Datasheet

LM25005
SNVS411C –JANUARY 2006–REVISED MARCH 2013 www.ti.com
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 (C
SS
, R
T
, C
RAMP
) 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
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
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