IND(ripple)

OUT

I

ESR

V

0.7

2

R1

R2

0.7

´

æ

ö

-

-

ç

÷

ç

÷

è

ø

=

´

UDG-09066

TPS51218

DRVL

4

VIN

1 mF

VFB

V5IN

V

OUT

2

TRIP

5

RF

# 2

# 1

# 3

5

6

Thermal Pad

GND

TPS51218

www.ti.com

SLUS935B

– MAY 2009 – REVISED FEBRUARY 2012

3. Determine the value of R1 and R2.

The output voltage is programmed by the voltage-divider resistor, R1 and R2, shown in Figure 17. R1 is

connected between the VFB pin and the output, and R2 is connected between the VFB pin and GND. Typical

designs begin with the selection of an R2 value between 10 k

Ω and 20 kΩ. Determine R1 using Equation 9.

(9)

LAYOUT CONSIDERATIONS

Figure 19. Ground System of DC/DC Converter Using the TPS51218

Certain points must be considered before starting a layout work using the TPS51218.

•

on one side of the PCB (solder side). Other small signal components should be placed on another side

(component side). At least one inner plane should be inserted, connected to ground, in order to shield and

isolate the small signal traces from noisy power lines.

•

All sensitive analog traces and components such as VFB, PGOOD, TRIP and RF should be placed away

from high-voltage switching nodes such as SW, DRVL, DRVH or VBST to

　avoid coupling. Use internal

layer(s) as ground plane(s) and shield feedback trace from power traces and

　components.

•

The DC/DC converter has several high-current loops. The area of these loops should be minimized in order to

suppress generating switching noise.

capacitor(s) and the source of the low-side MOSFET at ground as close as possible. (Refer to loop #1 of

Figure 19)

and back to source of the low-side MOSFET through ground. Connect source of the low-side MOSFET

– The third important loop is of gate driving system for the low-side MOSFET. To turn on the low-side

MOSFET, high current flows from V5IN capacitor through gate driver and the low-side MOSFET, and back

to negative node of the capacitor through ground. To turn off the low-side MOSFET, high current flows

Copyright

© 2009–2012, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Link(s): TPS51218