, and VOUT and

VCAP decoupling capacitors in close proximity to each

other.

4. Locate VCAP and VOUT dividers near the part and away

from switching components. Kelvin the top of resistor

dividers to the positive terminals of CCAP and COUT,

respectively. The bottom of the resistive dividers should

go back to the SGND pin. The feedback resistor con-

nections should not be run along the high current feeds

from the COUT capacitor.

5. Route ICAP and VCAP sense lines together, keep them

short. Same with VOUTSP and VOUTSN. Filter com-

ponents should be placed near the part and not near

the sense resistors. Ensure accurate current sensing

with Kelvin connections at the sense resistors. See

Figure 10.

6. The trace from the positive terminal of the input current

sense resistor, RSNSI, to the VOUTSP pin carries the

partâs quiescent and gate drive currents. To maintain

accurate measurement of the input current keep this

trace short and wide by placing RSNSI near the part.

DIRECTION OF SENSED CURRENT

RSNSC

RSNSI

3350 F10

TO ICAP TO VCAP

VOUTSP VOUTSN

Figure 10. Kelvin Current Sensing

7. Locate the DRVCC and BST decoupling capacitors in

close proximity to the IC. These capacitors carry the

MOSFET driversâ high peak currents. An additional

0.1Î¼F ceramic capacitor placed immediately next to

the DRVCC pin can help improve noise performance

substantially.

8. Locate the small-signal components away from high

frequency switching nodes (BST, SW, TG, and BG). All

of these nodes have very large and fast moving signals

and should be kept on the output side of the LTC3350.

9. The input ideal diode senses the voltage between VIN

and VOUTSP. VIN should be connected near the source

of the input ideal diode MOSFET. VOUTSP is used for

Kelvin sensing the input current. Place the input cur-

rent sense resistor, RSNSI, near the input ideal diode

MOSFET with a short, wide trace to minimize resistance

between the drain of the ideal diode MOSFET and RSNSI.

10. The output ideal diode senses the voltage between

VOUTSN and VCAP. VCAP is used for Kelvin sensing

the charge current. Place the output ideal diode near

the charge current sense resistor, RSNSC, with a short,

wide trace to minimize resistance between the source

of the ideal diode MOSFET and RSNSC.

11. The INFET and OUTFET pins for the external ideal diode

controllers have extremely limited drive current. Care

must be taken to minimize leakage to adjacent PC board

traces. 100nA of leakage from these pins will introduce

an additional offset to the ideal diodes of approximately

10mV. To minimize leakage, the INFET trace can be

guarded on the PC board by surrounding it with VOUT

connected metal. Similarly, the OUTFET trace should be

guarded by surrounding it with VCAP connected metal.

3350fb

For more information www.linear.com/LTC3350

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