LM25005_06 NSC [National Semiconductor], LM25005_06 Datasheet - Page 18

• Manufacturer Part Number: LM25005_06
• Description: 42V, 2.5A Step-Down Switching Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
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 cur-
rents 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 connec-
tions (C
Connect the AGND and PGND pins together through the top-
side copper area covering the entire underside of the device.
Place several vias in this underside copper area to the ground
plane.
C
C
C
C
C
C
C
C
C
C
C
C
D
R
R
R
R
R
R
R
U
L
ITEM
SS
10
11
12
1
2
3
4
5
6
7
8
9
1
1
1
2
3
4
5
6
7
1
, R
T
, C
C4532X7R2A225M
C4532X7R2A225M
C0805C331G1GAC
C2012X7R2A103K
C2012X7R2A103K
OPEN
C2012X7R2A223K
C2012X7R1C474M
C3225X7R1C226M
EEFHE0J151R
C0805C331G1GAC
OPEN
CSHD6-60C
6CWQ10FN
DR127-330
OPEN
OPEN
CRCW08052102F
CRCW08054992F
CRCW08055111F
CRCW08051651F
CRCW2512100J
LM25005
RAMP
) directly to the regulator AGND pin.
PART NUMBER
TABLE 1. 5V, 2.5A Demo Board Bill of Materials
CAPACITOR, CER, TDK
CAPACITOR, CER, TDK
CAPACITOR, CER, KEMET
CAPACITOR, CER, TDK
CAPACITOR, CER, TDK
NOT USED
CAPACITOR, CER, TDK
CAPACITOR, CER, TDK
CAPACITOR, CER, TDK
CAPACITOR, SP, PANASONIC
CAPACITOR, CER, KEMET
NOT USED
DIODE, 60V, CENTRAL
DIODE, 100V, IR (D1-ALT)
INDUCTOR, COOPER
NOT USED
NOT USED
RESISTOR
RESISTOR
RESISTOR
RESISTOR
RESISTOR
REGULATOR, NATIONAL SEMICONDUCTOR
18
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 approx-
imation for the output inductor power is P = I
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 obser-
vation 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 tem-
perature. Selecting a diode with an exposed pad will aid the
power dissipation of the diode.
DESCRIPTION
0.022µ, 100V
0.01µ, 100V
0.01µ, 100V
330p, 100V
330p, 100V
2.2µ, 100V
2.2µ, 100V
0.47µ, 16V
150µ, 6.3V
22µ, 16V
VALUE
10, 1W
49.9K
5.11K
1.65K
33µH
21K
OUT
2
x R x 1.1,