LTC3565 datasheet(18/22 Pages) LINER

18 / 22 page

LTC3565

3565fa

applications inForMation

The output voltage can now be programmed by choosing

the values of R1 and R2. To maintain high efficiency, the

current in these resistors should be kept small. Choosing

2µAwiththe0.6VfeedbackvoltagemakesR1~300k.Aclose

standard 1% resistor value is 294k then R2 is 931k.

The compensation should be optimized for these compo-

nentsbyexaminingtheloadstepresponsebutagoodplace

to start for the LTC3565 is with a 12.1kΩ and 680pF filter.

The output capacitor may need to be increased depending

on the actual undershoot during a load step.

ThePGOODpinisacommondrainoutputandrequiresapull-

up resistor. A 100k resistor is used for adequate speed.

The circuit on page 1 of this data sheet shows the complete

schematic for this design example.

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3565. These items are also illustrated graphically

in the layout diagram of Figure 4. Check the following in

your layout:

1. Does the capacitor CIN connect to the power VIN (Pin 6)

andpowerGND(Pin5)ascloseaspossible?Thiscapacitor

provides the AC current to the internal power MOSFETs

and their drivers.

2. Are the COUT and L1 closely connected? The (–) plate of

COUT returns current to PGND and the (–) plate of CIN.

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

between the (+) plate of COUT and a ground line. The

feedback signal VFB should be routed away from noisy

components and traces, such as the SW line (Pin 4), and

its trace should be minimized.

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

input capacitor CIN, the compensation capacitor CC and

CITH and all the resistors R1, R2, RT, and RC should be

routed away from the SW trace and the inductor L1. The

SW pin pad should be kept as small as possible.

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

thesignalandpowergroundssegregatedwithsmallsignal

components returning to the GND pin at one point.

6. Flood all unused areas on all layers with copper. Flood-

ing with copper will reduce the temperature rise of power

components. These copper areas should be connected to

one of the input supply rails: PVIN, SVIN or GND.

CITH

3565 F06

VIN

VOUT

BOLD LINES INDICATE HIGH CURRENT PATHS

CIN

COUT

PVIN

LTC3565

GND

SVIN

SYNC/MODE

PGOOD

VFB

RUN

ITH

Figure 4. LTC3565 Layout Diagram (See Board Layout Checklist)

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