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 (RFB1 or RFB2). 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 VIN.
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:
ICIN-(RMS) = ILOAD D(1 - D)
(1)
The variable D refers to the duty cycle, and can be approximated by:
D=
VOUT
VIN
(2)
From this equation, it follows that the maximum ICIN(RMS) requirement will occur at a full 4A load current with
the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by:
ICIN-(RMS) = 4A 0.5 x 0.5 = 2.0A
(3)
Ceramic capacitors feature a very large IRMS 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 IRMS 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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AN-1654 LM20124 Evaluation Board
1
Component Selection
2.2
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AVIN Filter
An RC filter should be added to prevent any switching noise on PVIN from interfering with the internal
analog circuitry connected to AVIN. These can be seen on the schematic as components RF and CF.
There is a practical limit to the size of the resistor RF as the AVIN pin will draw a short 60mA burst of
current during startup, and if RF is too large the resulting voltage drop can trigger the UVLO comparator.
For the demo board a 1Ω resistor is used for RF ensuring that UVLO will not be triggered after the part is
enabled. A recommended 1 µF CF capacitor coupled with the 1Ω resistor provides roughly 16dB of
attenuation at the 1 MHz switching frequency.
2.3
Inductor
As per the datasheet recommendations, the inductor value should initially be chosen to give a peak to
peak ripple current equal to roughly 30% of the maximum output current. The peak to peak inductor ripple
current can be calculated by the equation:
'Ip-p =
(VIN - VOUT) x D
L x fSW
(4)
Rearranging this equation and solving for the inductance reveals that for this application (VIN = 5V, VOUT =
1.2V, fSW = 1 MHz, and IOUT = 4A) the nominal inductance value is roughly 0.76 µH. Rounding up to the
nearest standard inductor value, a final inductance of 1 µH is selected. This results in a peak-to-peak
ripple current of 912 mA and 1.122A when the converter is operating from 5V and 3.3V respectively. Once
an inductance value is calculated, an actual inductor needs to be selected based on a tradeoff between
physical size, efficiency, and current carrying capability. For the LM20124 evaluation board, a Coilcraft
MSS1038-102NL inductor offers a good balance between efficiency (6 mΩ DCR), size, and saturation
current rating (9A ISAT rating).
2.4
Output Capacitor
The value of the output capacitor in a buck regulator influences the voltage ripple that will be present on
the output voltage, as well as the large signal output voltage response to a load transient. Given the peakto-peak inductor current ripple (ΔIP-P) the output voltage ripple can be approximated by the equation:
'VOUT = 'Ip-p x RESR +
1
8 x fSW x COUT
(5)
The variable RESR above refers to the ESR of the output capacitor. As can be seen in the above equation,
the ripple voltage on the output can be divided into two parts, one of which is attributed to the AC ripple
current flowing through the ESR of the output capacitor and another due to the AC ripple current actually
charging and discharging the output capacitor. The output capacitor also has an effect on the amount of
droop that is seen on the output voltage in response to a load transient event.
For the evaluation board, a Murata 100 µF ceramic capacitor is selected for the output capacitor to
provide good transient and DC performance in a relatively small package. From the technical
specifications of this capacitor, the ESR is roughly 2 mΩ, and the effective in-circuit capacitance is
approximately 55 µF (reduced from 100 µF due to the 1.2V DC bias). With these values, the peak to peak
voltage ripple on the output when operating from a 5V input can be calculated to be 3.9 mV.
2.5
CSS
A soft-start capacitor can be used to control the startup time of the LM20124 voltage regulator. The startup
time of the regulator when using a soft-start capacitor can be estimated by the following equation:
tSS =
CSS x 0.8
ISS
(6)
For the LM20124, ISS is nominally 5 µA. For the evaluation board, the soft-start time has been designed to
be roughly 5 ms, resulting in a CSS capacitor value of 33 nF.
2
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Component Selection
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2.6
CVCC
The CVCC capacitor is necessary to bypass an internal 2.7V subregulator. This capacitor should be sized
equal to or greater than 1 µF, but less than 10 µF. A value of 1 µF is sufficient for most applications..
2.7
CC1
The capacitor CC1 is used to set the crossover frequency of the LM20124 control loop. Since this board
was optimized to work well over the full input and output voltage range, the value of CC1 was selected to
be 3.3 nF. Once the operating conditions for the device are known, the transient response can be
optimized by reducing the value of CC1 and calculating the value for RC1 as outlined in the next section.
2.8
RC1
Once the value of CC1 is known, resistor RC1 is used to place a zero in the control loop to cancel the output
filter pole. This resistor can be sized according to the equation:
RC1 =
18 x D
CC1
IOUT
1-D
x
+
+
VIN
COUT
VOUT fSW x L
-1
(7)
For stability purposes the device should be compensated for the maximum output current expected in the
application.
2.9
CC2
A second compensation capacitor CC2 can be used in some designs to provide a high frequency pole,
useful for cancelling a possible zero introduced by the ESR of the output capacitor. For the LM20124
evaluation board, the CC2 footprint is unpopulated, as the low ESR ceramic capacitor used on the output
does not contribute a zero to the control loop before the crossover frequency. If the ceramic capacitor on
the evaluation board is replaced with a different capacitor having significant ESR, the required value of the
capacitor CC2 can be estimated by the equation:
CC2 =
COUT x RESR
RC1
(8)
2.10 RFB1 and RFB2
The resistors labeled RFB1 and RFB2 create a voltage divider from VOUT to the feedback pin that is used to
set the output of the voltage regulator. Nominally, the output of the LM20124 evaluation board is set to
1.2V, giving resistor values of RFB1 = 4.99 kΩ and RFB2 = 10 kΩ. If a different output voltage is required, the
value of RFB1 can be adjusted according to the equation:
RFB1 = RFB2 x
VOUT
0.8V
-1
(9)
RFB2 does not need to be changed from its value of 10 kΩ.
SNVA252B – October 2007 – Revised May 2013
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AN-1654 LM20124 Evaluation Board
3
Evaluation Board Schematic
3
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Evaluation Board Schematic
LM20124
PGOOD
L
PGOOD
VIN
CIN
RPG
SW
RFB1
PVIN
CBYP
VOUT
FB
EN
RF
COUT
RFB2
EN
AVIN
CF
COMP
VCC
RC1
PGND AGND
CC2
CVCC
SS/TRK
CSS
CC1
Figure 1. Evaluation Board Schematic
4
4
Bill of Materials
Designator
Description
Part Number
Manufacturer
Qty
U1
Synchronous Buck Regulator
LM20124
Texas Instruments
1
CIN
100 µF, 1210, X5R, 6.3V
GRM32ER60J107ME20
Murata
1
CBYP
1 µF, 0603, X5R, 6.3V
GRM188R60J105KA01
Murata
1
COUT
100 µF, 1210, X5R, 6.3V
GRM32ER60J107ME20
Murata
1
L
1 µH, 6 mΩ
MSS1038-102NL
Coilcraft
1
RF
1Ω, 0603
CRCW06031R0J-e3
Vishay-Dale
1
CF
100 nF, 0603, X7R, 16V
GRM188R71C104KA01
Murata
1
CVCC
1 µF, 0603, X5R, 6.3V
GRM188R60J105KA01
Murata
1
RPG
10 kΩ, 0603
CRCW06031002F-e3
Vishay-Dale
1
RC1
3.9 kΩ, 0603
CRCW06033901F-e3
Vishay-Dale
1
CC1
3.3 nF, 0603, X7R, 25V
VJ0603Y332KXXA
Vishay-Vitramon
1
CC2
OPEN
OPEN
N/A
0
CSS
33nF, 0603, X7R, 25V
VJ0603Y333KXXA
Vishay-Vitramon
1
RFB1
4.99 kΩ, 0603
CRCW06034991F-e3
Vishay-Dale
1
RFB2
10 kΩ, 0603
CRCW06031002F-e3
Vishay-Dale
1
Test Points
Test Points
160-1026-02-01-00
Cambion
7
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Connection Descriptions
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5
Connection Descriptions
Terminal Silkscreen
Description
VIN
This terminal is the input voltage to the device. The device will operation over the input voltage
range of 2.95V to 5.5V. The absolute maximum voltage rating for this pin is 6V.
GND
This terminal is the ground connection to the device. There are two different GND connections on
the PCB. One should be used for the input supply and the other for the load.
VOUT
This terminal connects to the output voltage of the power supply and should be connected to the
load.
EN
This terminal connects to the enable pin of the device. This terminal should be connected to VIN or
driven externally. If driven externally, a voltage typically greater than 1.18V will enable the device.
The operating voltage for this pin should not exceed 5.5V. The absolute maximum voltage rating
on this pin is 6V.
SS/TRACK
This terminal provides access to the SS/TRK pin of the device. Connections to this terminal are
not needed for most applications. The feedback pin of the device will track the voltage on the
SS/TRK pin if it is driven with an external voltage source that is below the 0.8V reference. The
voltage on this pin should not exceed 5.5V during normal operation. The absolute maximum
voltage rating on this pin is 6V.
PGOOD
This terminal connects to the power good output of the device. There is a 10 kΩ pull-up resistor
from this pin to the input voltage. The voltage on this pin should not exceed 5.5V during normal
operation and has an absolute maximum voltage rating of 6V.
SNVA252B – October 2007 – Revised May 2013
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AN-1654 LM20124 Evaluation Board
5
Performance Characteristics
6
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Performance Characteristics
Efficiency vs Load
Line Regulation (ILOAD = 4A)
Load Regulation (VIN = 5V)
0.5A to 4A Load Transient Response
(200 µs/DIV)
Startup Waveform
(2ms/DIV)
6
AN-1654 LM20124 Evaluation Board
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PCB Layout
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7
PCB Layout
Figure 2. Top Layer
Figure 3. Bottom Layer
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AN-1654 LM20124 Evaluation Board
7
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