LTC1775IGN datasheet(20/24 Pages) LINER

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LTC1775

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1775. These items are also illustrated graphically in

the layout diagram of Figure 12. Check the following in

your layout:

1) Connect the TK lead directly to the drain of the topside

MOSFET. Then connect the drain to the (+) plate of CIN.

This capacitor provides the AC current to the top

MOSFET.

2) The power ground pin connects directly to the source of

the bottom N-channel MOSFET. Then connect the source

to the anode of the Schottky diode (if used) and (–) plate

of CIN, which should have as short lead lengths as

possible.

3) The LTC1775 signal ground pin should connect to the

(–) plate of COUT. Connect the (–) plate of COUT to power

ground at the source of the bottom MOSFET. All small-

signal components (R2, CSS, CC, etc.) should return

directly to SGND.

Figure 12. LTC1775 Layout Diagram

CSS

OPTIONAL 5V EXTVCC

CONNECTION

CVCC

BOLD LINES INDICATE HIGH CURRENT PATHS

1775 F12

CIN

COUT

VIN

VOUT

INTVCC

OPEN

EXT

CLK

OUTPUT DIVIDER

REQUIRED

WITH VPROG OPEN

CC1

LTRACE

–

VIN

EXTVCC

LTC1775

SYNC

VPROG

BOOST

INTVCC

RUN/SS

FCB

ITH

SGND

VOSENSE

PGND

4) Keep the switch node SW away from sensitive small-

signal nodes. Ideally the switch node should be placed

on the opposite side of the power MOSFETs from the

LTC1775.

5) Connect the INTVCC decoupling capacitor CVCC closely

to the INTVCC pin and the power ground pin. This

capacitor carries the MOSFET gate drive current.

6) Does the VOSENSE pin connect as close as possible to

the load? In adjustable applications, the resistive di-

vider (R1, R2) must be connected between the load and

signal ground. Place the divider near the LTC1775 in

order to keep the high impedance VOSENSE node short.

7) For applications with multiple switching power con-

verters connected to the same VIN, ensure that the input

filter capacitance for the LTC1775 is not shared with the

other converters. AC input current from another con-

verter will cause substantial input voltage ripple that

may interfere with proper operation of the LTC1775. A

few inches of PC trace or wire (LTRACE ≈ 100nH)

between CIN and the VIN supply is sufficient to prevent

sharing.

APPLICATIO S I FOR ATIO

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