I
CIN-(RMS)
= 4A 0.5 x 0.5 = 2.0A
D =
V
OUT
V
IN
I
CIN-(RMS)
= I
LOAD
D(1 - D)
User's Guide
SNVA252B – October 2007 – Revised May 2013
AN-1654 LM20124 Evaluation Board
1
Introduction
The Texas Instruments LM20124 is a full featured buck switching regulator capable of driving up to 4A of
load current. The nominal 1 MHz switching frequency of the LM20124 reduces the size of the power stage
components while still allowing for highly efficient operation. The LM20124 is capable of converting an
input voltage between 2.95V and 5.5V down to an output voltage as low as 0.8V. Fault protection features
include cycle-by-cycle current limit, output power good, and output over-voltage protection. The dual
function soft-start/tracking pin can be used to control the startup response of the LM20124, and the
precision enable pin can be used to easily sequence the LM20124 in applications with sequencing
requirements. The LM20124 is available in a 16-pin HTSSOP package with an exposed pad for enhanced
thermal performance.
The LM20124 evaluation board has been designed to balance overall solution size with the efficiency of
the regulator. The evaluation board measures just under 1.3” × 1.1” on a two layer PCB, with all
components placed on the top layer. The power stage and compensation components of the LM20124
evaluation board have been optimized for an input voltage of 5V, but for testing purposes, the input can be
varied across the entire operating range. The output voltage of the evaluation board is nominally 1.2V, but
this voltage can be easily changed by replacing one of the feedback resistors (R
FB1
or R
FB2
). The control
loop compensation of the LM20124 evaluation board has been designed to provide a stable solution over
the entire input and output voltage range with a reasonable transient response. The EN pin must be above
1.18V (typ) on the board to initiate switching. If the EN function is not necessary, the EN pin should be
externally tied to V
IN
.
2
Component Selection
This section provides a walk-through of the design process of the LM20124 evaluation board. Unless
otherwise indicated, all equations assume units of Amps (A) for current, Farads (F) for capacitance,
Henries (H) for inductance, and Volts (V) for voltages.
2.1
Input Capacitor
The required RMS current rating of the input capacitor for a buck regulator can be estimated by the
following equation:
(1)
The variable D refers to the duty cycle, and can be approximated by:
(2)
From this equation, it follows that the maximum I
CIN(RMS)
requirement will occur at a full 4A load current with
the system operating at 50% duty cycle. Under this condition, the maximum I
CIN(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 100 µF X5R ceramic capacitor from Murata with a 5.4A I
RMS
rating provides the
necessary input capacitance for the evaluation board. For improved bypassing, a small 1 µF high
frequency capacitor is placed in parallel with the 100 µF bulk capacitor to filter high frequency noise
pulses on the supply.
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1
SNVA252B – October 2007 – Revised May 2013
AN-1654 LM20124 Evaluation Board
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