LM2853-1.2EVAL National Semiconductor, LM2853-1.2EVAL Datasheet - Page 3

• Manufacturer Part Number: LM2853-1.2EVAL
• Description: BOARD EVAL LM2853-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r

Specifications of LM2853-1.2EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3A
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 550kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2853
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Component Selection
L
The selection of the output filter components L
intrinsically linked, as both of these parameters affect the
stability of the system, and various characteristics of the
output voltage. First, a 4.7 µH inductor is chosen to allow
stable operation over the entire input voltage range (as per
the datasheet recommendations) from 3.0V to 5.5V. The size
of the inductor also directly affects the amplitude of the
inductor current ripple. This amplitude can be calculated
from the following equation:
From this, it follows that the maximum inductor current ripple
using standard operating conditions of the LM2853 and a 4.7
µH inductor will occur at V
these conditions the inductor current ripple is given as:
This means an inductor must be selected with a saturation
current higher than 3.264A to ensure that the inductor will
never saturate during normal operating conditions. A Coil-
craft DO3316P, 4.7 µH inductor provides the necessary cur-
rent handling capabilities (I
footprint.
O
and C
O
Top Layer (not to scale)
IN
SAT
= 5.5V, and V
= 5.4A) in a relatively small
(Continued)
OUT
O
= 2.5V. Under
and C
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O
, are
3
The ESR of the output capacitor affects both the ripple
voltage at the output and the overall stability of the loop. In
order to keep the output voltage ripple manageable under all
operating conditions, an ESR value of 50 mΩ is selected. As
per the datasheet recommendations, a capacitance of 220
µF will guarantee stability regardless of V
coupled with 4.7 µH inductor and 50 mΩ ESR. An AVX
low-ESR 6.3V tantalum capacitor provides the necessary
ESR and capacitance to stabilize the loop and control the
output voltage ripple, with suitable voltage derating for up to
a 3.3V output.
PCB Layout
The PCB layout of the LM2853 demo board was designed to
occupy as little board space as possible, while still following
sound layout guidelines and techniques. The input capacitor,
C
PGND pins, to minimize stray resistance and inductance
between C
capacitor is placed as close as possible to the AVIN and
SGND pins. PGND and SGND are connected to each other
and the ground plane at a single point, the exposed pad of
the LM2853. Also, in order to help conduct heat to the
ground plane and away from the LM2853, an array of vias is
used to connect the exposed pad to the ground plane,
instead of a single via. Finally, the sense pin trace is inten-
tionally routed away from the SW node to minimize any EMI
pickup.
IN
is placed as close as possible to the PVIN pins and the
IN
and the LM2853. Likewise, the AVIN bypass
Bottom Layer (not to scale)
IN
and V
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OUT
when