AN2559
Application note
System power supply board for digital solutions
Introduction
This document describes a power supply reference board designed for powering digital
applications, such as CPUs, FPGAs, memories, etc. The main purpose of the board is to
illustrate the basic principles used for the design of the power supply and to give designers a
usable prototype for testing and use.
The trend in recent years in the supplying of power to MCUs, CPUs, memories, FPGAs, etc.
is to reduce the supply voltage, increase the supply current and provide different voltage
levels for different devices in one platform. A typical example of this situation is the FPGA.
The FPGA contains a core part which works at a low level voltage, the interface part placed
between the core and the output, the system part, etc. It is important to note that each
FPGA family has a slightly different voltage level and the trend is to decrease the voltage for
each new family. The lowest operating voltage currently available is 1 V, and this can be
expected to decrease to 0.9 V or 0.8 V in the near future. A similar situation exists with other
digital applications. Typically, the main CPU, memory and interfaces require different supply
voltage levels. Low operating voltages also present another challenge - transient. Digital
devices are typically sensitive to voltage level. If the voltage drops below or crosses over a
specific limit, the device is reset. This limit is typically ± 3 or ± 5%. On the other hand, digital
device consumption can change very quickly (several amps in a few hundred nanoseconds).
A power supply must be able to react very quickly with a minimum of over (or under) voltage,
especially in cases where very low output voltage is required. There is additional stress
placed on power supplies for digital applications in the industrial environment.
The industrial standard bus is 24 V, but this voltage fluctuates and the maximum input
voltage level required can reach 36 V. Additional surge protection is also a mandatory part of
power supply input for industrial applications.
The goal of the board described in this application note is to cover all of the issues outlined
above. It is intended mainly to satisfy industrial input requirements (operating voltages up to
36 V) and generate several output voltages for mid-range power applications (up to several
amps). The main output voltage level can simply be set.
September 2007
Rev 1
1/35
www.st.com
Contents
AN2559
Contents
1
2
Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
Input part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
PM6680A block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.0.1
3.0.2
3.0.3
Power management block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Start-up/enable block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Step-down parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1
3.2
DC-DC converters based on the L5970AD . . . . . . . . . . . . . . . . . . . . . . . 14
Reset circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4
5
6
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1
PM6680A block - measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.1
6.1.2
6.1.3
6.1.4
Efficiency and light load consumption modes . . . . . . . . . . . . . . . . . . . . 23
Output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Start-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.2
L5970AD blocks - measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.2.1
6.2.2
6.2.3
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
7
8
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2/35
AN2559
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
The STEVAL-PSQ001V1 demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Block diagram of System Supply board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Schematic of input part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Location and correct polarity of the input supply connector on the board . . . . . . . . . . . . . . 6
Electrical diagram of the PM6680A section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
The placement of the jumpers for start-up/enable settings. . . . . . . . . . . . . . . . . . . . . . . . . . 9
Skip mode connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Jumper placement for V
CORE
voltage level setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Jumper placement for V
I/O
voltage level setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output voltages of L5970A parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Schematic of the two SMPS’s based on the L5970AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Jumper placement for enable/disable function of analog output and output3 . . . . . . . . . . 16
Schematic of the reset circuit and board placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
PCB top layer layout and first internal layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
PCB second internal layer and bottom layer layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Efficiency of the dual step-down converter at full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
PM6680A consumption at no load condition, in the different modes . . . . . . . . . . . . . . . . . 24
Output voltage ripple in different modes of light load operation . . . . . . . . . . . . . . . . . . . . . 24
Output voltage ripple of V
CORE
at the minimum input voltage (5 V) . . . . . . . . . . . . . . . . . . 25
Output voltage ripple of V
CORE
at the maximum output voltage (36 V) . . . . . . . . . . . . . . . 25
Output voltage ripple of V
I/O
at the minimum input voltage (5 V) . . . . . . . . . . . . . . . . . . . . 25
Output voltage ripple of V
I/O
at the maximum input voltage (36 V). . . . . . . . . . . . . . . . . . . 26
Start-up without setting the sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Start-up with a set sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Load transient response on V
CORE
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Load transient response on V
I/O
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Efficiency of output 3, by input voltage level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Efficiency of analog output, by input voltage level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Analog 5 V - output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
V
SYS
- output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Analog 3.3 V - output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
V
AUX
2.5 V - output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Transient response of V
SYS
based on the L5970AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Transient response of V
AUX
generated by the LDO KF25 . . . . . . . . . . . . . . . . . . . . . . . . . 33
3/35
Main characteristics
AN2559
1
Main characteristics
The main characteristics of the SMPS are listed below:
●
●
Input: 5 V - 36 V DC, surge protection
Outputs: the performance of the 6 outputs are described in
Table 1
below.
Output voltages (positive version)
V
OUT
Selectable from:
0.9, 1.0, 1.2, 1.5, 1.8 or 2.5 V
Selectable from:
1.0, 1.2, 1.5, 1.8, 2.5 V or 3.3 V
3.3 V
2.5 V
5V
3.3 V
I
OUT
max
4 A continuous
6 A peak
2 A continuous
3 A peak
0.4 A (0.8 A peak)
0.4 A
0.8 A
0.15 A
Tolerance
3%
3%
4%
2%
4%
2%
Table 1.
Label
Output1 (V
CORE
)
Output2 (V
I/O
)
Output3 V
SYS
Output3 V
AUX
Analog 5 V
Analog 3.3 V
4/35
AN2559
Description
2
Description
The System Supply board described in this application note is a dedicated design which
illustrates a typical solution for complete system supply, and can also be used as a direct
supply for customer solutions during the design process.
Figure 1.
The STEVAL-PSQ001V1 demo board
The block diagram of the System Supply board is shown in
Figure 2.
There are four DC-DC
converters, two linear regulators and a reset circuit. These parts are split into five relatively
independent units: the input part, a dual DC-DC converter based on the PM6680A and
generating 2 outputs (Output 1 and Output 2), two single DC-DC converters based on the
L5970A (Output 3 and Output 4) with linear regulator, and the reset circuit.
Figure 2.
Block diagram of System Supply board
STM6719
Reset signal
Input
5 - 36 V
Input
protection
E/D analog
L5970AD
Analog
5 V analog 500 mA
3.3 V analog 150 mA
Output 3
Vsys 3.3 V 400 mA
Vaux 2.5 V 400 mA
Output 2
Vi/o 1.0 - 3.3 V 2 A
Vi/o voltage settings
Output 1
Vcore 0.9 - 2.5 V 4 A
Vcore voltage settings
AI12693
LK112
M33
L5970AD
Skip mode
settings
KF25
E/D Vsys + Vaux
FB Vi/o
Vcore
Vi/o
PM6680A
E/D + start up
sequence settings
FB Vcore
5/35
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