dm00108726

AN4449
Application note
Buck-boost converter using the STM32F334 Discovery kit
Introduction
This application note describes the buck-boost DC/DC converter included in the
STM32F334 Discovery kit (32F3348DISCOVERY), a low-cost and easy-to-use
development kit to quickly evaluate and start application development with microcontrollers
of the STM32F3 series.
The STM32F334xx ARM® Cortex®-M4 microcontrollers, combining high integration and
performance, have been designed for digital power conversion applications and this buckboost DC/DC converter illustrates how they can efficiently control an H-bridge topology
converter. This application note demonstrates how the STM32F334xx products meet the
needs of this function thanks to their embedded high-resolution timer (HRTIM) and the
settings flexibility adapted for such switching-based converter application.
This demonstration example needs an external power supply that will be connected
independently from the mini-B USB cable connected to the host PC.
The firmware associated to this example needs to be programmed into your STM32F334
Discovery kit prior to the demonstration.
Reference documents:
• UM1733: Getting started with STM32F334 Discovery kit;
• DB2343: Discovery kit for STM32F334 microcontrollers;
• UM1735: Discovery kit for STM32F3 series with STM32F334C8 MCU User Manual.
These documents are available on STMicroelectronics web site (http://www.st.com).
Before installing and using the product, please accept the Evaluation Product License
Agreement from http://www.st.com/epla. For more information on the STM32F334
Discovery board and for demonstration software, visit www.st.com/stm32f3discovery.
September 2014
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www.st.com
1
Contents
AN4449
Contents
1
2
Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
Required hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2
Hardware settings of the STM32F334 Discovery kit . . . . . . . . . . . . . . . . . 5
1.3
Application schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4
Application principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.2
Non-inverting buck/boost converter basics . . . . . . . . . . . . . . . . . . . . . . . 8
1.4.3
Sizing the inductor of the buck-boost converter . . . . . . . . . . . . . . . . . . . 14
1.4.4
Setting the STM32F334 High-resolution timer . . . . . . . . . . . . . . . . . . . . 14
1.4.5
Software overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4.6
PI software regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.4.7
Getting started with the application . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.1
STM32F334xx peripherals used by the application . . . . . . . . . . . . . . . . . 26
2.1.1
2.2
Setting #define constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Application flowcharts description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.1
Application “main.c” flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.2
Application “HRTIM1_TIMA_IRQHandler()” flowchart . . . . . . . . . . . . . . 29
3
DC/DC converter main electrical characteristics . . . . . . . . . . . . . . . . . 31
4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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AN4449
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
HRTIM output signals through operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Event settings through operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Collection table for buck mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Collection table for boost mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Collection table for mixed mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Main electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
DocID025970 Rev 1
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List of figures
AN4449
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.
4/34
Connecting the external power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Overview of STM32F334 Discovery kit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
STM32F334 Discovery Buck/Boost converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Step-down operation (buck mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Step-down operation signals (buck mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Step-up operation (boost mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Step-up operation signals (boost mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Buck-boost operation (mixed mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Buck-boost cascaded transfer functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Operating modes according to VIN level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Buck-boost operation signals (mixed mode). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
ADC trigger event configuration in buck and boost modes. . . . . . . . . . . . . . . . . . . . . . . . . 16
DC/DC converter characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Duty cycle value vs. VIN for a given VOUT in buck mode . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Duty cycle value vs. VIN for a given VOUT in boost mode . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Duty cycle value vs. VIN for a given VOUT in mixed mode . . . . . . . . . . . . . . . . . . . . . . . . . 20
PI software controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Signal LEDs during standard operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Signal LEDs during limited operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Signal LEDs during fault condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
“main.c” flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
“HRTIM1_TIMA_IRQHandler()” flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
DocID025970 Rev 1
AN4449
Application description
1
Application description
1.1
Required hardware
This application uses STM32F334 Discovery kit on-board buck-boost DC/DC converter and
4 signal LEDs (LD3 to LD6). An external DC power supply (0-15 VDC 1 A max.) is required
for the demonstration.
The external power supply will be connected to CN5 (VIN) connector, while the VOUT signal
will be present on CN7.
The user will manage externally the current limitation to approximately 500 mA even if an
internal protection against overload is included in this firmware demonstration.
Caution:
Please comply with VIN polarity when connecting the power supply as well for VOUT when
connecting an external load or metering tools, as shown in Figure 1.
Figure 1. Connecting the external power supply
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Note:
This example and its hardware are totally independent from high brightness LED features
described in other documents.
1.2
Hardware settings of the STM32F334 Discovery kit
The SBx solder bridges should be set in their initial factory configuration. The user will
possibly connect an external current probe on CN6 pads to observe the inductor (L3)
current during operation. In that case, SB22 must be removed on the board bottom side.
Refer to STM32F334 Discovery kit user manual (UM1735) for further information related to
hardware.
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Application description
1.3
AN4449
Application schematics
Figure 2 shows the description of STM32F334 Discovery kit hardware.
Figure 2. Overview of STM32F334 Discovery kit
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1.4
Application principles
1.4.1
Overview
The STM32F334 Discovery embeds a buck-boost DC/DC converter that allows efficient
conversion of a DC voltage from one level to another.
The user can apply an input DC voltage from a range of 3 to 15 V maximum and use the
converter to get this voltage converted into a DC voltage ranging from 3 to 15 V maximum.
The VOUT target is set prior with a target constant defined into the firmware.
Due to the inductor size and the input/output varying conditions, the allowed output current
is given for 0.5 A typical. The input voltage can be converted to a lower voltage level using a
step-down mode, or converted to a higher level using a step-up mode. These 2 modes are
supported with a single H-bridge converter using a non-inverting buck-boost topology.
The STM32F334 microcontroller is interfaced to the power switches with a minimum
hardware, some acting as level shifters especially to adapt the low-level voltage of
microcontroller supply to the power stages connected to the external source. The buckboost converter topology is sketched in Figure 3.
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AN4449
Application description
Figure 3. STM32F334 Discovery Buck/Boost converter topology
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As shown above, the high-resolution timer controls the N and P MOS switches of H-bridge
converter. The P-MOSFETs are not directly connected to the high-resolution timer but
interfaced with level shifters, while the logic level N-MOSFETs are directly controlled by the
MCU. These level shifters are made up of a bipolar totem-pole driver. Please refer to
UM1735 for further details on converter schematics.
The buck-leg (left side of H-bridge) is driven by TA1 (for the PMOS) and TA2 (for the NMOS)
signals from HRTIM (high-resolution timer) and the boost-leg (right side of H-bridge) is
driven by TB1 (for the NMOS) and TB2 (for the PMOS) signals from HRTIM. This converter
structure uses 4 MOSFET switches to allow synchronous rectification that improves
converter efficiency and thermal performance. This consists to replace the rectification
diodes commonly used in buck or boost converter stages by MOSFET transistors ensuring
the current rectification and providing better efficiency in low-output-voltage and highcurrent power supplies. The conduction losses in the freewheeling diodes can represent a
significant part of the power losses of switching converters.
The internal HRTIM PWM switching frequency is 250 kHz. Both VIN and VOUT signals are
sensed by the ADC peripheral and the varying error vs. the VOUT target is evaluated.
This mode is called voltage mode control as the output regulation is based on voltage
measurement. The PWM duty is then computed internally by a proportional integral
controller (PI) based on ADC inputs.
The input or output currents are not sensed by any hardware current sensing method
(series sense resistor, MOSFET current sensing, current transformers, etc...) but evaluated
by a software solution described later in this document.
Caution:
For safety reasons, users must not modify the current design and must comply with
firmware operating limitations. Moreover, USB voltage source is not recommended as an
alternate power supply source for VIN as this can damage the board or the host USB PC
port if a high current is forced in the converter.
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Application description
1.4.2
AN4449
Non-inverting buck/boost converter basics
Step-down operation (buck mode)
This mode operates when VIN >> VOUT target and when the converter lowers the input
voltage level.
As shown in Figure 4, TA1 and TA2 are two PWM signals generated by the HRTIM and act
together as complementary signals. As evidenced by the converter topology, two transistors
from a same leg cannot be switched at the same time (thus risking to create a short-circuit
on the power line). Therefore Q1 and Q2 cannot be turned ON together and must be
managed one by one with the HRTIM peripheral by inserting dead-time periods between
conductions of Q1 and Q2 as shown in Figure 5. When Q1 is closed, the inductor charge
phase is observed. When Q2 is closed at its turn, the inductor discharge phase can start. On
the right side of the H-bridge, Q4 is maintained closed and Q3 open during all buck
operating phase.
Note:
In figures 4 and 5 the same color scheme has been used to indicate current flows.
Figure 4. Step-down operation (buck mode)
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Figure 5. Step-down operation signals (buck mode)
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Application description
During charge phase, Q1 is closed and the VIN voltage is applied to the inductor. The
current flows through the inductor and the output capacitor is charged through the ground. If
the voltage across Q1 and Q4 are ignored (ideal switches), the relation between VIN, VOUT,
L and the current IL flowing through L can be expressed by the formula below:
Vin − Vout = L
diL
dt
During discharge phase, Q1 is open and the current continues to flow through Q2 (now
closed) and through the inductor. In this condition and with Q2 and Q4 drop voltages
ignored as well, the formula between the current, L and VOUT becomes:
− Vout = L
diL
dt
If the HRTIM PWM switching period is TPWM (in seconds) then the charge phase tON and
the discharge phase tOFF are linked together by the formula:
tON + tOFF = TPWM
The PWM duty cycle is the ratio of time when Q1 is turned ON over the total TPWM period. D
is always included between 0 and 1. It can be expressed as:
D=
tON
TPWM
When Q1 is turned OFF, the OFF time is equivalent to:
tOFF = (1 − D) × TPWM
Considering that the average voltage across the inductor L is zero during a whole PWM
period and to prevent any saturation into the coil, the relation between VIN and VOUT for the
step-down converter (buck mode) is:
D × (Vin − Vout ) + (1 − D) × (−Vout ) = 0
D × Vin − D × Vout − Vout + D × Vout = 0
This sums up to:
Vout
=D
Vin
Step-up operation (boost mode)
This mode operates when VIN << VOUT target and when the converter raises the input
voltage level. As shown in Figure 6, TB1 and TB2 are two PWM signals generated by the
HRTIM and are working together as complementary signals. As it was the case in the buck
mode, and for the same H-bridge leg, Q3 and Q4 cannot be turned ON at the same time, as
shown in Figure 7. When Q3 is closed, the inductor charge phase takes place. When Q4 is
closed, the inductor discharge phase starts. On the left side of the H-bridge, Q1 is
maintained closed and Q2 open during all boost operating phase.
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Application description
Note:
AN4449
In figures 6 and 7 the same color scheme has been used to indicate current flows.
Figure 6. Step-up operation (boost mode)
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Figure 7. Step-up operation signals (boost mode)
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During charge phase, Q3 is closed and the VIN voltage is applied on the inductor through
the ground. If the voltages across Q1 and Q3 are ignored (ideal switches), the relation
between VIN, L and the current IL flowing through L can be expressed by the formula below:
Vin = L
diL
dt
During discharge phase, Q3 is open and the current continues to flow through the inductor
and Q4 (now closed) and finally through the output capacitor. In this condition and with Q1
and Q4 drop voltages ignored as well, the formula between VIN, the current IL, L and VOUT
becomes:
Vin − Vout = L
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diL
dt
AN4449
Application description
For the charge phase tON and the discharge phase tOFF the same formula can be applied:
tON + tOFF = TPWM
The PWM duty cycle is the ratio of time when Q3 is turned ON over the total TPWM period. It
can be expressed with D always included between 0 and 1as:
D=
tON
TPWM
When Q3 is turned OFF, the OFF time is equivalent to:
tOFF = (1 − D) × TPWM
For the boost mode, the following formula can be applied:
D × Vin + (1 − D) × (Vin − Vout) = 0
D × Vin + Vin − Vout − D × Vin + D × Vout = 0
Vin = Vout (1 − D)
That finally gives:
Vout
1
=
Vin 1 − D
Buck-boost operation (mixed mode)
This mode occurs when VIN is near VOUT (VIN ~ VOUT) and none of the buck or boost
modes can independently achieve VOUT target regulation. It uses both buck and boost sides
of the H-bridge converter and the 4 HRTIM outputs TA1, TA2, TB1 and TB2 generate PWM
signals where TA1 and TB1 are still complementary respectively to TA2 and TB2.
In this application example, during buck-boost operation, the buck duty cycle is maintained
fixed while the boost duty cycle is variable according to VIN and VOUT conditions.
Figure 8. Buck-boost operation (mixed mode)
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In the buck-boost mode, both operations are cascaded. Considering the buck mode as a
transfer function f1 with D1 as an input parameter and the boost mode as a transfer function
f2 with D2 as an input parameter, the equivalent scheme shown in Figure 9 can be applied:
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Application description
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Figure 9. Buck-boost cascaded transfer functions
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Now applying the consecutive equations for buck and boost converters:
V 0 = f 1( D1) × Vin
and
Vout = f 2( D 2) × V 0
then the global transfer function f is:
Vout
= f ( D1, D 2) = f 1( D1) × f 2( D 2)
Vin
As the transfer functions based on the converter duty cycles for buck and boost have been
mentioned above in this document, this finally gives for the buck-boost converter mode:
1
Vout
D1
= D1 ×
=
Vin
1 − D2 1 − D2
As an example, setting D1 buck duty cycle to 0.8 (fixed), with a variation of D2 boost duty
cycle between 0.05 and 0.45, allows to have a VOUT / VIN ratio varying from 0.85 to 1.45,
thus covering the entire range of VOUT close to VIN.
The purpose is to have 3 operating modes that are slightly overlapped between adjacent
ranges, as graphically shown in Figure 10.
Figure 10. Operating modes according to VIN level
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Application description
This overlap prevents from sporadic switching from one mode to another especially when
the converter operation is located at the threshold of the maximum or minimum operating
duty for the current mode. It acts like a hysteresis and improves application stability.
Consequently, if a VOUT target is set, the converter can operate in the 3 areas, for the cases
VIN << VOUT, VIN ~ VOUT, or VIN >> VOUT.
Concerning the buck-boost operating signals, the buck operation is achieved with a fixed
PWM waveform on TA1 (TA2 as complementary) and the boost one with a variable PWM
duty cycle on TB1 (TB2 as complementary) as shown in Figure 11, where the same color
scheme of figures 4 to 7 has been used for currents.
In this example, the PWM duty cycle D1 applied on buck side is fixed to 0.8 and the PWM
duty cycle D2 applied on the boost side may vary from 0.05 up to 0.45. Theoretically, if VIN =
VOUT, D1 and D2 should be complementary, as for instance, if D1 is fixed to 0.8, then D2
should be 0.2 referring to the relation between VIN, VOUT, D1 and D2 seen above. In real
application case, D2 needs some margins to introduce more flexibility between the
conversion modes and ensure a minimum hysteresis over operating ranges.
The IL current waveform can be broken down into 3 regions, as shown in Figure 11.
Figure 11. Buck-boost operation signals (mixed mode)
The first phase is equivalent to the boost time when phases 2 and 3 correspond to the buck
time. This curve is represented when VIN is very close to VOUT due to the horizontal line
observed in section 2. If VIN is slightly lower than VOUT, the line slope located in section 2
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Application description
AN4449
becomes negative and the current decreases through the inductor during this time slot. On
the opposite, if VIN is slightly higher than VOUT, the line slope located in section 2 is positive
and the current through the inductor increases for this same time slot.
Equations previously defined for buck and boost modes can be applied for the required
period.
1.4.3
Sizing the inductor of the buck-boost converter
Objectives of this buck-boost converter are to have an input voltage range from 3 VDC to
15 VDC maximum and the same parameters for the output voltage. For the converter, the
typical output current is 500 mA. The HRTIM switching frequency is selected to have a
PWM frequency of 250 kHz. A relevant estimation for the inductor ripple current is 30%
typical of the output current value.
For the buck mode, with a maximum of 15 VDC in input and a minimum of 3 VDC on the
output, the value of theoretical duty D is 3/15 = 0.2.
Based on these parameters and from the initial equation:
VL = L
diL
dt
The inductor L estimation value can be calculated with the formula below:
L min >
L min >
(Vin − Vout )
1
× D×
Fpwm
Iripple
(15 − 3)
1
× 0.2 ×
= 64µH
250000
0.5 × 0.3
For the boost mode, the input and output ranges are similar and the formula is now:
L min >
Vin 2 (Vout − Vin)
1
×
×
2
Fpwm
Vout
Iripple
The chosen value is L = 82 µH with a resistance rL of 460 mΩ typical, and typical saturation
current of 1.1 A.
1.4.4
Setting the STM32F334 High-resolution timer
This section describes how the STM32F334 high-resolution timer is set for this application
example. As described above, the N and P MOSFETs of the H-bridge converter are each
ones assigned to one of the HRTIM outputs. For the buck right side, there are TA1 and TA2
PWM waveforms connected to the buck-leg and for the boost left side, TB1 and TB2 PWM
waveforms connected to the boost-leg. TA1 and TA2, as well as TB1 and TB2 act as
complementary signals.
There are 4 modes that control the buck boost converter, as described in Table 1.
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Application description
Table 1. HRTIM output signals through operating modes
Converter mode
HRTIM
output
IDLE
BUCK
MIXED
BOOST
TA1
Low
PWMA
PWMA
High
TA2
Low
/PWMA
/PWMA
Low
TB1
Low
Low
PWMB
PWMB
TB2
Low
High
/PWMB
/PWMB
IDLE mode is a waiting mode when only timers A and B have been started but none of the
outputs has been set yet.
As soon as outputs from Timer A or Timer B are controlled, there is one state configured for
the timer period event and another one configured for a compare event (or a few ones). The
PWM period is selected to 4µs (fPWM = 250 kHz). Each 4 µs, the timer period event occurs.
Since both timer A and timer B are synchronized together, the period event is common for
the 2 timers. To activate the related outputs to timer A and B, one (or more) compare events
is (are) created, as shown in Table 2.
Table 2. Event settings through operating modes
Converter mode
ALL
BUCK
MIXED
Period
CMP1
CMP2
CMP4
CMP1
Timer A
4 µs
DutyA
-
ADC
trigger
DutyA
Timer B
4 µs
-
-
-
-
CMP2
DutyB
Mixed
BOOST
CMP4
CMP1
CMP2
CMP4
ADC
trigger
-
-
ADC
trigger
-
DutyB
-
-
Sliding ADC trigger event
Switching converters generate noise on the power lines due to the fast switch of the power
MOSFETs. Nevertheless, the application has to measure regularly the values of VIN and
VOUT voltages during operation and potentially during switching time periods.
According to the different operating modes, buck, mixed and boost modes, there are various
switching events on the timers outputs that can degrade the ADC measure when performed
at a regular time without looking at conditions on HRTIM outputs. For instance, if the ADC
measure is defined at the half of the PWM period while the duty cycle of an HRTIM output
signal reaches 0.5, both ADC and switching event are performed at the same time. To
ensure that the ADC measure is not polluted by the noise during a MOSFET transition, the
ADC trigger event has to be determined according to the current duty cycle value and also
depending on the various switching events due to the considered mode.
To improve VIN and VOUT ADC measurement accuracy and to carry out the sampling in a
non-transition period, the ADC trigger event is adapted every time the duty cycle changes.
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There are 2 conditions that are checked before applying the ADC trigger event time:
1.
Buck or Boost mode operation:
In this case, only one group of complementary outputs is active (TAx or TBx). The time
value of the ADC trigger event depends essentially on the value of the running duty
cycle. The ADC trigger time is assigned on the opposite sector of the signal transition
and its event also slides with the value of the duty cycle. It depends upon tON or tOFF
value, therefore the ADC trigger position varies with duty cycle as shown in Figure 12.
2.
Mixed (buck-boost) mode operation:
In this case, both groups TAx and TBx are active (see Figure 11). As the buck mode is
set with a fixed duty cycle of 0.8 and the boost mode duty cycle cannot exceed 0.45,
therefore the ADC trigger time event is set to a fixed value corresponding to a free
measuring window located in between. The value of ADC trigger time is typically 60%
of the overall PWM period.
Figure 12. ADC trigger event configuration in buck and boost modes
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1.4.5
Software overload protection
Buck-boost converters generally use different methods to evaluate the load current sourced
by the converter. There are several means but the most common method is to sense the
load current with a current sense resistor inserted serially into the output circuitry. This
allows controlling the maximum inductor current and to prevent any overload conditions.
The drawback is to introduce power losses caused by the inserted resistor and resulting in a
lower converter efficiency. Even if there are lossless current-sensing methods, here a
different approach has been used for this application example.
As described above in step-up or step-down operations in buck-boost basics chapter, the
theoretical duty cycle is known for a given operating mode (buck, boost, etc...) and can be
calculated by the VIN / VOUT ratio applied onto the converter. As for instance, a 10 VDC input
voltage can be converted into 5 VDC output voltage with a duty cycle equal to 0.5 (buck
mode). This basic assumption does not take into account the DC/DC converter efficiency for
a given output load current. The relation for buck mode is actually where η is the efficiency
factor:
Vout
×η = D
Vin
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Application description
Therefore, the application cannot anticipate itself the approximate value of the output
current based on the running mode and the duty cycle applied during operation.The idea is
to have this information available and stored into the microcontroller memory by anticipating
the efficiency based on the real case operation. For this purpose, the following configuration
is set as in Figure 13.
Figure 13. DC/DC converter characterization
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This DC/DC converter has been designed to have a typical current of 0.5 A for both Iin and
Iout.
The idea is to characterize the application in the different operating modes according to
various VIN / VOUT ratios: different (VIN, VOUT) pairs are applied to the converter, and two
ammeters inserted on input and output lines. For each pair of voltage values, the variable
Zload impedance is adjusted so that Iin and Iout are always within the desired range.
As an example, an input voltage of 10 VDC is applied on VIN, and VOUT target is set to
5 VDC. The impedance load is tuned to obtain Iout = 0.55 A. The duty cycle is recorded for
the operating point (VIN = 10 V, VOUT = 5 V). Similarly, for another (VIN, VOUT) pair, this time
in boost mode, the impedance load is tuned to have Iin not exceeding 0.55 A. Thus, a raw
table containing duty cycles D1 or D2 is reported for each (VIN, VOUT) pair. This allows
determining the functional mapping for the different modes.
The input range VIN is included between 3 to 15 VDC as well as for the output range. For
each (VIN, VOUT) pair, the converter applies the function f1(D1) or f2(D2) or f(D1,D2) =
f1(D1) X f2(D2) according to the VIN / VOUT ratio and the selected operating mode (buck,
boost or mixed).
The results are represented graphically in Table 3, Table 4 and Table 5, related to the three
operating modes. In each table, cells in green allow to easily identify the operating area for
the concerned mode (for readability purposes, duty cycles values have been removed). In
these tables VIN and VOUT are both expressed in Volts.
As an example, the duty cycles values Y0 to Y12 are obtained by f1 function:
y 0 = NA; y1 = f 1(4,3); y 2 = f 1(5,3); etc... y12 = f 1(15,3)
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Table 3. Collection table for buck mode
VIN
f1(D1)
3
3
4
5
6
7
8
9
10
11
12
13
14
15
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Y8
Y9
Y10
Y11
Y12
4
5
6
7
8
9
VOUT
10
11
12
13
14
15
The same duty cycle collections are gathered for boost and mixed modes (Table 4 and
Table 5, respectively). All these modes are indeed slightly overlapped together.
Table 4. Collection table for boost mode
VIN
f2(D2)
3
4
5
6
7
8
3
4
5
6
7
8
VOUT
9
10
11
12
13
14
15
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Application description
Table 5. Collection table for mixed mode
VIN
f(D1, D2)
3
4
5
6
7
8
9
10
11
12
13
14
15
3
4
5
6
7
8
VOUT
9
10
11
12
13
14
15
Going further, if the duty cycle values extracted from a same row table (Y0, Y1, Y2, etc... up
to Y12) are represented into a graph, it can be observed a corresponding curve or line for
any row of the table.
As shown in Figure 14, every buck function f1(D1) represents a curve for each VOUT value
chosen where other functions for respectively boost (Figure 15) and mixed (Figure 16)
modes as f2(D2) and f(D1,D2) are equivalent to simple lines.
Figure 14. Duty cycle value vs. VIN for a given VOUT in buck mode
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Figure 15. Duty cycle value vs. VIN for a given VOUT in boost mode
Figure 16. Duty cycle value vs. VIN for a given VOUT in mixed mode
The goal is to have all these data stored in three different tables, by keeping as information
only the curve or line equations needed to calculate the corresponding duty cycle for the
running (VIN, VOUT) pair. For the buck mode, the equations of these curves are computed
with polynomial interpolation while for both boost and mixed modes only regression lines
calculation is necessary.
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Application description
To compute the polynomial interpolation based on Lagrange’s interpolation, at least 4
distinct points are extracted from the raw table. This operation results in 4 polynomial factors
from degree included between 0 and 3 that are combined to get the final equivalent function.
As for instance, 4 points coordinates are extracted from the first buck curve: p1(5, 13605);
p2(8, 8521); p3(11, 6285); p4(15, 4731). Please notice that all duty values are expressed for
a whole PWM period equal to 18432 units. This number corresponds to 4 µs multiplied by
the timer frequency, equivalent to 144 MHz X 32. The corresponding duty cycle for 13605 is
indeed 13605/18432 = 73%. After Lagrange’s interpolation is computed, the equation of the
interpolated curve is:
L ( x) =
− 13513 3 130918 2 7199839
695647
×x +
×x −
×x+
1260
315
1260
21
Finally, these factors are arranged and simplified to limit the amount of data stored in
software tables with a slight loss of accuracy for the last constant term.
This gives:
L( x) = −10.725 × x 3 + 415.612 × x 2 − 5714.157 × x + 33126.047
The data stored in table for the equation curve corresponding to VOUT = 3 V for the buck
mode will be: {10725; 415612; 5714157; 33126}. From these data, the curve equation can
be retrieved easily and the last required operations will be performed by the MCU to
calculate the corresponding duty from an input value of VIN.
It’s now possible to calculate any duty cycle for the buck mode for the VIN range [3;15] VDC.
Which would be the duty cycle for VIN = 9 V to obtain VOUT = 3 V in buck mode? This means
for the MCU to extract the 4 factors and to calculate:
L(9) = −10.725 × (9)3 + 415.612 × (9) 2 − 5714.157 × 9 + 33126
This finally gives:
L(9) = 7544
Thus, if VIN = 9 V and VOUT = 3 V then 7544 (corresponds to 41%) represents the duty cycle
register value that application shouldn’t exceed to keep the output (or input) current below
typical value. If the duty cycle order is set beyond, therefore the output current will raise over
it. In that case, overload conditions on the converter are reached.
As well for boost and mixed modes, the regression line is computed based on the
coordinates of all different points collected in the raw data tables.
In that case, only 2 factors a and b will result from the regression line such as:
f ( x) = ax + b
All these data are stored in 3 tables, the first one for buck mode including the 4 polynomial
factors (x3 to x0) for each of the 0 to 12 rows (3 to 15 V). The second and third one
(respectively assigned to boost and mixed mode) contain the 2 regression line factors a and
b for each of the 0 to 12 rows. If user has chosen an intermediate value for VOUT (not an
integer one, e.g. VOUT = 5.7 V), the MCU computes the value for two adjacent curves (or
lines) and performs a linear interpolation between these 2 curves. As an example, if the user
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has targeted VOUT = 5.7 V, then the duty cycles from adjacent curves or lines for 5 and 6 V
are computed with VIN value and a linear interpolation is performed to find the nearest
approximation of maximum duty cycle.
Basically, for a given mode, the application uses the corresponding equation to evaluate the
maximum duty cycle admissible for the running operation. Subsequently, the value of the
duty cycle applied into the converter is compared with the value of the computed duty cycle
limit. If the duty cycle applied into the converter exceeds the value of the theoretical limit for
a given time, then the converter is stopped and the overload is detected. As a drawback,
there is no possibility to change the overload limit as the data entered into tables, then the
interpolation functions, have been characterized for a defined current value.
Nevertheless, this method does not use external hardware that would degrade converter
efficiency such as current-sense resistors but can be easily used for current limitation
purpose. This allows stopping the converter and anticipating any hardware damages if an
external output load is forcing the converter beyond its capability.
1.4.6
PI software regulation
This DC/DC converter uses the measured and the target VOUT voltages to set the converter
duty cycle in the corresponding operating mode. As mentioned earlier, the ADC peripheral is
regularly triggered by the HRTIM to sample VIN and VOUT values. Based on this information,
a software based PI controller (proportional-integral) is used to apply the optimized value of
duty cycle according to VOUT target value.
As shown in Figure 17, the application set point which is the VOUT target of the converter is
compared with the VOUT fed back by ADC conversion.
Figure 17. PI software controller
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The error is computed by the difference between the target value and the ADC measured
value. This error is then amplified with Kp and Ki gain parameters from respectively
Proportional and Integral terms and summed together to set the new duty cycle order.
Depending on the considered converter mode (buck, boost or mixed), the PI controller
setting value is limited to the maximum or minimum duty cycle value allowed for the related
mode by a saturation method. This saturation can take place for very low or very high values
of duty cycle. Every time the saturation point is reached during a PI request, one of the two
counters, CTMax or CTMin, respectively assigned to maximum and minimum duty cycle is
incremented. If the system does not reach the set-point for an unknown reason, the PI order
tends to return maximum or minimum duty cycle values along the consecutive PI
computation. In that case, the counters for maximum or minimum duty cycle reach their
overflow value and then the converter may be stopped immediately.
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Application description
This method ensures fast response from converter and shuts it down as for instance when a
short circuit is present on VOUT signal output. These counters are also used to detect that
one operating mode has reached its own operating limits, and to accordingly manage the
converter mode changes.
1.4.7
Getting started with the application
As soon as the buck-boost firmware has been programmed into the microcontroller, the
application is ready to start. As mentioned in Section 1.1: Required hardware, an external
power supply is needed for the demonstration and the mini-B USB cable connected on the
PC side.
In order to start the converter, follow the steps:
1.
connect the mini-B USB cable;
2.
connect the external power supply to CN5 connector (VIN);
3.
press the B2 button to reset the microcontroller.
The VOUT target value is set by the firmware and can be modified manually for all other
values included from 3 to 15 VDC. This modification has to be performed in main.h file
containing the VOUT target parameter as shown below:
#define VOUT_TARGET ((uint16_t) 5000)
Please notice that this value is expressed in mV.
There is an optional update of the firmware that can be done, which consists in entering the
real value of application 3V3 reference voltage. In the same main.h file, the following
parameter (always expressed in mV) is mentioned:
#define REAL_3V3 (uint16_t) 3300
For accuracy purpose, this variable can be replaced by the current value of 3V3 voltage of
the STM32F334 Discovery kit. A small variation on the 3V3 regulator can be detected, this
parameter is used to account for it, and must be replaced by the actual value of the 3V3 that
can be measured with a voltmeter connected in parallel with C19 capacitor. Otherwise, this
value must be left as default.
There are 2 remaining constants in main.h file that can be precisely set for compensation of
VIN and VOUT resistors bridges.
The user has to measure respectively VIN and R45 voltages, and VOUT and R46 voltages.
The VIN_RESISTOR_RATIO is obtained by computing (VR45/VIN) *10000, similarly the
VOUT_RESISTOR_RATIO uses (VR46/VOUT)*10000.
For instance, the user sets precisely VIN voltage to 10 V and reads the voltage value on
R45, e.g. VR45 = 2.010 V. In this case VIN_RESISTOR_RATIO is (2.010/10)*10000 = 2010.
For VOUT, the user reads both VOUT and R46 voltage and computes similarly the
VOUT_RESISTOR_VALUE.
New computed values are replaced in constants definition, otherwise, these values must be
left as default ones as shown below:
#define VIN_RESISTOR_RATIO(uint16_t) 2012
#define VOUT_RESISTOR_RATIO(uint16_t) 1988
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A load or any electrical component complying with converter output characteristics can be
connected to VOUT.
Once the converter has started, the signal LEDs (green, blue, orange and red) are active
and the following displays are observed according to VIN / VOUT conditions and to context:
•
Standard operation (Figure 18):
–
Green LED toggles for buck mode
–
Blue LED toggles for boost mode
–
both Green and Blue LEDs toggle for mixed mode.
Figure 18. Signal LEDs during standard operation
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•
Limited operation (Figure 19):
–
The same display is performed but the Orange LED is also toggling when
converter operates near maximum duty cycles limits. Application can sustain such
conditions (above 95% of allowed operation range), but user is warned that
operating limits are close.
Figure 19. Signal LEDs during limited operation
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Application description
•
Stopped Operation (Figure 20): A fault condition has been detected into the converter:
–
Orange LED toggling:
VIN is not in the expected range [3;15] VDC
–
Red LED toggling:
Converter stopped (PI controller limits reached for a given time)
Overload detected (current exceeds limit)
Software initialization fault.
Figure 20. Signal LEDs during fault condition
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2
Firmware description
2.1
STM32F334xx peripherals used by the application
This application example uses the following STM32F334xx peripherals with the settings
described below:
GPIOs
GPIOs are essentially used for the 4 signal LEDS and few others for further development or
debug purposes:
•
PB6 to PB9 set as output GPIOs to drive the signal LEDs
•
PA0 set as input GPIO and connected to the User push-button B1 but the button
function is not implemented yet in this example. Users can place their own code into
the mentioned section if needed.
ADC
ADC peripheral manages VIN and VOUT 12-bit ADC sampling. An ADC calibration is
performed at application startup.
Two channels are used in injected conversion mode with different sampling times according
to respective VIN and VOUT hardware adaptation. The ADC conversion is triggered by the
HRTIM Timer A compare event CMP4 at every PWM period and the VIN and VOUT
conversions stored.
HRTIM High-resolution timer
As mentioned earlier in overview section, the high-resolution timer is connected to the
power interface of the DC/DC buck-boost converter.
The 4 HRTIM TA1, TA2, TB1, TB2 outputs are used for the 4 PWM signals generation of the
H-bridge topology converter.
In debug mode, HRTIM Timer D is also used to highlight ADC conversion trigger event on
TD1. “DEBUG” variable in firmware header must be set accordingly.
A HRTIM DLL calibration is performed at the beginning of HRTIM initialization function
(duration 14 µs).
The HRTIM frequency is set to 250 kHz (period t = 4 µs) and determines the main switching
frequency of the buck-boost converter.
The repetition counter for HRTIM Timer A is set to have an ISR every 8 PWM periods
(4 µs x 8 = 32 µs). It can be easily lowered up to 4 if the overload protection is removed, this
is freeing some MCU activity during interrupt routine.
The converter uses 4 different modes that need special initialization functions such as Idle,
Buck, Mixed and Boost. By these dedicated functions, the HRTIM outputs are configured to
select the set and reset sources combined with related events.
As a requirement of this application example, HRTIM Timer A and B use dead time
configuration to set rising and falling dead times. This prevents overlap of switching
command from MOSFET transistors from a same leg.
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Firmware description
Please refer to Section 1.4.4: Setting the STM32F334 High-resolution timer for further
information.
Interrupts
As a major process of this application firmware, the HRTIM Timer A interrupt
HRTIM1_TIMA_IRQHandler() manages all relative tasks to the converter.
The PI functions are called from this interrupt routine and the converter duty cycles are
updated accordingly. Depending on the ongoing operating mode, the converter also
updates the ADC trigger at each interrupt. As soon as the converter reaches the limits of its
operating mode, the mode change is performed during the interrupt.
A state machine is managing the LEDs display during the interrupt to highlight modes and
other features.
If the overload detection is ON (#define OVERLOAD_ON is set), the duty cycle limit is
computed into the interrupt every 1.6ms which correspond to 50 x 32 µs (repetition period of
HRTIM1_TIMA interrupt). This task has been limited to avoid overloading the MCU at each
repetition periods. As mentioned earlier in this document, the duty cycle limit is calculated
with functions based on polynomial interpolation or regression lines. This is the major
consuming task in terms of MCU resources even if this operation does not exceed 30% of
MCU activity only every 1.6 ms.
2.1.1
Setting #define constants
There are 3 defined constants in header of the application firmware:
1.
#define DEBUG: this constant is used to enable HRTIM Timer D and allowing the ADC
trigger conversion event visible on TD1. This is only for debug purpose and to highlight
interaction between timers’ events.
2.
#define RANGE_MONITORING_ON: this constant is used to enable the detection on
VIN and VOUT voltages when one of these two voltages exceeds range limits (3 to
15 VDC). If the application detects such eventuality for a given time, then the converter
is stopped and the LED display is updated.
3.
#define OVERLOAD_ON: this constant is used to enable the detection of any overload
in the converter. The current duty cycle has reached the limit value for a given time
(overload) and the converter must be stopped due to fault detection. The LED display
is also updated.
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2.2
Application flowcharts description
2.2.1
Application “main.c” flowchart
The “main.c” software (sketched in Figure 21) is limited to the initialization of the different
peripherals used during this demonstration example. Mainly ADC and HRTIM peripherals
are set at startup. Then the application firmware checks roughly what is the appropriate
converter mode to start and puts a request mode change that will be serviced inside the
interrupt routine. The application waits for any stop of the converter in case of fault
detection. If any fault is detected during the HRTIM interrupt, then the converter stop
request is set immediately but this stop is managed in the main sequence once the interrupt
has been serviced and has ended. Different LEDs displays are performed.
Figure 21. “main.c” flowchart
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2.2.2
Firmware description
Application “HRTIM1_TIMA_IRQHandler()” flowchart
As shown in Figure 22, the major process is performed during HRTIM Timer A interrupt
routine.
Once the interrupt has been serviced, the related IT is cleared immediately. The process
identifies which is the running converter mode. Any change of the converter occurs in
interrupt routine. This is to ensure smooth transitions between modes and applying the new
waveforms in synchronization with the timer period. For each active mode such as buck,
boost or mixed, a different display is managed and a dedicated PI function is called to
compute the value of duty cycle. This duty cycle is updated at each interrupt and the ADC
trigger event is set as well.
Figure 22. “HRTIM1_TIMA_IRQHandler()” flowchart
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Then the converter checks for any information returned by the PI software controller that
would overflow CTMax or CTMin counters. In these situations, a specific action is performed
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Firmware description
AN4449
according to the current context. As for instance, in the buck mode converter, the application
checks the CTMax counter overflow but also checks if the VOUT output signal has started to
rise at a minimum level. This is to distinguish a situation where the maximum duty cycle is
requested to reach the target rapidly but also when the maximum duty cycle is applied and
unfortunately VOUT signal does not show any variation. In the first situation, the buck mode
will change to the mixed mode while for the second one the converter will be stopped as a
fault has appeared.
Basically, the duty cycle high and low limits for each mode are defined as main variables
into the firmware based on the performances of this converter. As the 3 converter modes
have adjacent operating area, the converter has to move from one mode to another when it
reaches the operating limits of its own mode, precisely when counters CTMax and CTMin
inform that a maximum or minimum duty cycle order is permanently returned by the PI
controller. Any converter mode change is actually triggered by the overflow of CTMax or
CTMin.
Regarding fault detection, the overload limit which is computed by the interrupt software is
performed in two steps. The first step consists to collect the data from the relevant table and
applying the right calculation according to the interpolation type. The second step is a
common part of the interrupt task and checks for any duty cycle value that would exceed the
overload limit. If the applied duty cycle operates near limits, then users are alerted by a
display. If the applied duty cycle exceeds the overload limit for a specified time then the
converter is stopped.
For further information on the firmware, refer to the complete firmware package available at
http://www.st.com.
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3
DC/DC converter main electrical characteristics
DC/DC converter main electrical characteristics
Table 6. Main electrical characteristics
Symbol
Min
Typ
Max
Input voltage (CN5)
3
-
15
Output voltage (CN7)
3
-
15
Iin
Input current on VIN
0
-
0.55(1)
Iout
Output current on VOUT
0
-
0.55(1)
Inductor value
-
82
-
Average current in L inductor
-
-
0.75
Ripple current through inductor
-
0.3 x Iout
-
Minimum Input voltage software detection
2.70
2.9
3.1
VOUT min
Minimum Output voltage software detection
2.70
2.9
3.1
VIN max
Maximum Input voltage software detection
14.9
15.1
15.3
VOUT max
Maximum Output voltage software detection
14.9
15.1
15.3
VOUT ripple
Output ripple voltage
(Iout = 525 mA VIN = 12 VDC, VOUT = 5 VDC)
-
100
-
mVp-p
-
0.5
1
%
Converter switching frequency
-
250
-
kHz
PWM Duty cycle in buck mode
15
-
90
PWM Duty cycle in boost mode
5
-
90
PWM Duty cycle in mixed mode
5
-
45
Iout = 400 mA, VIN = 12 VDC, VOUT = 5 VDC
-
88
-
Iout = 180 mA, VIN = 7 VDC, VOUT = 12 VDC
-
87
-
VIN
VOUT
L
IL AVG
Iripple
VIN min
Parameter
VOUT accuracy Output voltage accuracy (at 100% load)
fPWM
Duty cycle
Efficiency
Unit
VDC
A
µH
A
VDC
%
%
1. If overload detection is ON the maximum current is detected for this value +/- 10%.
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33
Conclusions
4
AN4449
Conclusions
DC-to-DC converters are widespread in industrial, consumers or automotive applications.
They require high integration, efficiency, reliability but also more and more flexibility that
analog solutions cannot always offer. Digital power conversions systems are taking the lead
to address this market by offering a wide combination of technical solutions based on highefficient digital timers.
This example around the STM32F334xx advanced ARM®-based 32-bit MCUs highlights
some of the benefits of this microcontroller and especially the embedded high-resolution
timer mainly designed for digital power conversions applications. It includes a very high
range of settings that makes this buck-boost converter application example easier to
develop. It also demonstrates its ability to setup and switch from one running configuration
to another in one PWM cycle time.
This demonstration firmware combined with the low cost and high-performance STM32F334
Discovery kit are the easiest ever distributed tools on how to get started with STM32F334xx
microcontroller and to experience its power-oriented solutions based on the brand new highresolution timer.
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5
Revision history
Revision history
Table 7. Document revision history
Date
Revision
08-Sep-2014
1
Changes
Initial release.
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AN4449
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