LTC3631 datasheet(20/28 Pages) LINER

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LTC3646/LTC3646-1

36461fc

For more information www.linear.com/LTC3646

The following list summarizes the three possible connec-

tions for EXTVCC:

1. EXTVCCgrounded.ThiswillcauseINTVCCtobepowered

from the internal 5.0V regulator.

2. EXTVCC connected directly to VOUT. This is the normal

connection for a 4.5V to 6V regulated output and pro-

vides the highest efficiency.

3. EXTVCC connected to an external supply. If an external

supply is available in the 4.5V to 6V range, it may be

used to power EXTVCC. Ensure that EXTVCC < VIN.

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3646.

1. Does the capacitor CPVIN connect to PVIN and PGND

as close to the pins as possible? These capacitors

provide the AC current to the internal power MOSFETs

and drivers. The (–) plate of CPVIN should be closely

connected to PGND and the (–) plate of COUT.

2. The output capacitor, COUT, and inductor L1 should

be closely connected to minimize loss. The (–) plate

of COUT should be closely connected to PGND and the

(–) plate of CIN.

3. The resistive divider, R1 and R2, must be connected

between the (+) plate of COUT and a ground line termi-

nated near SGND. The feedback signal, VFB, should be

routedawayfromnoisycomponentsandtracessuchas

the SW line, and its trace length should be minimized.

In addition, RT and the loop compensation components

should be terminated to SGND.

4. Keep sensitive components away from the SW pin. The

RRT resistor, the feedback resistors, the compensation

components, and the INTVCC bypass capacitor should

all be routed away from the SW trace and the inductor.

5. A ground plane is preferred, but if not available the

signal and power grounds should be segregated with

both connecting to a common, low noise reference

point. The point at which the ground terminals of the

VIN and VOUT bypass capacitors are connected makes a

good, low noise reference point. The connection to the

PGND pin should be made with a minimal resistance

trace from the reference point.

6. Flood all unused areas on all layers with copper in order

to reduce the temperature rise of power components.

Thesecopperareasshouldbeconnectedtotheexposed

backside connection of the IC.

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