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Texas Instruments Efficient Super-Capacitor Charging with TPS62740 Application notes
Application Report
SLVA678 – December 2014
Efficient Super-Capacitor Charging with TPS62740
Florian Feckl ................................................................................... ALPS, Low Power DC/DC Converter
ABSTRACT
Long-life batteries with LiSOCl2 chemistry in a bobbin type cell construction have a very high specific
energy (Wh/kg) but are unable to provide currents higher than 20 mA, for example. Long-life batteries also
suffer with reduced operation runtimes when higher currents are drawn.
The TI Design PMP9753 shows a concept to buffer energy in a super capacitor and therefore decouples
load peaks from the battery.
This application note helps designers to calculate and define the parameters like minimum and maximum
voltage levels, storage capacitor size or maximum battery current.
Contents
Introduction ................................................................................................................... 2
Circuit Concept Description ................................................................................................ 3
2.1
Detailed Block Diagram ............................................................................................ 4
3
Parameter Calculation....................................................................................................... 5
3.1
Voltage Levels....................................................................................................... 5
3.2
Storage Capacitor .................................................................................................. 5
3.3
Current Limit Resistor .............................................................................................. 6
3.4
Start-up Procedure ................................................................................................. 8
4
Summary ...................................................................................................................... 9
Appendix A
List of Parameters and Acronyms ............................................................................... 10
1
2
List of Figures
...............................................
.......................................................................................
Recharge Cycle Sequencing ...............................................................................................
Application Circuit Block Diagram .........................................................................................
Application Circuit During Start-up Sequence ...........................................................................
1
Typical Discharge Curve of a TADIRAN SL-360 Over Several Loads
2
2
Simplified Charging Block Diagram
3
3
4
5
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4
8
1
Introduction
1
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Introduction
Battery chemistries like the LiSOCl2, offer great benefits in terms of application runtime. These battery
chemistries bring operation runtimes of 15 years and more to reality. The technology, however, has a
limited characteristic in terms of supporting higher loads.
Higher load pulses cannot be supported by the battery itself due to the internal impedance. As well, the
higher the drawn current, the shorter the battery operation. Figure 1 shows how different battery currents
translate to different battery runtimes.
NOTE: http://www.tadiranbatteries.de/pdf/lithium-thionyl-chloride-batteries/SL-360.pdf
Figure 1. Typical Discharge Curve of a TADIRAN SL-360 Over Several Loads
To overcome these limitations, peak power assistance concepts must be considered.
For example, a wireless sensor node transmits its gathered data once a day to a base station. The data
transmission requires 500 mA for 200 ms. This short power peak cannot be supported by the primary cell
itself. The pulse needs to be buffered somewhere else.
2
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Circuit Concept Description
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2
Circuit Concept Description
This reference design shows an energy buffering concept based on the TPS62740, a 360-nA quiescent
current buck converter, in combination with an electric double-layer capacitor (EDLC) or a so called super
capacitor.
Voltage control (µController)
VSEL
Charge Current
e.g. 3mA max.
TPS62740
Primary
Cell
Storage
Capacitor,
EDLC
Figure 2. Simplified Charging Block Diagram
The circuit uses a resistor at the output of the TPS62740 to limit the current into the storage capacitor as
well as the battery current drawn from the primary cell. The resistor will be selected in a way to keep the
load, and thereby the battery current, below a level the primary battery can support. The TPS62740
features a digital input to adjust the output voltage by four VSEL Pins. During the charging of the EDLC,
the output voltage can be stepped up in 100-mV steps. This helps to minimize the power losses caused
by the resistor.
In an application like a wireless sensor, the µController will be supplied from the output of the TPS62740
step-down converter. Therefore, the voltage must stay above the µController minimum supply voltage (for
example, 1.9 V). The maximum voltage of a single layer super capacitor is typically 2.7 V, which leads to a
usable capacitor voltage range of 1.9 V to 2.7 V. Figure 3 shows the basic flow of a recharge cycle.
Most of the time the voltage is kept at 1.9 V to minimize the losses of the micro-controller and other
leakage currents in the application (Phase 1). Prior to a wireless data transmission, the capacitor is
charged up to 2.7 V (Phase 2). During transmission, the stored energy in the capacitor can be extracted
down to 1.9 V (Phase 3).
For appropriate measurement results, see the PMP9753 Test Report (TIDU628).
VMAX
2.7V
1.9V
VMIN
idle
1
VSEL [V]
1.9
charge
2
Stepping 1.9 .. 2.7
radio transmission
TIME [~]
discharge
idle
3
1
1.9
1.9
active
13 min
200ms
Figure 3. Recharge Cycle Sequencing
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Circuit Concept Description
2.1
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Detailed Block Diagram
Figure 4 shows the block diagram of the energy buffering reference design consisting of following main
blocks:
• Primary cell
• TPS62740 buck converter
• Current limiting resistor network
• Storage capacitor
• Connection to the µController
The LiSoCl2 primary cell is directly connected to the TPS62740. The buck converter is controlled by the
available µController in the application. The MCU enables/disables the buck converter, adjusts the output
voltage, and enables the efficient charging as shown in Figure 3.
The output of the DC/DC converter is connected to the current limiting resistor. Figure 4 shows two
resistors, one resistor can be connected by a switch. This is designed to handle the start-up procedure
which is necessary to pre-charge the EDLC to the minimum voltage of 1.9 V. To not exceed the maximum
battery current, only the 300-Ω resistor is used.
Once the storage capacitor is pre-charged, the switch is turned on and the current is limited by the
combined resistance.
A load like a radio power amplifier can now be directly connected to the storage capacitor which does
support larger peak currents to be drawn from it.
µController
LowIq-Buck
TPS62740
Control
Power
Load
(Boost + RF PA)
Figure 4. Application Circuit Block Diagram
4
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Parameter Calculation
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3
Parameter Calculation
This section describes how to calculate the necessary parameters of the circuit for the final endapplication.
The parameters like the minimum and maximum voltage at the super capacitor and its capacitance are
calculated as well as the resistance of the limiting resistor.
All subchapters present the dedicated formula and how to use it. For further details, the included subsubchapters provide derivations and examples.
3.1
Voltage Levels
Before selecting the capacitance value of the storage capacitor, the minimum and maximum voltage are
chosen.
The minimum voltage is mainly defined by the lowest supply voltage of the micro controller. The voltage at
the capacitor must not fall below this voltage to guarantee the operation of the MCU. This value can be
extracted from the controllers’ datasheet.
An example value is VMIN = 1.9 V.
The maximum voltage is defined by the highest voltage that the EDLC is allowed to be exposed to without
any lifetime issues. The voltage is depending on the capacitor and can be found in its datashett.
A ypical maximum value of an EDLC is VMAX = 2.7 V.
The voltage difference between VMAX and VMIN is the voltage range in which the capacitor can be charged
and discharged
From the example above, the voltage difference is ΔV = 2.7 – 1.9 V = 800 mV.
3.2
Storage Capacitor
The size of the capacitor is determined by following parameters:
ELOAD
VMIN
VMAX
The required energy from the load (for example, the radio power amplifier)
The minimum system voltage (minimum supply, for example, MSP430)
The maximum capacitor voltage
The capacitor size can be calculated according to the parameters previously given, as in: Equation 1.
ELOAD ´ 2
C=
(VMAX ² - VMIN ² )
(1)
For calculating the capacitance to buffer the energy for a radio transmission pulse, the following Equation
2 can be used. Here the boost converter losses are already considered.
ERADIO ´ 2
C=
(VMAX ² - VMIN ² ) ´ hBOOST
(2)
With:
ERADIO = Energy of the radio power amplifier during transmission
ηBOOST = Efficiency factor of the boost converter
ELOAD = The required energy from the load (for example, the radio power amplifier stage)
VMIN = The minimum system voltage (minimum supply; for example, MSP430)
VMAX = The maximum EDLC voltage
3.2.1
Equation Derivation
First, the required energy for a radio telegram transmission is calculated:
ERADIO = PRADIO × tRADIO = VPA × IRADIO × tRADIO
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Parameter Calculation
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The energy, which is required by the radio power amplifier, is the multiple of the voltage level needed by
the PA, the drawn current and the duration.
As the PA needs a higher voltage, a step-up converter needs to be placed between the storage capacitor
and the amplifier. Therefore, the conversion efficiency is taken into account as well:
E
ELOAD = RADIO
hBOOST
(3)
With this equation, the desired energy from the capacitor ELOAD is calculated.
Next, the extractable energy from the storage capacitor is calculated as follows:
ECAP = EMAX - EMIN
(4)
1
1
2
2
ECAP = ´ C ´ VMAX
- ´ C ´ VMIN
2
2
1
2
2
ECAP = ´ C ´ (VMAX
- VMIN
)
2
Equalizing Equation 5 and the Equation 4 leads to:
ERADIO
= ECAP
h BOOST
(5)
ERADIO 1
2
2
= ´ C ´ Δ (VMAX
- VMIN
)
h BOOST 2
Dissolving the equation to isolate C, gives the equation to calculate the minimum capacity of the EDLC:
C=
3.2.2
ERADIO ´ 2
h BOOST ´ (VMAX ² - VMIN ² )
(6)
Calculation Example
A radio power amplifier in a wM-bus application has the following parameters:
VPA = 3.5 V
IRADIO = 300 mA
tRADIO = 200 ms
ηBOOST = 0.93 for 2.2 V to 3.5 V conversion (worst-case condition)
VMIN = 1.9 V
VMAX = 2.7 V
With Equation 7 and Equation 8:
C=
C=
VPA ´ I RADIO ´ tTX ´ 2
(VMAX ² - VMIN ²) ´h BOOST
(7)
3.5 V ´ 300 mA ´ 200 ms ´ 2
(2.7
2
)
- 1.92 V ´ 0.92
C = 0.12 F
(8)
This means the theoretical required storage capacitance has to be 120 mF to store the required energy.
3.3
Current Limit Resistor
In this circuit, the maximum battery current is limited by a resistor placed at the output of the DC/DC
converter. The TPS62740 is able to set the output voltage according to the levels at the VSEL pins in a
resolution of 100 mV.
6
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Parameter Calculation
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For calculating the resistance, the maximum voltage over the resistor of 0.1 V is considered. The
resistance value can be calculated with Equation 9:
RLIM =
Δ ´ MAX
I BAT ´ VBAT ´ BUCK
(9)
With:
ΔV = Maximum voltage across the resistor, 100 mV
VMAX = Maximum voltage of the storage capacitor
IBAT = Maximum battery current
VBAT = Primary battery voltage
ηBUCK = Efficiency factor of TPS62740 for the current previously listed
3.3.1
Equation Derivation
The resistor limits the current at the converter output; the input current limit needs to be transformed to the
input current of TPS62740, which is the battery current:
´ hBUCK
I
ICHARGE = BAT
(10)
D
For the duty-cycle D, the value which gives the highest battery current is chosen:
D =
VMAX
VBAT
ICHARGE
(11)
I
´ VBAT ´ hBUCK
= BAT
VMAX
(12)
The resistor is calculated by the voltage across itself and the charge current as shown in Equation 13.
Equation 12 is inserted for the charge current to calculate it according to the desired battery current.
DV
RLIM =
ICHARGE
RLIM =
3.3.2
DV ´ VMAX
IBAT ´ VBAT ´ hBUCK
(13)
Calculation Example
For a LiSOCl2 primary battery, a battery current of approximately 3 mA is the optimum discharge current
for the battery to extract the most energy and achieve the longest lifetime.
Considering this value gives Equation 14:
0.1 V ´ 2.7 V
RLIM =
3 mA ´ 3.6 V ´ 0.92
RLIM = 27.2 W
(14)
According to use-typical E-Series resistors, the following resistor is chosen:
RLIM = 30 Ω
(15)
Due to the fact that, the about ten times higher, start-up resistor is in parallel, the effective resistance is
slightly smaller.
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Parameter Calculation
3.4
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Start-up Procedure
Considering a maximum battery current as well in the pre-charge (or start-up) phase, a different resistor
must be chosen.
Figure 5 shows the block diagram using a bigger resistor to charge the EDLC to the minimum system
voltage.
µController
Control
Power
LowIq-Buck
TPS62740
Storage
Capcitor
Figure 5. Application Circuit During Start-up Sequence
To calculate the resistor value, the higher ∆V from zero to VMIN needs to be considered as in Equation 15:
VMIN ´ VMIN
RSTART =
IBAT ´ VBAT ´ hBUCK
(16)
3.4.1
Equation Derivation
Please refer to Section 3.3.1.
The stepsize of the voltage is now changing due to the step from zero to e.g. 1.9V.
3.4.2
Calculation Example
With the example values used in the previous sections, the following example is calculated:
1.9 V ´ 2.7 V
RSTART =
4 mA ´ 3.6 V ´ 0.92
RSTART = 272.5 W
(17)
According to typical-use E-Series resistors, the following resistor is chosen:
RSTART = 300 Ω
8
(18)
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Summary
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4
Summary
Due to the characteristics of certain batteries, applications with ultra-long runtimes need new concepts for
buffering energy.
Using an EDLC in combination the TPS62740 brings the following main advantages:
• Single cell super capacitor with a maximum voltage of below 3 V can be used.
• Storage capacitors like Murata DMF series enable an application runtime of > 15 years.
• Pulsed currents are decoupled from batteries leading to more extractable energy.
• Application runtimes are extended because of the high efficiency of the solution.
The charging sequence can be implemented efficiently due to the digital output voltage selection feature
of the TPS62740 device.
This buck converter is designed to operate with a quiescent current of typical 360 nA and is ideal for a
direct connection to the battery and ultra-long runtimes.
Applications like a wireless sensor node can use the existing μController to handle the charging sequence
of the EDLC.
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Appendix A
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List of Parameters and Acronyms
A.1
List of Parameters
VMIN
VMAX
VBAT
IBAT
VDC/DC
VCAP
PRADIO
CCAP
RCHARGE
RSTART
ERADIO
ΔV
tRADIO
VPA
ηBOOST
ηBUCK
EPA
ECAP
A.2
List of Acronyms
ALPS
TI
VSEL
EDLC
MCU
MSP430
PA
10
Minimum voltage in the system, minimum discharge voltage
Maximum voltage in the system, maximum voltage for the EDLC
Voltage of the battery
Current drawn from the battery
DC/DC-converter output voltage
Storage capacitor (EDLC) voltage
Power required by the load, for example, a radio power amplifier
Storage capacitor (EDLC) capacitance
Resistor to limit the current during charging cycle
Resistor to limit the current during start-up sequence
Energy required by the load, for example, a transmission of a radio telegram
Voltage delta between the minimum and maximum voltage at the EDLC
Telegram transmission duration
Supply-voltage for the radio power amplifier
Conversion efficiency of the boost converter stage
Conversion efficiency of the buck converter stage
Energy required by the radio power amplifier for one telegram transmission
Extractable energy from the storage capacitor
Advanced Low Power Solutions
Texas Instruments
Voltage selection, input pins of TPS62740
Electric Double Layer Capacitor
Micro Controlling Unit
Texas Instruments’ Micro Controller family
Power Amplifier
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