Texas Instruments LM20133 User manual

PVIN SW
AGND
FB
PGOOD
RFB1
RFB2
COUT
EN
CSS
SS/TRK
AVIN
CF
CC1
COMP
RC1
VIN
LM20133
L
RF
VCC CVCC
VOUT
PGND
PGOOD
RPG
EN
CC2
SYNC SYNC
CIN CBYP
User's Guide
SNVA274B–October 2007–Revised May 2013
AN-1688 LM20133 Evaluation Board
1 Introduction
The LM20133 is a full featured buck switching regulator capable of driving up to 3A of load current. This
device features a clock synchronization input that allows the switching frequency to be synchronized to an
external clock source. The flexibilty to synchronize the switching frequency from 500 kHz to 1.5 MHz
allows the size of the power stage components to be reduced while still allowing for high efficiency. The
LM20133 is capable of converting an input voltage between 2.95 V and 5.5 V down to an output voltage
as low as 0.8 V. 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 LM20133, and the precision enable pin can be used to easily sequence the LM20133 in
applications with sequencing requirements. The LM20133 is available in a 16-pin HTSSOP package with
an exposed pad for enhanced thermal performance.
The LM20133 evaluation board has been designed to balance overall solution size with the efficiency of
the regulator. The evaluation board measures just under 1.3” x 1.1” on a two layer PCB, with all
components placed on the top layer. The power stage and compensation components of the LM20133
evaluation board have been optimized for an input voltage of 5 V, but for testing purposes, the input can
be varied across the entire operating range. The output voltage of the evaluation board is nominally 1.2 V,
but this voltage can be easily changed by replacing one of the feedback resistors (RFB1 or RFB2). The
control loop compensation of the LM20133 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 1.18 V (typ) on the board to initiate switching. If the EN function is not necessary, the EN
pin should be externally tied to VIN.
Figure 1. Evaluation Board Schematic
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Bill of Materials www.ti.com
2 Bill of Materials
Designator Description Part Number Qty Manufacturer
U1 Synchronous Buck Regulator LM20133 1 Texas Instruments
CIN 47 µF, 1210, X5R, 6.3 V GRM32ER60J476ME20 1 Murata
CBYP 1 µF, 0603, X5R, 6.3 V GRM188R60J105KA01 1 Murata
COUT 47 µF, 1210, X5R, 6.3 V GRM32ER60J476ME20 1 Murata
L 2.5 µH, 10 mΩMSS1038-252NL 1 Coilcraft
RF1Ω, 0603 CRCW06031R0J-e3 1 Vishay-Dale
CF100 nF, 0603, X7R, 16 V GRM188R71C104KA01 1 Murata
CEXT 1 µF, 0603, X5R, 6.3 V GRM188R60J105KA01 1 Murata
RPG 10 kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale
RC1 1 kΩ, 0603 CRCW06031001F-e3 1 Vishay-Dale
CC1 5.6 nF, 0603, X7R, 25 V VJ0603Y562KXXA 1 Vishay-Vitramon
CC2 OPEN OPEN 0 N/A
CSS 33 nF, 0603, X7R, 25 V VJ0603Y333KXXA 1 Vishay-Vitramon
RFB1 4.99 kΩ, 0603 CRCW06034991F-e3 1 Vishay-Dale
RFB2 10 kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale
Test Points Test Points 160-1026-02-01-00 8 Cambion
3 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 2.95 V to 5.5 V. The absolute maximum voltage rating for this pin is 6 V.
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 1.18 V will enable the
device. The operating voltage for this pin should not exceed 5.5 V. The absolute maximum
voltage rating on this pin is 6 V.
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 0.8 V reference. The
voltage on this pin should not exceed 5.5 V during normal operation. The absolute maximum
voltage rating on this pin is 6 V.
PGOOD This terminal connects to the power good output of the device. There is a 10 kΩpull-up resistor
from this pin to the input voltage. The voltage on this pin should not exceed 5.5 V during
normal operation and has an absolute maximum voltage rating of 6 V.
SYNC This terminal connects to the SYNC pin of the device. If this pin is left open the switching
frequency will default to approximately 400kHz. The voltage on this pin should not exceed 5.5
V during normal operation and has an absolute maximum voltage rating of 6 V.
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www.ti.com Performance Characteristics
4 Performance Characteristics
Efficiency
vs
Load Line Regulation (ILOAD = 3A)
0.5A to 3A Load Transient Response
Load Regulation (VIN = 5 V) (200 µs/DIV)
Startup Waveform
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'VOUT = 'IP-P x 1
8 x fSW x COUT
RESR +
'IP-P = (VIN - VOUT) x D
L x fSW
ICIN(RMS) = 3A 0.5 x 0.5 = 1.5A
D = VOUT
VIN
ICIN(RMS) = IOUT D(1 - D)
Component Selection www.ti.com
5 Component Selection
This section provides a walk-through of the design process of the LM20133 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.
5.1 Input Capacitor
The required RMS current rating of the input capacitor for a buck regulator can be estimated by
Equation 1:
(1)
The variable D refers to the duty cycle, and can be approximated by:
(2)
From Equation 3, it follows that the maximum ICIN(RMS) requirement will occur at a full 3A load current with
the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by:
(3)
Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal
for this application. A 47 µF X5R ceramic capacitor from Murata 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 47 µF bulk capacitor to filter high frequency noise pulses on the supply.
5.2 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 1 Ωresistor is used for RFensuring that UVLO will not be triggered after the part is enabled.
A recommended 1 µF CFcapacitor coupled with the 1 Ωresistor provides roughly 16dB of attenuation at
the 1 MHz switching frequency.
5.3 Inductor
As per the device-specific data sheet 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 Equation 4:
(4)
Rearranging this equation and solving for the inductance reveals that for this application (VIN =5V,VOUT =
1.2 V, fSYNC = 500 kHz, and IOUT = 3A) the nominal inductance value is roughly 2.03 µH. Rounding up to
the nearest standard inductor value, a final inductance of 2.5 µH is selected. This results in a peak-to-
peak ripple current of 730 mA and 898 mA when the converter is operating from 5 V and 3.3 V,
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 LM20133 evaluation
board, a Coilcraft MSS1038-252NL inductor offers a good balance between efficiency (10 mΩDCR), size,
and saturation current rating (5.7A ISAT rating).
5.4 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 peak-
to-peak inductor current ripple (ΔIP-P) the output voltage ripple can be approximated by Equation 5:
(5)
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CC2 = COUT x RESR
RC1
x
CC1
COUT
IOUT
VOUT +15 x D
VIN
-1
+
1 - D
fSW x L
RC1 =
tSS = 0.8V x CSS
ISS
www.ti.com Component Selection
The variable RESR above refers to the ESR of the output capacitor. As can be seen in Equation 5, 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 47 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 3 mΩ, and the effective in-circuit capacitance is approximately 32 µF
(reduced from 47 µF due to the 1.2 V DC bias). With these values, the peak-to-peak voltage ripple on the
output when operating from a 5 V input can be calculated to be 8 mV.
5.5 CSS
A soft-start capacitor can be used to control the startup time of the LM20133 voltage regulator. The startup
time of the regulator when using a soft-start capacitor can be estimated by Equation 6:
(6)
For the LM20133, 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.
5.6 CVCC
The CVCC capacitor is necessary to bypass an internal 2.7 V 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..
5.7 CC1
The capacitor CC1 is used to set the crossover frequency of the LM20133 control loop. Since this board
was optimized to work well over the full input, output voltage, and frequency range, the value of CC1 was
selected to be 5.6 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.
5.8 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 Equation 7:
(7)
For stability purposes the device should be compensated for the maximum output current expected in the
application.
5.9 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 LM20133
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 Equation 8:
(8)
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RFB1 = - 1
VOUT
0.8 x RFB2
PCB Layout www.ti.com
5.10 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 LM20133 evaluation board is set to 1.2
V, giving resistor values of RFB1 = 4.99 kΩand RFB2 = 10 kΩ. If a different output voltage is required, the
value of RFB1 can be adjusted according to Equation 9:
(9)
RFB2 does not need to be changed from its value of 10 kΩ.
6 PCB Layout
Figure 2. Top Layer
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Figure 3. Bottom Layer
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