LM2853-3.3EVAL National Semiconductor, LM2853-3.3EVAL Datasheet - Page 2

• Manufacturer Part Number: LM2853-3.3EVAL
• Description: BOARD EVAL LM2853-3.3
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r

Specifications of LM2853-3.3EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• 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: -

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Performance
Component Selection
C
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
occur at a full 3A load current with the system operating at
50% duty cycle. Under this condition, the maximum I
given by:
IN
and C
BYP
0.5A to 3A Load Step Response (V
Horizontal Resolution: 200 µs/Div.
Efficiency vs. I
Trace 1: V
Trace 2: I
RMS
RMS
will
is
OUT
LOAD
2
LOAD
(100 mV/Div.)
(1 A /Div.)
Ceramic capacitors feature a very large I
footprint, making a ceramic capacitor ideal for this applica-
tion. A 47 µF ceramic capacitor from Murata with a 4.9A I
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
to filter high frequency noise pulses on the supply, and
prevent those pulses from disturbing the analog control cir-
cuitry of the chip.
C
The soft-start capacitor has been chosen to provide a soft-
start time of roughly 3 ms. Using the internal soft-start resis-
tance of 450 kΩ and the external soft-start capacitor value of
2.2 nF, the approximate soft-start time can be calculated as
follows:
A 3 ms soft-start time will allow the LM2853 to start up
gracefully without triggering over-current protection, regard-
less of the operating conditions.
SS
(V
IN
IN
= 5V, V
= 5V)
3 x 2.2 nF x 450 kΩ = 2.97 ms ≈ 3 ms
20201805
20201802
OUT
T
= 1.8V)
SS
IN
= 3 x C
to GND. This small capacitor helps
SS
x R
SS
RMS
=
rating in a small
RMS