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Texas Instruments DRV8662, DRV2665, and DRV2667 Application notes
Application Report
SLOA198 – September 2014
DRV8662, DRV2665, and DRV2667 Configuration Guide
Brian Burk .................................................................................................... Haptic and Piezo Products
ABSTRACT
The DRV8662, DRV2665, and DRV2667 are high-voltage, Piezo drivers each with an integrated boost
converter and amplifier. The integrated high-voltage solution eliminates design complexities and reduces
the overall solution size for driving Piezos.
These devices have been designed for simplicity and are easy to configure; however, there are some
common design issues that should be avoided. This application note discusses boost converter basics,
hysteretic boost operation, and describes the proper steps for configuring the boost converter including
calculating the load requirements, selecting the inductor, current limit resistor, input capacitor, and output
capacitor.
1
2
3
4
5
6
7
8
9
10
11
Contents
Boost Converter Basics ..................................................................................................... 3
DRV8662, DRV2665, and DRV2667 Boost Converter ................................................................. 4
2.1
DRV8662, DRV2665, and DRV2667 Boost Converter Efficiency ............................................ 5
2.2
DRV8662, DRV2665, and DRV2667 Boost Converter Load Regulation .................................... 6
Configuring the Boost Converter........................................................................................... 7
Boost Converter Output Voltage ........................................................................................... 7
Calculating the Load Current ............................................................................................... 8
Selecting an Inductor ........................................................................................................ 8
6.1
Inductance Rating .................................................................................................. 9
6.2
Saturation Current Rating.......................................................................................... 9
6.3
Thermal Current Rating .......................................................................................... 10
6.4
Choosing REXT ...................................................................................................... 11
6.5
What to Avoid: Using Incorrect Inductor Current Ratings .................................................... 11
Calculate the Maximum Boost Current .................................................................................. 13
Output Capacitor Selection ............................................................................................... 13
Input Capacitor Selection.................................................................................................. 14
PCB Layout ................................................................................................................. 16
10.1 What to Avoid: Incorrect Inductor Placement .................................................................. 17
Examples .................................................................................................................... 20
11.1 Example: Based on the DRV8662EVM ........................................................................ 20
11.2 Example: Based on the DRV2667EVM-CT with 25-nF Piezo Module ..................................... 24
List of Figures
1
Efficiency with VBST = 30 V .................................................................................................. 5
2
Efficiency with VBST = 55 V .................................................................................................. 5
3
Efficiency with VBST = 80 V .................................................................................................. 5
4
Efficiency with VBST = 105 V ................................................................................................ 5
5
Load Regulation with VBST = 30 V .......................................................................................... 6
6
Load Regulation with VBST = 55 V .......................................................................................... 6
7
Load Regulation with VBST = 80 V .......................................................................................... 6
8
Load Regulation with VBST = 105 V ........................................................................................ 6
9
Inductance vs DC Current................................................................................................. 10
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10
SW Node Parasitic Capacitance ......................................................................................... 17
11
Inductor Charging with Parasitic Capacitance .......................................................................... 18
12
Simplified Version of the DRV8662EVM ................................................................................ 20
13
Actual Load Regulation and Boost Efficiency
14
15
16
17
2
..........................................................................
Boost Efficiency.............................................................................................................
Simplified Version of the DRV2667EVM-CT Schematic ..............................................................
Boost Voltage vs. Boost Current - DRV2667EVM-CT-80 V ..........................................................
Efficiency vs. Boost Current - VBST = 80 V ..............................................................................
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23
24
27
27
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Boost Converter Basics
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1
Boost Converter Basics
A boost converter, as the name implies, converts a low voltage power rail and boosts it to a higher
voltage. A boost converter consists of three main components: an inductor, a switch (MOSFET), and a
diode. The basic boost converter schematic is shown in the following figure.
L
D
VDD
VBST
SW
COUT
To convert the low voltage to a high voltage, the boost converter operates in two phases. In the first phase
the inductor is charged by closing the switch and forcing current through the inductor to ground. During
this period the current through the inductor increases allowing the inductor to store charge.
L
D
VDD
VBST
IL
ISW
SW
COUT
Once the inductor current reaches a maximum threshold, the switch opens and forces the inductor to
dump the stored charge through the diode and onto the output capacitor and load.
L
D
VDD
VBST
IL
ID
SW
COUT
By repeating this charge and dump process, the boost converter is able to increase the output voltage.
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DRV8662, DRV2665, and DRV2667 Boost Converter
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DRV8662, DRV2665, and DRV2667 Boost Converter
The DRV8662, DRV2665, and DRV2667 use a hysteretic boost converter design to generate the high
voltage needed to drive Piezos. This section describes the basic operating principle of the hysteretic boost
converter.
The hysteretic boost converter uses fairly simple feedback to control the timing and frequency of the
switch. The ILIM value in the following figure is the peak current through the inductor every time the switch
turns on. Once the peak current is reached, the switch opens. The peak inductor current of the DRV8662,
DRV2665, and DRV2667 is set by the resistor on the REXT pin (pin 15).
Inductor Current (A) - ISW
ILIM
REXT = K
VREF
- RINT
ILIM
Diode
Reverse-Recovery
Time (s)
1/fSW
The boost converter only switches when the output voltage (VBST) is below the final target value, meaning
that it will only switch when it needs to. Unlike a fixed-frequency boost converter design, the hysteretic
boost converter design has a continually varying switching frequency and is load-dependent. Note that the
DRV8662, DRV2665, and DRV2667 have forced switching at approximately 37 kHz.
4
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2.1
DRV8662, DRV2665, and DRV2667 Boost Converter Efficiency
The boost converter efficiency for the DRV8662, DRV2665, and DRV2667 is shown in Figure 1 through
Figure 4. The measurements were taken using the DRV8662EVM.
2.1.1
Boost Efficiency vs Boost Current
100
100
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
80
80
70
Efficiency (%)
70
Efficiency (%)
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
60
50
40
60
50
40
30
30
20
20
10
10
0
0
0
10
20
30
40
Boost Current (mA)
50
60
68.9
0
Figure 1. Efficiency with VBST = 30 V
20
30
Boost Current (mA)
40
45.6
D001
Figure 2. Efficiency with VBST = 55 V
100
100
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
80
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
80
70
Efficiency (%)
70
Efficiency (%)
10
D001
60
50
40
60
50
40
30
30
20
20
10
10
0
0
0
5
10
15
Boost Current (mA)
20
0
2
D003
Figure 3. Efficiency with VBST = 80 V
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4
6
8
10
Boost Current (mA)
12
14
16
D004
Figure 4. Efficiency with VBST = 105 V
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DRV8662, DRV2665, and DRV2667 Boost Converter
2.2
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DRV8662, DRV2665, and DRV2667 Boost Converter Load Regulation
The boost converter load regulation for the DRV8662, DRV2665, and DRV2667 is shown in Figure 5
through Figure 8. The measurements were taken using the DRV8662EVM.
Boost Regulation vs Current
35
60
30
50
25
Boost Voltage (V)
Boost Voltage (V)
2.2.1
20
15
10
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
5
40
30
20
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
10
0
0
0
10
20
30
40
Boost Current (mA)
50
60
68.9
0
Figure 5. Load Regulation with VBST = 30 V
20
30
40
Boost Current (mA)
50
60
67.6
D006
Figure 6. Load Regulation with VBST = 55 V
120
90
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
80
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
100
Boost Voltage (V)
70
Boost Voltage (V)
10
D005
60
50
40
30
80
60
40
20
20
10
0
0
0
10
20
30
40
Boost Current (mA)
50
60
68.9
Figure 7. Load Regulation with VBST = 80 V
6
0
10
D007
20
30
40
Boost Current (mA)
50
60
67.6
D008
Figure 8. Load Regulation with VBST = 105 V
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Configuring the Boost Converter
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3
Configuring the Boost Converter
This section describes the basic steps for configuring the boost converter. See the following sections for
more details.
1. Select the boost voltage.
Based on the rated voltage of the load, set the feedback resistors to the appropriate boost voltage
(Section 4).
2. Calculate the load current.
Estimate the maximum required load current to support the voltage, capacitance, and frequency of the
load (Section 5).
3. Select an inductor.
Choose an inductor with an inductance value between 3.3 µH and 22 µH, and a saturation current
rating more than 1 A (Section 6).
4. Set the current limit and REXT resistor.
Calculate the REXT resistor based on the saturation current of the inductor (Section 6.4).
5. Compare the maximum boost current.
Compare the maximum boost current with the required load current calculated in step 2 (Section 7).
6. Choose an input and output capacitor.
Input and output capacitors help reduce the effects of ripple and transient load currents on the supply
and output voltages (Section 8).
7. Verify performance.
Verify the performance of the components selected.
4
Boost Converter Output Voltage
The boost converter should be set based on the rated or maximum voltage required by the load. An
additional 5 V should be added to provide headroom when using the amplifier. Use the following equation
to calculate the boost voltage.
VBOOST = VPEAK + 5 V
Symbol
VBOOST
VPEAK
(1)
Description
Boost output voltage
Peak amplifier output voltage
Value
-
Unit
V
V
The boost output voltage is programmed by two external resistors shown in the following diagram:
VBST
DRVxxxx
R1
FB
R2
The boost feedback resistors can be calculated using equation Equation 2:
æ
R ö
V
= V ç1 + 1 ÷
FB ç
BOOST
÷
è R2 ø
(2)
NOTE: Ensure that the sum of R1 and R2 is greater than 400 kΩ to prevent large leakage currents
due to high voltages on VBOOST.
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Calculating the Load Current
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Calculating the Load Current
The peak load current of the DRV8662, DRV2665, or DRV2667 on the OUT+ and OUT– terminals can be
approximated using Equation 3.
I(load-peak) = 2π × CLOAD × VBOOST × ƒMAX
Symbol
CLOAD
VBOOST
fMAX
(3)
Description
Load capacitance
Boost voltage
Maximum output frequency
Value
–
–
–
Unit
F
V
f
Use the load parameters capacitance, boost voltage (peak output voltage), and maximum output
frequency to estimate the peak current required at the load. This provides a rough current approximation
that can be used to estimate the inductor size for the boost design.
6
Selecting an Inductor
The inductor plays a critical role in the performance of the DRV8662, DRV2665, and DRV2667, so
selecting and testing a suitable inductor is important to ensure the best performance.
An inductor can be described with relatively few parameters. The following table shows the typical
parameters listed in an inductor datasheet:
Part Number
Inductance
(µH)
CIG22E3R3SNE
3.3
DC
Resistance
(Ω)
0.200
ISATURATION
(A)
IRMS
SRF
(MHz)
1.1
1.30
42
DEFINITIONS
Inductance – the primary functional parameter of an inductor.
DC Resistance (DCR) – the resistance in the inductor due to the wire
ISATURATION or Saturation Current – the peak current flowing through the inductor that causes the inductance to drop due to core
saturation.
IRMS or RMS Current or Thermal Current – the amount of continuous RMS current flowing through the inductor that causes the
maximum allowable temperature rise.
SRF or Self Resonant Frequency – the frequency at which the inductance of the inductor winding resonates with the capacitance
of the inductor winding.
To help narrow the number of inductors quickly, begin by looking at these three parameters:
spaceer
1. Inductance – the range of recommend inductances is from 3.3 µH to 22 µH.
2. Saturation Current (ISAT, 30% decrease in inductance due to DC current) – saturation current should
typically be above 1 A for most applications, but will vary depending on the load. Use the examples at
the end of this document as a reference for choosing an inductor.
3. RMS Current or Thermal Limit – the RMS current is less of an issue for haptic applications because of
the low duty cycle of operation; however, in other applications where continuous operation is likely, be
sure to select an appropriate RMS current rating.
spaceer
Inductor current ratings are always the biggest source of confusion when selecting an inductor, because
there are multiple, non-standardized current ratings to look for. See Section 6.5 for more information on
inductor current ratings.
8
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Selecting an Inductor
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See the following sections for more information on each parameter.
NOTE: The inductor will see high voltages (VBST – VDD) during normal operation. Ideal inductors do
not have a voltage rating and thus most manufacturers will not publish a voltage rating;
however, certain inductor core materials have voltage limitations. Please contact the
manufacturer and ensure that the inductor material can operate at high voltages.
6.1
Inductance Rating
The inductance sets the maximum switching frequency of the boost converter. The general trade off with
inductances between 3.3 µH and 22 µH are:
• Larger inductances (10 µH or greater)
– Advantage: Cause the boost converter to run at a lower switching frequency meaning less
switching losses.
– Disadvantage: Larger values typically have higher series resistance and lower saturation currents,
requiring physically larger inductors.
• Smaller inductances (Less than 10 µH)
– Advantage: Typically have higher saturation currents and are a better choice for maximizing output
current of the boost converter per inductor area.
– Disadvantage: Higher switching frequencies can lead to more losses. Switching losses are not a
major concern in most applications, but if thermal dissipation is a concern because of a small PCB
or extreme temperatures, then consider using a larger inductance.
The approximate switching frequency can be calculated using Equation 4:
æ 1
ö
1
ƒ switching = ILIMIT ´ L ç
+
÷
è VIN VIN - VBOOST ø
Symbol
fswitching
VIN
VBOOST
ILIMIT
(4)
Description
DRVxxx switching frequency
Minimum VDD voltage applied to the DRVxxxx
Maximum boost output voltage
Current limit set by the DRVxxxx REXT resistor
Value
–
–
–
–
Unit
Hz
V
V
Ap
Tip
Smaller inductances (3.3 – 4.7 µH) are often preferred in space-constrained applications because of their
size, higher saturation current, and ability to deliver more charge to the load.
6.2
Saturation Current Rating
Saturation current is the second-most important parameter of an inductor when using a hysteretic boost
converter. Inductor saturation current is typically measured as the peak current that causes the inductance
to decrease by 30%. This is the maximum operating current of the inductor.
In Figure 9, the saturation current for the 3.3-µH inductor is approximately 400 mA, which is the current
that causes the inductance to reduce to 2 µH or by about 30%. A graph like Figure 9 can be used to
determine and verify the saturation current rating of a specific inductor.
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Selecting an Inductor
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Figure 9. Inductance vs DC Current
The inductor saturation current value affects two things in the DRV8662, DRV2665, and DRV2667 boost
design:
1. The amount of current that can be delivered to the load; the larger the saturation current, the larger
amount of current can be delivered to the load.
2. The value of the current limit resistor (REXT) for the DRV8662, DRV2665, and DRV2667. It should be
set equal to or less than the saturation current of the inductor. Section 6.4 describes how to choose the
correct current limit resistor.
ILIM_DRVxxxx ≤ ISaturation_inductor
(5)
Remember that the current limit on the DRV8662, DRV2665, and DRV2667 is not a safety mechanism,
but a threshold to signal when the boost switch should open.
Tip
Often the saturation current is listed on the front page of an inductor datasheet; however, it is good
practice to verify this value using an “Inductance vs DC Current” graph similar to Figure 9.
6.3
Thermal Current Rating
The thermal current rating is typically measured as the RMS current that causes some fixed rise in
temperature (typically 20 – 30°C). Do not exceed this RMS current value.
Calculate the RMS current through the inductor using Equation 6. This is an approximation based on a
triangular RMS waveform.
I
IRMS = LIMIT
3
(6)
Symbol
IRMS
ILIMIT
10
Description
Inductor RMS Current
Peak current limit set by the DRVxxxx REXT resistor
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Value
–
–
Unit
ARMS
Ap
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Selecting an Inductor
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6.4
Choosing REXT
The resistor on the REXT pin is found using Equation 7:
æ V
ö
REXT = ç K REF ÷ - RINT
è ILIM ø
Symbol
K
VREF
RINT
ILIMIT
(7)
Description
Value
10500
1.35
60
–
Internal reference voltage on REXT
Internal resistance
Inductor current limit from inductor datasheet
Unit
V
Ω
Ap
The following graph shows the relationship between REXT and the current limit.
2.5
VDD = 3.6 V
Inductor = 3.3 PH
Not Loaded
Gain = 41 dB
ILIM - Inductor Current (A)
2
1.5
1
0.5
0
6
8
10
12
14
16
18
20
REXT (k:)
22
24
D001
D009
Use the REXT resistor to set the current limit for the inductor. This can serve two purposes:
1. Prevent Inductor Saturation – the current limit must be set equal to or lower than the inductor
saturation current. If the inductance were to drop below 2.2 µH because of saturation, the boost
converter may have difficulty regulating the output voltage.
2. Limit Peak Battery Current – the resistor can help limit the maximum current from the battery;
however, a lower current limit decreases the amount of current delivered to the load each switch cycle.
Tip
The best way to lower peak battery currents is by adding a bulk input capacitor.
6.5
What to Avoid: Using Incorrect Inductor Current Ratings
Manufacturers provide multiple current ratings, because boost converters typically operate with a very high
peak-to-average ratio. This means there are very large current spikes, but the average current flowing
through the inductor is very low. Because these are very different current operating points, the inductor
manufacturers provide two current ratings to help make inductor selection easier: one for peak currents
and one for RMS currents. The thermal current rating indicates maximum RMS currents, while the
saturation current rating indicates maximum peak currents that could cause core saturation.
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Selecting an Inductor
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While inductor saturation current and thermal rating are readily available in most inductor datasheets, they
are often obscured by manufacturer operating conditions and nomenclature. In some cases, one
manufacturer may refer to one parameter as the rated current and another manufacturer may refer to a
completely different parameter as the rated current. The point of this section is to dispel any confusion and
clarify what to look for when choosing the saturation current and thermal rating.
The following parameters are taken from two different inductor datasheets compared side-by-side. Both
are good inductors, but only one has sufficient current ratings for a 1-A boost design.
Inductor 1
Murata – LQM2MPN3R3MG0L
Inductor 2
TDK – VLS3010
Datasheet Front Page “Rated DC Current”
Rated DC Current: 1.2 A
Datasheet Front Page “Rated DC Current”
Rated DC Current: 1.1-1.3 A
Datasheet Graph Data
ISAT : 400 mA
ITHERMAL : 1.2 A
Datasheet Graph Data
ISAT : 1.1–1.3 A
ITHERMAL : 1.5 A
Inductance vs Peak Current
Inductance vs Peak Current
Inductor 1 (on the left) has a Rated DC Current of 1.2 A and Inductor 2 has a Rated DC Current between
1.1–1.3 A. Initially, both seem to work if our boost configuration requires 1 A; however, they are not equal.
If you notice the two parameters underneath “Rated DC Current” labeled ISAT and ITHERMAL are different.
You can see that Inductor 1 has a thermal rating of 1.2 A, but only a 400 mA saturation current. Inductor 2
has a rated current of 1.1–1.3 A and a saturation current of 1.1–1.3 A and a thermal rating of 1.5 A. It is
apparent from this comparison that “Rated DC Current” does not always refer to the same parameter, so
be careful when choosing an inductor based on its current ratings.
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7
Calculate the Maximum Boost Current
The maximum support boost current is estimated using Equation 8 and Equation 9.
First calculate the maximum duty cycle of the hysteretic converter. Equation 8 assumes worst-case duty
cycle during maximum current.
Vin _ min ´ h
D = 1Vboost
(8)
Symbol
D
Vin_min
ETA (η)
Vboost
Description
Boost Duty Cycle
Minimum VDD
Boost Efficiency
Boost Voltage
Value
–
–
60
–
Unit
–
V
%
V
If the approximate and the actual measurements do not align perfectly, then you can adjust η, which is the
efficiency of the boost converter.
I
æ
ö
Iboost _ max = ç Ilimit - limit ÷ ´ (1 - D )
2
è
ø
Symbol
Iboost_max
Ilimit
D
(9)
Description
Maximum boost current
Boost current limit (REXT)
Boost duty cycle
Value
–
–
–
Unit
A
A
–
Iboost_max is the maximum boost current the boost can deliver to the amplifier. Compare this current to the
current calculated for the load current. The boost current should be higher than the load current.
8
Output Capacitor Selection
The output capacitor is important for decreasing output voltage ripple and reducing the effects of load
transients on the boost voltage. The boost output voltage can be configured from 20 V up to 105 V, so the
boost output capacitor must have a voltage rating equivalent to the boost output voltage or higher. A 250V rated, 100-nF capacitor of X5R or X7R type is recommended for a boost converter voltage of 105 V.
The selected capacitor should have a minimum working capacitance of 50 nF.
To estimate the absolute minimum capacitance required, use Equation 10. Typically the DRVxxxx devices
operate with a switching frequency between 800 kHz to 1 MHz. To include additional margin for the device
loop response, it is best to use one-sixth of the switching frequency (ƒ).
DI
C=
2 ´ fSW ´ VDROOP
(10)
Symbol
DELTA I
VDROOP
fSW
Description
Boost transient current
Maximum boost output voltage droop
Boost switching frequency
C
Output capacitor
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Value
0 – 0.070
–
Typically
800kHz-1MHz
–
Unit
mA
V
Hz
F
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Input Capacitor Selection
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Tip
A guideline for ceramic capacitors: the de-rated capacitance is approximately equal to the rated
capacitance multiplied by one minus the applied voltage over the rated voltage.
Cde-rated = Crated (1 – Vapplied/Vrated)
For example, when 50 V is applied to a 100-V rated capacitor, the capacitance will decrease by about
50%. Most capacitor vendors provide a capacitance versus voltage curve for reference.
9
Input Capacitor Selection
The input capacitor provides current to the DRVxxxx when there are large current transients during startup
and heavy load periods.
Charge Required from
Bulk Capacitor
ITransient
Current Supplied by Host
t
tr
When a bulk input capacitor is included, the following diagram shows that the battery actually sees a
filtered version of ILIM.
IAVG
L1
RBAT
ISW
SW
CBULK
VBAT
When the boost converter enters a heavy load condition or during a startup sequence, the switching
frequency reaches a maximum value set by the slope of the charge/discharge curve and the ILIM value.
Inductor Current (A) - ISW
ILIM
IAVG
Time (s)
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Calculating the exact IAVG value is rather difficult, but as long as the bulk decoupling capacitance in the
system is sufficiently high, the average current drawn from the battery will be less than one-half of ILIM as a
general guideline.
To estimate the bulk input capacitor value, use Equation 11.
CBULK =
Symbol
CBULK
ITR
LTrace
VDROOP
1.21´ ITR 2 ´ L Trace
VDROOP2
(11)
Description
Minimum VDD bulk capacitance required
Input transient current (maximum is ILIMIT)
Input trace inductance (estimate 50 nH, if unknown)
Maximum boost output voltage droop
(Ex.: 3.6 V – 0.1 V = 3.5 V, 0.1 V is the allowable droop)
Value
–
–
–
–
Unit
F
A
H
V
This equation was derived from the RLC circuit formed by the trace resistance and inductance combined
with the bulk capacitance and capacitor ESR.
RTRACE
LTRACE
ESR
VDD
+
VIN
CBULK
CAP
Keep in mind that it is difficult to calculate the exact input current required for the DRVxxxx, so a minimum
capacitance between 22 µF and 47 µF is recommend.
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PCB Layout
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PCB Layout
Use the following guidelines for PCB layout.
CVREG
GND
IN-
IN+
SCL
SDA
VREG
CPUMP
CVDD
REXT
PUMP
REXT
VDD
OUT-
FB
OUT+
GND
PVDD
GND
VBST
R1
R2
VBST
NC
SW
SW
GND
GND
CVBST
L1
CBULK
VDD
GND
1. Place the feedback resistors, R1 and R2, close to the FB pin. If the resistors are placed far from
the FB pin, then noise can enter the FB trace and causes instability or unwanted ripple.
2. Protect the feedback trace (Red trace). Isolate the feedback trace on a different layer and shield
from the SW node using a ground plane.
3. Minimize the size of SW node trace. The trace between the SW pins and the inductor terminal
should be as physically small as possible. A large switch node can add parasitic capacitance and slow
the switching frequency of the boost converter, preventing it from delivering the required current. A
small SW node also helps prevent radiated emissions.
4. Place the bulk input capacitor near the inductor. The bulk input capacitor provides the current
during quick transient current spikes. Closer means less resistance between the bulk capacitor and
inductor.
5. Place the output capacitor near the VBST pin.
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10.1 What to Avoid: Incorrect Inductor Placement
In space-constrained applications, PCB real estate often trumps correct component placement. While
layout guidelines were provided in the previous section, this section specifically covers inductor placement
in more detail. The inductor should be placed as close to the SW node as possible; this helps reduce
parasitic resistances, inductances, and most importantly, parasitic capacitances.
L
D
VDD
VBST
CParasitic
Q
COUT
Figure 10. SW Node Parasitic Capacitance
What happens if the SW node has too much parasitic capacitance? Two things:
1. The switching frequency decreases due to a higher RC constant, resulting in less current delivery to
the load
2. The SW node stores charge in the parasitic capacitor, resulting in less current delivery to the load
The switching frequency changes as a result of the charging and discharging of the parasitic capacitor
each cycle. Figure 11 shows how the current charge cycle of the inductor changes when parasitic
capacitance is present.
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PCB Layout
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Difference in
Switching Period
ILIM
Current
Without
Parasitic
Capacitance
Current
ILIM
With Parasitic
Capacitance
Time
Figure 11. Inductor Charging with Parasitic Capacitance
Figure 11 shows that the period of the inductor charge cycle increases with parasitic capacitance. This
longer charge cycle results in a slower boost-switching frequency.
In addition to a slower switching frequency, the parasitic capacitance on the SW node consumes charge.
If, for example, the DRVxxxx boost design normally supports 10-mA current and there is parasitic
capacitance present, a significant portion – sometimes up to 1 mA of the current – can be consumed by
the parasitic capacitance. This means that 10% of the current intended for the load is being consumed by
the parasitic capacitance.
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1A
9mA
L
D
VDD
VBST
CParasitic
Q
1mA
COUT
Both of these issues can have drastic effects on the output waveform. To identify switch node parasitic
capacitance, look for continuous switching on the SW pin. During normal operation, the switching turns on
and off depending on the current required; however, with parasitic capacitance the SW node often
continuously switches to recover for lost switch cycles and current.
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Examples
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Examples
This section provides two examples based on the DRV8662EVM and DRV2667EVM-CT and a 25-nF
Piezo actuator.
11.1 Example: Based on the DRV8662EVM
The DRV8662EVM is the evaluation module for the DRV8662 analog high-voltage Piezo haptic driver. The
board includes a MSP430 to control the DRV8662.
Figure 12 illustrates a simplified version of the DRV8662EVM.
L1
4.7uH / 1.8A
3.0 V to 5.5 V
110uF
DRV8662
VDD
SW
VREG
VBST
C1
0.1uF
C5
0.1uF / 250V
PVDD
FB
GAIN0
GAIN
R1
768k
R2
13k
GAIN1
Analog
Input
IN+
OUT+
IN-
OUT-
VPUMP
C2
0.1uF
REXT
Piezo
Actuator
25nF
R5
13k
GND
Figure 12. Simplified Version of the DRV8662EVM
The following table contains the primary components of the design. The board was configured for a
SEMCO Piezo actuator.
Reference Designator
Value
Manufacturer Part #
Manufacturer
L1
4.7 µH / 1.8 ASAT
LPS4018-472MLB
Coilcraft
Piezo Actuator
25 nF at 150 V, 200 Hz
PHAT423535XX
SEMCO
The full design documents including the schematic, layout, and BOM can be found in the DRV2667EVMCT User’s Guide (SLOU323).
11.1.1
Configure the Boost Voltage
The values in this section were calculated using the DRV Design Equations excel file available on
www.ti.com.
The SEMCO actuator has a rated voltage of 150 Vpp, which is defined as the voltage required for 100%
vibration. The following values were entered into the design equation spreadsheet:
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Actuator Properties
Actuator Model: SEMCO 42mm
Cactuator = 25 nF
Vactuator = 150 Vpp
Next, the settings section was completed to reflect a typical board environment. Vboost was set to 80 V
using Equation 12. The additional 5 V provides headroom for the amplifier.
V
Vboost = actuator + 5V
(12)
2
Settings
Vin = 3.6 V
Vboost = 80 V
ƒin_min = 100 Hz
Vin_min = 3 V
Vforward = 0.7 V
ƒin_max = 300 Hz
Vin_max = 5 V
µboost = 70% (efficiency)
Iripple_% = 20%
To configure the boost voltage, R1 and R2 must be set so that the voltage divider equals 1.32 V. The
values are calculated using the Feedback Resistors section of the design equations excel file. R1 was
selected to be 768 kΩ so that the total resistance of the resistor divider is large, preventing high leakage
currents. R2 is calculated automatically and then R2_actual is the closest standard resistor value. Finally, the
boost voltage is back-calculated to show the expected boost voltage.
Feedback Resistors
Vfeedback = 132 V
R1 ´ Vfeedback
R2 =
Vboost - Vfeedback
VBST
R1 = 768 kΩ
R2 = 12.885 kΩ
R2_actual = 13 kΩ
DRVxxxx
R1
FB
R2
æ
R1
Vboost _ actual = Vfeedback ç 1 +
ç R2 _ actual
è
11.1.2
ö
÷
÷
ø
Vboost_actual = 79.302 V
Configure the Inductor Current
The Piezo load requires approximately 1.88 mA of average current based on the following equation:
Piezo Actuator Current Requirements (Approximate)
Iout_max = 2π × Cactuator × Vboost × ƒin_max
Iout_max = 1.884956 mA
Maximum required amplifier current (estimate)
æ Vboost ö
Ibat _ max = 2p ´ ƒin _ max ´ Cactuator ´ Vboost ç
÷
è Vin ´ mboost ø
Ibat_max = 139.626 mA
Maximum VBAT current
This means that the inductor current limit (REXT) must be sufficiently large to support 1.88 mA.
The DRV8662EVM inductor is already fixed, but from here forward you may choose a different inductor
that fits your load requirements. The maximum boost current with 80 Vp boost voltage is 20.3 mA based
on the 4.7 µH / 1.8 A Coilcraft inductor selected using the following equations:
Boost Converter Current Capacity (Approximate)
Vin _ min ´ h
D = 1Vboost
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D = 0.978
Estimated duty cycle at maximum frequency
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Boost Converter Current Capacity (Approximate)
I
æ
ö
Vboost _ max = ç Ilimit - limit ÷ ´ (1 - D )
2
è
ø
Iboost_max = 20.289 mA
Maximum boost output current estimate
350
Vboost (V)
25
50
75
100
300
Iboost_max (mA)
250
200
150
100
50
IOUT (mA)
64.92
32.46
21.64
16.23
0
0
5
10
15
20
25
30
35
40
45
50
55
60
VBoost (V)
65
70
75
80
85
90
95
100
105
110
D013
The inductor on the DRV8662 is the LPS4018-472MLB from Coilcraft. The saturation current is 1.8 A and
the current limit due to continuous current is 1.8 A. Enter these values into the Boost Current Limit
Resistor section of the design equations excel file.
Boost Current Limit Resistor
Inductor Model Coilcraft LPS4018-472MLB
L = 4.7 µH
Isat = 1.8 A
Imin = min (ISAT, Ithermal)
Imin = 1.8 A
Ilimit = Imin
Ilimit = 1.8 A
Ithermal = 1.8 A
æ
V
Rext = ç k ´ ref
Ilimit
è
Rext_actual = 7.87 kΩ
Ilimit _ actual
ö
÷ - Rint
ø
Vref
= k´
Iext _ actual + Rint
Rext = 7.815 kΩ
Ilimit_actual = 1.788 A
The resulting current-limit resistor is calculated as 7.815 kΩ in Rext. The nearest standard resistor value is
7.87 kΩ shown in the Rext+actual box. The current limit was then back-calculated to show the expected
current limit, which is less than the saturation current.
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11.1.3
Boost Performance Results
The actual load regulation and boost efficiency for the DRV8662EVM are shown in Figure 13. The load
regulation is a measure of the boost voltage versus the boost output current. In Figure 13, the voltage
regulation begins to drop above approximately 31 mA of boost output current.
90
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
80
70
Boost Voltage (V)
60
50
40
30
20
10
0
0
10
20
30
40
Boost Current (mA)
50
60
69.6
D014
D014
Figure 13. Actual Load Regulation and Boost Efficiency
The boost efficiency is shown in Figure 14. The efficiency data was removed after the boost voltage
decreased by more than 5 V.
100
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
80
Efficiency (%)
70
60
50
40
30
20
10
0
0
5
10
15
Boost Current (mA)
20
25
D015
D015
Figure 14. Boost Efficiency
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11.2 Example: Based on the DRV2667EVM-CT with 25-nF Piezo Module
The DRV2667EVM-CT is the evaluation module for the DRV2667 high-voltage piezo haptic driver with
digital front-end. The kit includes all the components to evaluate and test the DRV2667 including a Piezo
haptics module manufactured by Samsung Electric and Mechanical.
Figure 15 is a simplified version of the DRV2667EVM-CT schematic.
L1
3.3uH / 1.1A
110uF
DRV2667
VDD
SW
VREG
VBST
C1
0.1uF
C5
0.1uF / 250V
PVDD
FB
SDA
I2C
R1
768k
R2
13k
SCL
Analog
Input
IN+
OUT+
IN-
OUT-
VPUMP
C2
0.1uF
REXT
Piezo
Actuator
25nF
R5
13k
GND
Figure 15. Simplified Version of the DRV2667EVM-CT Schematic
The following table contains the primary components of the design:
Reference Designator
Value
Manufacturer Part #
Manufacturer
L1
3.3 µH / 1.1 ASAT
VLS3010ET-3R3M
TDK
Piezo Actuator
25 nF at 150 V, 200 Hz
PHAT423535XX
SEMCO
The full design documents including the schematic, layout, and BOM, can be found in the DRV2667EVMCT User’s Guide (SLOU323).
11.2.1
Configure the Boost Voltage
The values in this section were calculated using the DRV2667 Design Equations excel file available on
www.ti.com.
The SEMCO actuator has a rated voltage of 150 Vpp, which is defined as the voltage required for 100%
vibration. The following values were entered into the design equation spreadsheet:
Actuator Properties
Actuator Model: SEMCO 42mm
Cactuator = 25 nF
24
Vactuator = 150 Vpp
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Next, the settings section was completed to reflect a typical board environment. Vboost was set to 80 V
using Equation 13. The additional 5 V provides headroom for the amplifier.
V
Vboost = actuator + 5V
(13)
2
Settings
Vin = 3.6 V
Vboost = 80 V
ƒin_min = 100 Hz
Vin_min = 3 V
Vforward = 0.7 V
ƒin_max = 300 Hz
Vin_max = 5 V
µboost= 70% (efficiency)
Iripple_% = 20%
To configure the boost voltage, R1 and R2 must be set so that the voltage divider equals 1.32 V. The
values are calculated using the Feedback Resistors section of the design equations excel file. R1 was
selected to be 768 kΩ so that the total resistance of the resistor divider is large, preventing high leakage
currents. R2 is calculated automatically and then R2_actual is the closest standard resistor value. Finally, the
boost voltage is back-calculated to show the expected boost voltage.
Feedback Resistors
Vfeedback = 1.32 V
R1 ´ Vfeedback
R2 =
Vboost - Vfeedback
R1 = 768 kΩ
R2 = 12.885 kΩ
R2_actual = 13 kΩ
VBST
DRVxxxx
R1
FB
R2
æ
R1
Vboost _ actual = Vfeedback ç 1 +
ç R2 _ actual
è
11.2.2
ö
÷
÷
ø
Vboost_actual = 79.302 V
Configure the Inductor Current
The Piezo load requires approximately 1.88 mA of average current based on the following equation:
Piezo Actuator Current Requirements (Approximate)
Iout_max = 2π × Cactuator × Vboost × ƒin_max
Iout_max = 1.884956 mA
Maximum required amplifier current (estimate)
æ Vboost ö
Ibat _ max = 2p ´ ƒin _ max ´ Cactuator ´ Vboost ç
÷
è Vin ´ mboost ø
Ibat_max = 139.626 mA
Maximum VBAT current
This means that the inductor current limit (REXT) needs to be sufficiently large to support 1.88 mA.
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The DRV2667EVM-CT inductor is already fixed, but from here forward you may choose a different
inductor that fits your load requirements. The maximum boost current with 80 Vp boost voltage is 12.4 mA,
based on the 3.3-µH /1.1-A TDK inductor selected using the following equations:
Boost Converter Current Capacity (Approximate)
Vin _ min ´ h
D = 1Vboost
D = 0.978
Estimated duty cycle at maximum frequency
I
æ
ö
Iboost _ max = ç Ilim it - limit ÷ ´ (1 - D )
2
è
ø
Iboost_max = 12.400 mA
Maximum boost output current estimate
250
Vboost (V)
25
50
75
100
Iboost_max (mA)
200
150
100
IOUT (mA)
39.68
19.84
13.23
9.92
50
0
0
5
10
15
20
25
30
35
40
45
50
55
60
VBoost (V)
65
70
75
80
85
90
95
100
105
110
D016
D016
The inductor on the DRV2667EVM-CT is the VLS3010ET-3R3M from TDK. The saturation current is 1.1 A
and the current limit due to continuous current is 1.5 A. Enter these values into the Boost Current Limit
Resistor section of the design equations excel file.
Boost Current Limit Resistor
Inductor model VLS3010ET-3R3M
L = 3.3 µH
Isat = 1.1 A
Imin = min (ISAT, Ithermal)
Imin = 1.1 A
Ilimit = Imin
Ilimit = 1.1 A
æ
V ö
Rext = ç k ´ ref ÷ - Rint
Ilimit ø
è
Vref
Ilimit _ actual = k ´
Iext _ actual + Rint
Rext = 12.83 kΩ
Ithermal = 1.5 A
Rext_actual = 13 kΩ
Ilimit_actual = 1.085 A
The resulting current limit resistor is calculated as 12.83 kΩ in Rext. The nearest standard resistor value is
13 kΩ shown in the Rext+actual box. The current limit was then back-calculated to show the expected current
limit, which is less than the saturation current.
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11.2.3
Boost Performance Results
The actual load regulation and boost efficiency for the DRV2667EVM-CT are shown in Figure 16. The
load regulation is a measure of the boost voltage versus the boost output current. In Figure 16, the voltage
regulation begins to drop above approximately 10 mA of boost output current. Comparing the voltage
regulation of the DRV2667EVM-CT to the DRV8662EVM, the DRV2667EVM-CT boost has much less
current capacity because of the smaller inductor and current limit resistor settings.
90
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
80
70
Boost Voltage (V)
60
50
40
30
20
10
0
0
10
20
30
40
Boost Current (mA)
50
60
69.6
D017
D017
Figure 16. Boost Voltage vs. Boost Current - DRV2667EVM-CT-80 V
The boost efficiency is shown in Figure 17. The efficiency data was removed after the boost voltage
decreased by more than 5 V.
100
VDD = 3.6 V
VDD = 4.7 V
VDD = 5.5 V
90
80
Efficiency (%)
70
60
50
40
30
20
10
0
0
1
2
3
4
5
Boost Current (mA)
6
7
8
9
10
D018
D018
Figure 17. Efficiency vs. Boost Current - VBST = 80 V
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