Texas Instruments | Powering the TPS546D24A Device Family from a Single 3.3 V Input Power Supply | Application notes | Texas Instruments Powering the TPS546D24A Device Family from a Single 3.3 V Input Power Supply Application notes

Texas Instruments Powering the TPS546D24A Device Family from a Single 3.3 V Input Power Supply Application notes
Application Report
SLUAA03 – December 2019
Powering the TPS546D24A Device Family from a Single
3.3 V Input Power Supply
ABSTRACT
The 3.3 V, 5 V, and 12 V rails have been the most popular voltages for microelectronic circuits for
decades. Years ago, many components and integrated circuits used a 12 V or 5 V supply, and when the
process technology shrunk, the required voltage dropped to 3.3 V. Today, integrated circuits use a much
wider variety of voltages, and many are far below 1 V. Interestingly, the 12 V, 5 V and 3.3 V rails still
remain despite many process technology reductions. For example, most modern computer power supplies
follow the ATX (Advanced Technology Extended) motherboard convention and provide +3.3 V, +5 V, +12
V, and -12 V for the use of the circuit board. Many power supplies still provide these voltages and rely on
other DC/DC converters to support the ever-growing number of point-of-load voltages. There are
advantages and disadvantages to using a particular input voltage rail for point-of-load conversion, and
explaining these are beyond the scope of this document. However, this application note will explore
several techniques using an available 3.3-V rail when the internal circuitry of the DC/DC converter does
not support 3.3-V operation. The TPS546D24A, a 40-A step-down converter with PMBus™ and telemetry
will be used as an example, but the techniques can be useful for other DC/DC converters that provide
split-rail support.
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Contents
Split-Rail Support ...........................................................................................................
3.3-V Operation with a Discrete Charge-pump .........................................................................
3.3-V Operation with a Charge-pump Integrated Circuit ...............................................................
3.3-V Operation with a Boost Converter Power Module ...............................................................
Summary .....................................................................................................................
Resources ....................................................................................................................
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List of Figures
..........................................
................................................
140-mA, 5-V TPS60150 Charge-pump Circuit ..........................................................................
140-mA, 5-V TPS60150 Charge-pump Layout Example ..............................................................
TPS81256 550-mA, 5-V Power Module Circuit .........................................................................
TPS81256 550-mA, 5-V Power Module Layout Example .............................................................
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TPS546D24A Demonstrating Split-rail Support with AVIN and PVIN Pins
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Discrete Charge-pump Circuit Driven from TPS546D24A SYNC Pin
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List of Tables
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Comparison of 3.3-V input to 5-V Output Solutions ....................................................................
Discrete Components for Charge-pump Circuit
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Split-Rail Support
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Trademarks
PMBus is a trademark of SMIF, Inc.
All other trademarks are the property of their respective owners.
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Split-Rail Support
Interestingly, the term ‘split-rail’ has been used historically to describe a decorative fence made out of
timber logs, usually split lengthwise into rails. In power supply terms, it is used to describe multiple input
voltage pins. Figure 1 shows the TPS546D24A providing split-rail support with AVIN and PVIN pins. The
AVIN pin supplies power to the controller, and the PVIN pin supplies power to the power stage or power
MOSFETs. The TPS546D24A PVIN pin allows 2.95-V to 16-V input for conversion, and the AVIN pin
allows 2.95-V to 18-V input for operation, and 4-V to 18-V for switching. Note that the VDD5 pin is the
output of the 5-V internal low drop out (LDO) regulator, which powers the driver stage of the controller.
One advantage of split-rail operation, when using a higher PVIN voltage such as 12 V, is to over-drive the
5 V LDO regulator with an external 5 V supply to improve efficiency and reduce power dissipation. In this
case, the LDO power loss is avoided. Another advantage is the ability to use a 3.3 V rail for PVIN when
other rails are not available or to increase the duty cycle and switch at a higher frequency to reduce the
size of the output inductor and capacitors.
Figure 1. TPS546D24A Demonstrating Split-rail Support with AVIN and PVIN Pins
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3.3-V Operation with a Discrete Charge-pump
The required bias current of a DC/DC converter depends on its operating frequency and characteristics of
the power MOSFETs. The discrete charge-pump must be able to provide the required current. According
to Equation 1, the TPS546D24A draws approximately 132 mA from the AVIN supply while switching at
325 kHz. An external circuit shown in Figure 2 can be easily added to implement a discrete voltagedoubling charge-pump, driven by the SYNC pin of the TPS546D24A. When the 3.3-V input rail ramps, the
controller becomes active at 2.95 V, begins the configuration sequence, and loads the external resistor
2
Powering the TPS546D24A Device Family from a Single 3.3 V Input Power
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3.3-V Operation with a Charge-pump Integrated Circuit
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pin-strapping values. With the pin-strap option is selected for SYNC_OUT, the SYNC pin starts switching
while AVIN completes the ramp up to 3.3 V. The SYNC pin drives the base of the 0.22-µF capacitors 3.3
V at a 50% duty cycle, which charges up the 2.2-µF capacitor connected to AVIN, boosting the voltage
higher than 4 V. Then, the device begins switch-mode conversion. In other words, when AVIN is greater
than 2.95 V and the SYNC pin starts switching, the charge-pump boosts the AVIN pin above 4 V and the
TPS546D24A begins switching the power MOSFETs for voltage conversion. If the 3.3 V rail has a wide
tolerance or if there is any risk of overshoot on the input voltage rail, it is recommended to place a Zener
diode to clamp the voltage applied to the AVIN pin. The Diodes Incorporated SD103ATW-7-F is suggested
for the Schottky diodes, and integrates all three diodes in a small SOT-363 package. The routing of the
diodes in one package is very straight forward. The SI869DH is suggested for the dual MOSFET and is
available in the same SOT-363 package as the diode array, which allows a small compact solution. The
recommended 10-µF input capacitor is a 10-V X5R rating in a 0805 package. The flying capacitor
suggestion is a 220 nF with a 10-V rating in a 0402 package. Table 1 summarizes the bill of materials for
the discrete charge-pump circuit.
(1)
Figure 2. Discrete Charge-pump Circuit Driven from TPS546D24A SYNC Pin
Table 1. Discrete Components for Charge-pump Circuit
COMPONENT
PART/TYPE
PACKAGE
Dual MOSFET
SI869DH
SOT-363
SD103ATW-7-F
SOT-363
Input Capacitor
10 µF, 10 V, X5R
0805
Flying Capacitor
220 nF, 10 V
0402
Triple Schottky Diodes
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3.3-V Operation with a Charge-pump Integrated Circuit
A simple 6-pin charge-pump integrated circuit shown in Figure 3 can also be used, especially when the
SYNC pin function of the TPS546D24A is needed for another purpose. The 140-mA TPS60150 has
several advantages, such as its regulation accuracy, integrated protection, and small size while avoiding
the use of an inductor. Only four components are needed as shown in Figure 3. The circuit occupies about
15 in2 of board space. The IC is 2-mm × 2-mm, each capacitor is in a 0603 package, and several vias are
used that occupy a small space. The output voltage is fixed at 5 V to eliminate external resistors. The
TPS60150 starts up in less than 150 µs and does not interfere with the operation of the TPS546D24A
configuration sequence. Additionally, the charge-pump begins operation when the AVIN input is 2.7 V,
which is lower than the 2.95 V start-up voltage of the TPS546D24 AVIN pin.
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3.3-V Operation with a Boost Converter Power Module
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Figure 3. 140-mA, 5-V TPS60150 Charge-pump Circuit
Figure 4. 140-mA, 5-V TPS60150 Charge-pump Layout Example
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3.3-V Operation with a Boost Converter Power Module
If the TPS546D24 needs to operate at a higher frequency, an inductive boost solution can be required to
supply more current to AVIN. The TPS81256 is a 5-V output boost module as shown in Figure 5 and
integrates the inductor and input and output capacitors in a small 2.6-mm × 2.9-mm × 1-mm package.
Since the inductor is integrated within the package, the total solution is smaller than the discrete chargepump and the charge-pump integrated circuit. The current capability of the module is 550 mA with a 3.3-V
input and a 5-V output. According to Equation 1, the TPS81256 provides enough current to TPS546D24A
AVIN to the approximate 1.5-MHz support operating frequency. The start-up is in 400 µs from active
enable to turn on. The start-up voltage is 2.5 V, which does not interfere with the TPS546D24A
configuration sequence. Figure 6 shows the small size of the integrated power module and all of the
included components within the dotted line of Figure 5.
4
Powering the TPS546D24A Device Family from a Single 3.3 V Input Power
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Summary
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Figure 5. TPS81256 550-mA, 5-V Power Module Circuit
Figure 6. TPS81256 550-mA, 5-V Power Module Layout Example
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Summary
The 3.3-V rail has not gone away. Certain applications like optical modules or X-haul transport equipment
can use a 3.3-V rail and require a point-of-load power management solution for high-current processors.
In some cases, the 5-V and 12-V rails of an ATX power supply can be exhausted without remaining
current capacity. This document reviewed several solutions to provide a 5 V bias to the split-rail
TPS546D24A from a simple discrete solution to an integrated power module. The key trade-offs of each
solution are highlighted in Table 2.
Table 2. Comparison of 3.3-V input to 5-V Output Solutions
5 V SOURCE FROM 3.3 V
SOLUTION SIZE
SOLUTION COST AT 1 Ku
FSW SUPPORT FOR
TPS546D24A
Boost module – TPS81256
9 mm2
$1.00
1500 kHz
Charge-pump IC – TPS60150
15 mm2
$0.63
350 kHz
2
$0.40
1000 kHz
Discrete charge- pump
6
20 mm
Resources
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•
Texas Instruments, When and How to Supply an External Bias for Buck Controllers – Part 3 Blog
TPS60150 Product Page
TPS81256 Product Page
TPS546D24A Product Page
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