LM25005 datasheet(19/21 Pages) NSC

19 / 21 page

Application Information (Continued)

PCB LAYOUT AND THERMAL CONSIDERATIONS

The circuit in Figure 1 serves as both a block diagram of the

LM25005 and a typical application board schematic for the

LM25005. In a buck regulator there are two loops where

currents are switched very fast. The first loop starts from the

input capacitors, to the regulator VIN pin, to the regulator SW

pin, to the inductor then out to the load. The second loop

starts from the output capacitor ground, to the regulator

PGND pins, to the regulator IS pins, to the diode anode, to

the inductor and then out to the load. Minimizing the loop

area of these two loops reduces the stray inductance and

minimizes noise and possible erratic operation. A ground

plane in the PC board is recommended as a means to

connect the input filter capacitors to the output filter capaci-

tors and the PGND pins of the regulator. Connect all of the

low power ground connections (C

SS,RT,CRAMP) directly to

the regulator AGND pin. Connect the AGND and PGND pins

together through the topside copper area covering the entire

underside of the device. Place several vias in this underside

copper area to the ground plane.

The two highest power dissipating components are the re-

circulating diode and the LM25005 regulator IC. The easiest

method to determine the power dissipated within the

LM25005 is to measure the total conversion losses (Pin –

Pout) then subtract the power losses in the Schottky diode,

output inductor and snubber resistor. An approximation for

the Schottky diode loss is P = (1-D) x Iout x Vfwd. An

approximation for the output inductor power is P = I

OUT

2 xR

x 1.1, where R is the DC resistance of the inductor and the

1.1 factor is an approximation for the ac losses. If a snubber

is used, the power loss can be estimated with an oscillo-

scope by observation of the resistor voltage drop at both

turn-on and turn-off transitions. The regulator has an ex-

posed thermal pad to aid power dissipation. Adding several

vias under the device to the ground plane will greatly reduce

the regulator junction temperature. Selecting a diode with an

exposed pad will aid the power dissipation of the diode.

TABLE 1. 5V, 2.5A Demo Board Bill of Materials

ITEM

PART NUMBER

DESCRIPTION

VALUE

C4532X7R2A225M

CAPACITOR, CER, TDK

2.2µ, 100V

C4532X7R2A225M

CAPACITOR, CER, TDK

2.2µ, 100V

C0805C331G1GAC

CAPACITOR, CER, KEMET

330p, 100V

C2012X7R2A103K

CAPACITOR, CER, TDK

0.01µ, 100V

C2012X7R2A103K

CAPACITOR, CER, TDK

0.01µ, 100V

OPEN

NOT USED

C2012X7R2A223K

CAPACITOR, CER, TDK

0.022µ, 100V

C2012X7R1C474M

CAPACITOR, CER, TDK

0.47µ, 16V

C3225X7R1C226M

CAPACITOR, CER, TDK

22µ, 16V

EEFHE0J151R

CAPACITOR, SP, PANASONIC

150µ, 6.3V

C0805C331G1GAC

CAPACITOR, CER, KEMET

330p, 100V

OPEN

NOT USED

CSHD6-60C

DIODE, 60V, CENTRAL

6CWQ10FN

DIODE, 100V, IR (D1-ALT)

DR127-330

INDUCTOR, COOPER

33µH

OPEN

NOT USED

OPEN

NOT USED

CRCW08052102F

RESISTOR

21K

CRCW08054992F

RESISTOR

49.9K

CRCW08055111F

RESISTOR

5.11K

CRCW08051651F

RESISTOR

1.65K

CRCW2512100J

RESISTOR

10, 1W

LM25005

REGULATOR, NATIONAL SEMICONDUCTOR

LM25005 Datasheet(PDF) 19 Page - National Semiconductor (TI)

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