Texas Instruments | AN-1747 3A LM20k Reference Designs (Rev. A) | Application notes | Texas Instruments AN-1747 3A LM20k Reference Designs (Rev. A) Application notes

Texas Instruments AN-1747 3A LM20k Reference Designs (Rev. A) Application notes
Application Report
SNVA297A – November 2007 – Revised May 2013
AN-1747 3A LM20k Reference Designs
Ron Lerner
1
.....................................................................................................................................
Introduction
The LM20123, LM20133, and LM20143 are a full-featured family of high performance 3A synchronous
buck converters. These devices are tailored to operate over an input voltage range of 2.95 V to 5.5 V and
each can be optimized to meet many different performance requirements. The LM20123 operates at a
fixed frequency and only requires 11 components to generate a solution. The LM20143 is similar to the
LM20123 except the frequency of the device can be varied from 460 kHz to 1.5 MHz with an external
resistor. This gives the power supply designer the flexibility to trade-off inductor size and efficiency. The
LM20133 features a synchronization input pin that synchronizes the internal oscillator to an external signal
to keep the switching regulators operating with the same phase, which is critical in many noise sensitive
designs.
The reference designs discussed show how the 3A devices can be optimized for size, efficiency and
transient response. The trade-offs made for each design discuss the various 3A devices. Test results for
including efficiency, output voltage ripple, and transient response are shown for each design.
2
Solution Optimized for Efficiency
The LM20143 was selected for a high-efficiency design because the operating frequency can be adjusted
to minimize switching losses. A low switching frequency minimizes the switching losses; however, this also
requires a solution with a higher value inductance with a higher DC series resistance (DCR). Since the
LM20k family of devices feature very low switching losses, a switching frequency of 620 kHz was
selected. This choice of operating frequency balances the inductance DCR losses with the switching
frequency losses to give a small, highly efficient solution. This design can also work with the LM20133
synchronizing to 620 kHz.
2.1
Inductor Selection
As per the data sheet recommendations, the inductor value should initially be chosen to give a peak-topeak ripple current equal to roughly 30% of the maximum output current. The peak-to-peak inductor ripple
current can be calculated by Equation 1:
'IP-P =
(VIN - VOUT) x D
L x fSW
(1)
Rearranging Equation 1 and solving for the inductance reveals that for this application (VIN = 5 V, VOUT =
3.3 V, D = VIN/VOUT = .66, fSW = 620 kHz, and IOUT = 3A) the nominal inductance value is roughly 2 µH.
Once an inductance value is calculated, an actual inductor needs to be selected based on a trade-off
between physical size, efficiency, and current carrying capability. The purpose of this design is to
maximize the efficiency therefore, the choice of small value inductors with lower series resistance (DCR)
can be examined provided the output current plus one-half the peak-to-peak ripple current does not
exceed the device current limit. Examining several inductor vendors a TDK SPM6530T-1R5M100 inductor
was selected. This 1.5 µH inductor results in a peak-to-peak ripple current of 1.2A when the converter is
operating from 5 V and 3.3 V. For the a design targeting high efficiency the TDK SPM6530T-1R5M100
inductor offers a good balance between efficiency (9.7 mΩ DCR), size, and saturation current rating (10A
ISAT rating).
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Solution Optimized for Efficiency
2.2
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Output Capacitor Selection
The value of the output capacitor in a buck regulator influences the voltage ripple that is present on the
output voltage, as well as the large signal output voltage response to a load transient. Given the peak-topeak inductor current ripple (that can be calculated using Equation 1) the output voltage ripple can be
approximated by Equation 2:
'VOUT = 'IP-P x RESR +
1
8 x fSW x COUT
(2)
The variable RESR above refers to the ESR of the output capacitor. As can be seen in Equation 2, 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 this design, a TDK 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 at the 1.5 MHz switching frequency is roughly 2 mΩ, and the effective in-circuit
capacitance is approximately 45 µF (reduced from 100 µF due to the 3.3 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.
2.3
Compensation Selection
The compensation network was selected using the excel calculator to give a crossover frequency of 50
kHz. For this target crossover frequency, operating conditions, and filter components the excel design tool
(available online) suggests a value of CC1 of 2.7 nF and a value of RC1 of 9.31 kΩ.
The final schematic for a 5 V or 3.3 V conversion optimized for efficiency is shown in Figure 1.
L
LM20143
VOUT
SW
PVIN
VIN
U1
CIN
RF
EN
RFB1
FB
COUT
AVIN
CF
RT
COMP
RC1
CC1
RFB2
PGOOD
RT
VCC
SS/TRK
PGND AGND
CVCC
CSS
(optional)
Figure 1. 3.3 V Output Solution Optimized for Efficiency
2
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Solution Optimized for Efficiency
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Table 1. High Efficiency Bill of Materials (BOM) (VIN = 5 V, VOUT = 3.3 V, IOUT(MAX) = 3A)
Qty
Designator
Description
Part Number
Manufacturer
1
U1
1
CIN
Synchronous Buck Regulator
LM20143
Texas Instruments
47 µF, 1206, X5R, 6.3 V
C3216JB0J476M
1
TDK
COUT
100 µF, 1210, X5R, 6.3 V
C3225X5R0J107M
TDK
1
L
1.5 µH, 9.7 mΩ
SPM6530T-1R5M100
TDK
1
RF
1 Ω, 0603
CRCW06031R0J-e3
Vishay-Dale
1
CF
1 µF, 0603, X7R, 10 V
GRM188R71A105KA01
Murata
1
CVCC
1 µF, 0603, X7R, 10 V
GRM188R71A105KA01
Murata
1
RC1
9.31 kΩ,0603
CRCW06039312F
Vishay-Dale
1
CC1
2.7 nF,0603,X7R,25 V
VJ0603Y272KXA
Vishay-Vitramon
1
CSS
33 nF,0603,X7R,25 V
VJ0603Y333KXXA
Vishay-Vitramon
1
RFB1
30.9 kΩ,0603
CRCW06033092F
Vishay-Dale
1
RFB2
10 kΩ, 0603
CRCW06031002F-e3
Vishay-Dale
1
RT
196 kΩ, 0603
CRCW06031963F
Vishay-Dale
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Optimizing a Solution for Size
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The calculated component PCB area for this design is 104 mm2. The efficiency vs. IOUT, output voltage
ripple and transient response are shown in Figure 2 through Figure 4.
Figure 2. Efficiency vs IOUT
Figure 3. Output Voltage Ripple
Figure 4. Transient Response
3
Optimizing a Solution for Size
For the smallest possible solution size, the fixed frequency LM20123 device was selected since the 1.5
MHz free running oscillator minimizes both the component count and inductor size. To minimize the
solution size, careful attention needs to be paid to the selection of the external components such as the
inductor, input, and output capacitors.
4
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Optimizing a Solution for Size
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3.1
Inductor Selection
The inductor should be sized for approximately 30% ripple current. For VIN = 5 V, VOUT = 1.2 V, fSW = 1.5
MHz, and IOUT = 3A, the ideal inductance value can be calculated from Equation 1 to be 0.68 µH. 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. Since the purpose of this design is minimize the
size, it is possible to select inductor values smaller than 0.68 µH, as long as the output current plus onehalf the peak-to-peak ripple current does not exceed the device current limit. After examining several
inductor vendors, a Coilcraft LPS4018-561 inductor was selected. This 0.56 µH inductor results in a peakto-peak ripple current of 1.09A and 909 mA when the converter is operating from 5 V and 3.3 V,
respectively. For a design where size is critical, the Coilcraft LPS4018-561 inductor offers an extremely
small size (3.9 mm x 3.9 mm) with relatively low DC losses (30 mΩ DCR), and saturation current rating
(5.2A ISAT rating) that exceeds the device current limit.
3.2
Output Capacitor Selection
A TDK 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 at
the 1.5 MHz switching frequency is roughly 3 mΩ and the effective in-circuit capacitance is approximately
28 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 6 mV.
3.3
Compensation Selection
The compensation network was selected using the excel calculator to give a crossover frequency of 119
kHz. For this target crossover frequency, operating conditions, and filter components selected above, the
excel calculator suggested a value of CC1 of 2.2 nF and a value of RC1 of 3.65 kΩ.
The final schematic for a 5 V to 1.2 V or 3.3 V to 1.2 V solution is shown in Figure 1.
L
LM20123
PVIN
VIN
CIN
VOUT
RFB1
EN
RF
U1
SW
FB
AVIN
COUT
RFB2
PGOOD
CF
COMP
RC1
CC1
VCC
SS/TRK
PGND AGND
CVCC
CSS
(optional)
Figure 5. 1.2 V Output Solution Optimized for Size
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Optimizing a Solution for Size
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Table 2. Bill of Materials (BOM) (Small solution size, VIN = 5 V, VOUT = 1.2 V, IOUT(MAX) = 3A)
6
Qty
Designator
Description
Part Number
Manufacturer
1
U1
Synchronous Buck
Regulator
LM20123
Texas Instruments
1
CIN
47 µF, 1206, X5R, 6.3 V
C3216JB0J476M
TDK
1
COUT
47 µF, 1206, X5R, 6.3 V
C3216JB0J476M
TDK
1
L
.56 µH, 30 mΩ
LPS4018-561MLC
Coilcraft
1
RF
1 Ω, 0402
CRCW04021R0J-e3
Vishay-Dale
1
CF
1 µF, 0402, X5R, 10 V
GRM155R61A105KE15
Murata
1
CVCC
1 µF, 0402, X5R, 10 V
GRM155R61A105KE15
Murata
1
RC1
3.65 kΩ, 0402
CRCW04023652F-e3
Vishay-Dale
1
CC1
2.2 nF,0402,X7R, 25 V
VJ0402Y222KXXA
Vishay-Vitramon
1
CSS
33 nF,0402,X5R, 25 V
VJ0402G333KXJA
Vishay-Vitramon
1
RFB1
4.99 kΩ,0402
CRCW04024992F-e3
Vishay-Dale
1
RFB2
10 kΩ, 0402
CRCW04021002F-e3
Vishay-Dale
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Solution Optimized for Transient Response
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The calculated component PCB area for this design is 64 mm2. The efficiency vs. IOUT, output voltage
ripple, and transient response are shown in Figure 6 through Figure 8.
Figure 6. Efficiency vs IOUT
Figure 7. Output Voltage Ripple
Figure 8. Transient Response
4
Solution Optimized for Transient Response
To optimize the transient response, the switching frequency should be as high as possible. A high
switching frequency allows the crossover frequency to maximized and the inductor size to be minimized. A
small inductor permits also permits the inductor current to quickly ramp during a load step change.
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Solution Optimized for Transient Response
4.1
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Inductor Selection
Again selecting the inductor to be sized for approximately 30% ripple current for VIN = 5 V, VOUT = 1.2 V,
fSW = 1.5 MHz, and IOUT = 3A, the ideal inductance value can be calculated from Equation 1 to be 0.68 µH.
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. Since the purpose of this design is to
select the smallest possible inductor values smaller than 0.68 µH can be examined as long as the output
current plus one-half the peak-to-peak ripple current does not exceed the device current limit. Examining
several inductor vendors a TDK SPM6530T-R47M170 inductor was selected. This 0.47 µH inductor
results in a peak-to-peak ripple current of 1.29A and 1.08A when the converter is operating from 5 V and
3.3 V, respectively. For optimization of the transient response, the TDK SPM6530T-R47M170 inductor
offers a good balance between efficiency (3.3 mΩ DCR), size, and saturation current rating (17A ISAT
rating).
4.2
Output Capacitor Selection
A Sanyo 470 µF POSCAP capacitor with 10 mΩ of series resistance (ESR) is selected for the output
capacitor to provide good transient and DC performance in a relatively small package. If desired, the
output voltage ripple can be further reduced by placing a 47 µF ceramic capacitor in parallel with the
Sanyo POSCAP.
4.3
Compensation Selection
The compensation network was selected using the excel design tool (available online) to give a crossover
frequency of 110 kHz. For this target crossover frequency, operating conditions, and filter components a
value for CC1 of 0.47 nF and a value for RC1 of 57.6 kΩ were selected. Since the output capacitor value
and ESR are large an additional capacitor CC2 is recommended. For this design, CC2 is 100 pF.
The final schematic for a 5 V to 1.2 V or 3.3 V to 1.2 V conversion is shown in Figure 1.
L
LM20143
CIN
EN
RF
VOUT
SW
PVIN
VIN
RFB1
U1
+
FB
COUT
AVIN
CF
RT
RT
COMP
RC1
CC2
CC1
RFB2
PGOOD
VCC
SS/TRK
PGND AGND
CVCC
CSS
(optional)
Figure 9. 1.2 V Output Solution Optimized for Load Transients
8
AN-1747 3A LM20k Reference Designs
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Solution Optimized for Transient Response
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Table 3. Transient Response Bill of Materials (BOM) (VIN = 5 V, VOUT = 3.3 V, IOUT(MAX) = 3A, fSW = 500
kHz)
Qty
Designator
Description
Part Number
Manufacturer
1
U1
Synchronous Buck
Regulator
LM20143
Texas Instruments
1
CIN
47 µF, 1206, X5R, 6.3 V
C3216JB0J476M
TDK
1
COUT
470 µF, D4D, 6.3 V
4TPD470M
Sanyo
1
L
0.47 µH,3.3 mΩ
SPM6530T-R47M170
TDK
1
RF
1 Ω, 0603
CRCW06031R0J-e3
Vishay-Dale
1
CF
1 µF, 0603, X7R, 10 V
GRM188R71A105KA01
Murata
1
CVCC
1 µF, 0603, X7R, 10 V
GRM188R71A105KA01
Murata
1
RC1
57.6 kΩ, 0603
CRCW06035762F-e3
Vishay-Dale
1
CC1
.47 nF,0603,X7R, 25 V
VJ0603Y471KXXA
Vishay-Vitramon
1
CC2
100 pF,0603, 50 V,COG
GRM1885C1H101JA01
Murata
1
CSS
33 nF,0603,X7R, 25 V
VJ0603Y333KXXA
Vishay-Vitramon
1
RFB1
4.99 kΩ,0603
CRCW06034992F-e3
Vishay-Dale
1
RFB2
10 kΩ, 0603
CRCW06031002F-e3
Vishay-Dale
1
RT
48.7 kΩ, 0603
CRCW06034872F-e3
Vishay-Dale
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Solution Optimized for Transient Response
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The calculated component PCB area for this design is 126 mm2. The efficiency vs. IOUT, output voltage
ripple, and transient response are shown in Figure 10 through Figure 12.
Figure 10. Efficiency vs IOUT
Figure 11. Output Voltage Ripple
Figure 12. Transient Response
10
AN-1747 3A LM20k Reference Designs
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