Texas Instruments LM20124, LM20133 User Manual

I

CIN-(RMS)

= 4A

0.5 x 0.5

= 2.0A

D =

V

OUT

V

IN

I

CIN-(RMS)

= I

LOAD

D(1 - D)

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
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.

User's Guide

SNVA252B–October 2007–Revised May 2013

AN-1654 LM20124 Evaluation Board

FB1

or R

). The control

FB2

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:

The variable D refers to the duty cycle, and can be approximated by:

From this equation, it follows that the maximum I
the system operating at 50% duty cycle. Under this condition, the maximum I

Ceramic capacitors feature a very large I
for this application. A 100 µF X5R ceramic capacitor from Murata with a 5.4A I
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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CIN(RMS)

rating in a small footprint, making a ceramic capacitor ideal

RMS

requirement will occur at a full 4A load current with

CIN(RMS)

is given by:

rating provides the

RMS

(1)

(2)

(3)

SNVA252B–October 2007–Revised May 2013 AN-1654 LM20124 Evaluation Board

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tSS =

CSS x 0.8

I

SS

'V

OUT

= 'I

p-p

x

R

ESR

+

1

8 x fSW x C

OUT

'I

p-p

=

(VIN - V

OUT

) x D

L x f

SW

Component Selection

2.2 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 RFand CF.
There is a practical limit to the size of the resistor RFas the AVIN pin will draw a short 60mA burst of
current during startup, and if RFis too large the resulting voltage drop can trigger the UVLO comparator.
For the demo board a 1Ω resistor is used for RFensuring that UVLO will not be triggered after the part is
enabled. A recommended 1 µF CFcapacitor 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:

Rearranging this equation and solving for the inductance reveals that for this application (VIN= 5V, V

1.2V, fSW= 1 MHz, and I
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 I

SAT

= 4A) the nominal inductance value is roughly 0.76 µH. Rounding up to the

OUT

rating).

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=

OUT

(4)

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 (ΔI

2.5 C

The variable R
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.

SS

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:

For the LM20124, ISSis nominally 5 µA. For the evaluation board, the soft-start time has been designed to
be roughly 5 ms, resulting in a CSScapacitor value of 33 nF.

above refers to the ESR of the output capacitor. As can be seen in the above equation,

ESR

) the output voltage ripple can be approximated by the equation:

P-P

(5)

(6)

2

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R

FB1

= R

FB2

x

-1

V

OUT

0.8V

CC2 =

C

OUT

x R

ESR

R

C1

x

C

C1

C

OUT

I

OUT

V

OUT

+

18 x D

V

IN

-1

+

1-D

f

SW

x L

R

C1

=

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Component Selection

2.6 C

2.7 C

2.8 R

2.9 C

VCC

The C

capacitor is necessary to bypass an internal 2.7V subregulator. This capacitor should be sized

VCC

equal to or greater than 1 µF, but less than 10 µF. A value of 1 µF is sufficient for most applications..

C1

The capacitor CC1is 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 CC1was 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 CC1and calculating the value for RC1as outlined in the next section.

C1

Once the value of CC1is known, resistor RC1is used to place a zero in the control loop to cancel the output
filter pole. This resistor can be sized according to the equation:

(7)

For stability purposes the device should be compensated for the maximum output current expected in the
application.

C2

A second compensation capacitor CC2can 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 CC2footprint 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 CC2can be estimated by the equation:

2.10 R

The resistors labeled R
set the output of the voltage regulator. Nominally, the output of the LM20124 evaluation board is set to

1.2V, giving resistor values of R
value of R

R

and R

FB1

does not need to be changed from its value of 10 kΩ.

FB2

FB2

and R

FB1

can be adjusted according to the equation:

FB1

create a voltage divider from V

FB2

= 4.99 kΩ and R

FB1

to the feedback pin that is used to

OUT

= 10 kΩ. If a different output voltage is required, the

FB2

(8)

(9)

SNVA252B–October 2007–Revised May 2013 AN-1654 LM20124 Evaluation Board

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Copyright © 2007–2013, Texas Instruments Incorporated