LM25005

SNVS411C – JANUARY 2006 – REVISED MARCH 2013

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PCB LAYOUT AND THERMAL CONSIDERATIONS

The circuit in theTypical Application Circuit and Block Diagram 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 capacitors and the PGND pins of the regulator. Connect all of the low power ground

connections (CSS, 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 = IOUT

2 x R 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 oscilloscope by observation of

the resistor voltage drop at both turn-on and turn-off transitions. The regulator has an exposed 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.

5V, 2.5A Demo Board Bill of Materials

ITEM

PART NUMBER

DESCRIPTION

VALUE

C

1

C4532X7R2A225M

CAPACITOR, CER, TDK

2.2µ, 100V

C

2

C4532X7R2A225M

CAPACITOR, CER, TDK

2.2µ, 100V

C

3

C0805C331G1GAC

CAPACITOR, CER, KEMET

330p, 100V

C

4

C2012X7R2A103K

CAPACITOR, CER, TDK

0.01µ, 100V

C

5

C2012X7R2A103K

CAPACITOR, CER, TDK

0.01µ, 100V

C

6

OPEN

NOT USED

C

7

C2012X7R2A223K

CAPACITOR, CER, TDK

0.022µ, 100V

C

8

C2012X7R1C474M

CAPACITOR, CER, TDK

0.47µ, 16V

C

9

C3225X7R1C226M

CAPACITOR, CER, TDK

22µ, 16V

C

10

EEFHE0J151R

CAPACITOR, SP, PANASONIC

150µ, 6.3V

C

11

C0805C331G1GAC

CAPACITOR, CER, KEMET

330p, 100V

C

12

OPEN

NOT USED

D

1

CSHD6-60C

DIODE, 60V, CENTRAL

6CWQ10FN

DIODE, 100V, IR (D1-ALT)

L

1

DR127-330

INDUCTOR, COOPER

33µH

R

1

OPEN

NOT USED

R

2

OPEN

NOT USED

R

3

CRCW08052102F

RESISTOR

21K

R

4

CRCW08054992F

RESISTOR

49.9K

R

5

CRCW08055111F

RESISTOR

5.11K

R

6

CRCW08051651F

RESISTOR

1.65K

R

7

CRCW2512100J

RESISTOR

10, 1W

U

1

LM25005

REGULATOR, TEXAS INSTRUMENTS

20

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