Texas Instruments 3A 550 kHz Synchronous SIMPLE SWITCHER® Buck Regulator Evaluation Module LM2853-1.8EVAL/NOPB LM2853-1.8EVAL/NOPB Datenbogen
Produktcode
LM2853-1.8EVAL/NOPB
I
RMS
= 3A 0.5 x 0.5 = 1.5A
D =
V
IN
V
OUT
I
RMS
= I
LOAD
D(1-D)
Performance
4
Performance
Figure 1. Efficiency vs. I
LOAD
(V
IN
= 5V)
Figure 2. Horizontal Resolution: 200 µs/Div.
Trace 1: V
OUT
(100 mV/Div.)
Trace 2: I
LOAD
(1 A /Div.)
5
Component Selection
5.1
C
IN
and C
BYP
The necessary RMS current rating of the input capacitor can be estimated by the following equation:
(1)
where the variable D refers to the duty cycle, and can be approximated by:
(2)
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:
(3)
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
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.
control circuitry of the chip.
5.2
C
SS
The soft-start capacitor has been chosen to provide a soft-start time of roughly 3 ms. Using the internal
soft-start resistance of 450 k
soft-start resistance of 450 k
Ω
and the external soft-start capacitor value of 2.2 nF, the approximate soft-
start time can be calculated as follows:
T
SS
= 3 × C
SS
× R
SS
= 3 × 2.2 nF × 450 k
Ω
= 2.97 ms
≈
3 ms
(4)
A 3 ms soft-start time will allow the LM2853 to start up gracefully without triggering over-current protection,
regardless of the operating conditions.
regardless of the operating conditions.
2
AN-1513 LM2853 Evaluation Board
SNVA180A – October 2006 – Revised April 2013
Copyright © 2006–2013, Texas Instruments Incorporated