NSC LM2853-3.3EVAL, LM2853-1.2EVAL Datasheet
LM2853 Evaluation Board
Introduction
The LM2853 synchronous SIMPLE SWITCHER®buck regulator is a synchronous switching regulator capable of delivering up to 3A of current into a load. The LM2853 represents
the ultimate in ease of use, as internal type-3 compensation
minimizes the necessary external components and eases
the selection of those components. The LM2853 is capable
of accepting an input voltage between 3.0V and 5.5V and
delivering an output voltage that is factory programmable
from 0.8V to 3.3V in 100mV increments. The nominal switching frequency of the LM2853 is 550 kHz.
The LM2853 Evaluation Board was designed to accommodate three standard output voltage options (1.2V/1.8V/3.3V)
using the same layout and external components. Just five
external components are included on the board, and the
entire 3A power supply occupies a minimum amount of
space (1.2” by 0.82”) on a two layer PCB without sacrificing
efficiency or performance. The input voltage can be varied
over the entire operating range of the LM2853 (3.0V to 5.5V)
for testing purposes. Also, the board is designed to be stable
with all standard LM2853 voltage options, so if another
voltage option needs to be tested, the LM2853 IC can be
removed and replaced with the desired option.
Schematic
20201803
Bill Of Material
ID Part Number Type Size Parameters QTY Vendor
U1 LM2853MH-x.x/NOPB 3A Buck ETSSOP-14 x.xV 1 NSC
C
IN
GRM31CR60J476ME19 Capacitor 1206 47 µF 1 Murata
C
BYP
GRM21BR71C105KA01 Capacitor 0805 1 µF 1 Murata
C
SS
VJ0805Y222KXXA Capacitor 0603 2.2 nF 1 Vishay-Vitramon
L
O
DO3316P-472 Inductor DO3316P 4.7 µH 1 Coilcraft
C
O
TPSD227X06R0050 Capacitor D Case 220 µF
(50 mΩ)
1 Vishay-Sprague
National Semiconductor
Application Note 1513
Jeff Kissinger
October 2006
LM2853 Evaluation Board AN-1513
© 2006 National Semiconductor Corporation AN202018 www.national.com
Performance
Efficiency vs. I
LOAD(VIN
= 5V)
20201802
0.5A to 3A Load Step Response (VIN= 5V, V
OUT
= 1.8V)
20201805
Horizontal Resolution: 200 µs/Div.
Trace 1: V
OUT
(100 mV/Div.)
Trace 2: I
LOAD
(1 A /Div.)
Component Selection
CINand C
BYP
The necessary RMS current rating of the input capacitor can
be estimated by the following equation:
where the variable D refers to the duty cycle, and can be
approximated by:
From this equation, it follows that the maximum I
RMS
will
occur at a full 3A load current with the system operating at
50% duty cycle. Under this condition, the maximum I
RMS
is
given by:
Ceramic capacitors feature a very large I
RMS
rating in a small
footprint, making a ceramic capacitor ideal for this application. A 47 µF ceramic capacitor from Murata with a 4.9A I
RMS
rating provides the necessary input capacitance for the
evaluation board. For improved load regulation and transient
performance, an extra 1 µF ceramic capacitor is placed near
to the AVIN pin from V
IN
to GND. This small capacitor helps
to filter high frequency noise pulses on the supply, and
prevent those pulses from disturbing the analog control circuitry of the chip.
C
SS
The soft-start capacitor has been chosen to provide a softstart time of roughly 3 ms. Using the internal soft-start resistance of 450 kΩ and the external soft-start capacitor value of
2.2 nF, the approximate soft-start time can be calculated as
follows:
T
SS
=3xCSSxRSS=
3 x 2.2 nF x 450 kΩ = 2.97 ms ≈ 3ms
A 3 ms soft-start time will allow the LM2853 to start up
gracefully without triggering over-current protection, regardless of the operating conditions.
AN-1513
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