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Page 4-1
Buck-Boost Converters for Portable Systems
Michael Day and Bill Johns
ABSTRACT
Fig. 1 graphically depicts the design challenge
of producing 3.3 V from a Li-ion battery. The
voltage-discharge profile of a typical Li-ion
battery starts at 4.2 V when the battery is fully
charged and no current is flowing. When time, t, is
less than 0 minutes, the battery is unloaded and
has no current flow. At t = 0, a load is applied to
the battery, and its voltage drops due to its internal
impedance and protection circuitry. The battery
voltage drops gradually until it reaches about 3.4 V,
where it starts to drop rapidly as it nears the end of
its discharge cycle. The Li-ion battery’s discharge
curve presents the designer with two challenges:
how to generate a regulated output voltage over
the full discharge curve, and how to fully utilize
the battery’s stored energy. When the battery’s
discharge curve is both above and below the
power supply’s required output voltage, the
system must be capable of both buck (step-down)
and boost (step-up) conversion. Figs. 2 and 3
show that the discharge curves of dual-cell
alkaline and NiMH batteries present the same
problem for system voltages between 1.8 and 3.0 V.
This design issue is not new, and designers
have solved it using many solutions over the
years. These include cascaded buck and boost
converters, single low-dropout regulators (LDOs),
3.5
3
Voltage – V
I. DESIGN CHALLENGE
2.5
2
1.5
1
0
100
50
150
200
250
300
Time – min
Fig. 2. 500-mA discharge curve of dual-cell
NiMH battery.
3.5
4.3
3
3.9
3.7
Voltage – V
Battery Voltage – V
4.1
3.5
3.3
3.1
2.9
2.5
2
1.5
2.7
2.5
–5
1
45
95
145
0
195
Fig. 1. 500-mA discharge curve of 1650-mAh
18650 Li-ion battery.
50
100
150
Time – min
Discharge Time – min
Fig. 3. 500-mA discharge curve of dual-cell
alkaline battery.
Workbook 4-1
200
Topic 4
This topic presents several solutions to a typical problem encountered by many designers of portable
power—how to produce 3.3 V from a single-cell Li-ion battery. The advantages and disadvantages of each
solution are provided along with measured data on overall battery runtime. This data helps the designer
select the best overall solution for specific system requirements. This topic also provides a detailed
comparison of the Texas Instruments (TI) fully integrated TPS63000 buck-boost converter and other buckboost solutions. The efficiency, overall ease of use, and operation in “transition mode”—when the
converter switches from buck to boost mode—are discussed.
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Page 4-2
Topic 4
single buck converters, single-ended primaryinductance converters (SEPICs), and buck-boost
converters. The authors discuss some of the more
popular solutions, compare advantages and
disadvantages, and then show how improvements
in integrated power-converter technology provide
superior performance that eliminates the
disadvantages of classical buck-boost converters.
A. Solution 1: Cascaded Buck and Boost
Converter
Fig. 4 shows a popular solution, cascaded
buck and boost converters that are separate and
discrete. A boost converter can follow a buck
converter, or vice versa. Most designers choose to
have the boost converter follow the buck
converter. The buck converter operates directly
from the battery voltage and generates an
intermediate voltage such as 1.8 V. A separate
boost converter operates from the intermediate
voltage to generate the regulated 3.3-V output.
This architecture has several advantages, such as
providing the intermediate voltage, which may
already be needed in the system. It also utilizes
100% of the battery’s capacity when the buck
converter operates down to the battery’s end-ofdischarge voltage.
A disadvantage of this solution is the two
stages of power conversion. The effective powerconversion efficiency is the product of the
individual buck and boost converters’ efficiencies.
The typical efficiency of buck and boost converters operating at these voltage levels is 90%; so
the effective 3.3-V conversion efficiency is
90% • 90% = 81%, which is relatively low for a
switching converter. Additional disadvantages of
this architecture are the higher cost associated
with two separate converters, and the increased
parts count and solution size that are prohibitive in
small, portable products.
B. Solution 2: Single Buck Converter
A single buck converter is a solution that
many designers overlook. Although this topology
has not gained much widespread use, it has clear
benefits that should not be ignored. Designers
typically dismiss this solution when they realize it
cannot provide a regulated output voltage over the
battery’s full discharge curve. When a buck
converter’s input voltage drops below its
programmed output voltage, the output voltage
falls out of regulation. Despite this drawback, this
solution is very efficient, approaching 96% in
many cases. Although the single buck converter
cannot utilize 100% of the battery’s energy, the
energy it does use is converted at a very high
efficiency. This keeps the overall system runtime
comparable to other solutions with lower
efficiency. This topology’s solution size is smaller
and less expensive compared to the cascaded buck
and boost solution.
To understand this topology’s limitation,
consider what happens when the battery voltage
drops close to the converter’s output voltage.
Many buck converters enter a 100% duty-cycle
mode in this condition. The converter stops
switching and passes the input voltage directly
through to the output. Fig. 5 shows that in 100%
duty-cycle mode, the power metal-oxidesemiconductor field-effect transistor (MOSFET)
turns on fully and behaves like a resistor with a
value equal to the MOSFET’s on resistance,
RDS(on). The catch diode—or, in the case of a
synchronous converter, the synchronous
MOSFET—turns off and becomes an open
circuit. The output voltage equals the input
VIN
VOUT
+
–
1.8-V
VOUT
100% Duty-Cycle Mode
VIN
+
1.8-V
Buck
Converter
3.3-V
Boost
Converter
–
Fig. 4. Cascaded buck and boost converter.
3.3-V
VOUT
VIN
VOUT
+
–
MOSFET/
RDS(on)
Inductor
DCR
Fig. 5. Buck converter and 100% duty-cycle mode.
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Page 4-3
voltage minus a voltage drop across the converter.
This voltage drop is a function of the power
MOSFET’s RDS(on), the output inductor’s DC
resistance (DCR), and the load current, IOUT. The
converter voltage drop in 100% duty-cycle mode
sets the minimum allowable battery voltage
required to keep the output voltage in regulation.
This minimum battery voltage is calculated as
⎛ 100% − VOUT(Tol) ⎞
VBAT ( min ) = VOUT ( nom ) ⎜
⎟⎠ +
100
⎝
(
)
+ R DS(on ) + DCR IOUT ,
where VOUT(nom) is the nominal 3.3-V setpoint,
RDS(on) is the power MOSFET’s on resistance,
DCR is the output inductor’s DC resistance,
V OUT(Tol) is the minimum allowable outputvoltage tolerance, and IOUT is the converter’s 3.3-V
output current.
Using a typical TPS62020 design, an
appropriately sized inductor, and a maximum 3%
VOUT drop yields
⎛ 100% − 3% ⎞
VBAT ( min ) = 3.3 V ⎜
⎟⎠
⎝
100
+ (0.210 Ω + 0.05 Ω) 500 mA
= 3.331 V.
When the battery voltage drops to VBAT(min), the
system must shut down so it does not run with the
3.3-V rail below its minimum tolerance, which
could corrupt the data. With this topology, the
system shuts down even though the battery still
contains anywhere from 5 to 15% of its rated
capacity. The actual unused capacity depends on
many factors, including component resistances,
load currents, battery age, and ambient
temperature. Depending on the desired battery
shutdown voltage, the buck-only topology
provides superior efficiency with a reasonable cost
and board area.
C. Solution 3: LDO
Another solution that doesn’t get much
widespread use is the low-dropout regulator
(LDO). An LDO typically provides the smallest
solution size and is usually the cheapest. Many
designers dismiss the LDO due to its low
efficiency, but close examination of an LDO’s
efficiency equation shows a respectable efficiency
for this application:
Eff =
PWR OUT VOUT • IOUT VOUT
=
=
,
PWR IN
VIN • I IN
VIN
when IOUT ≈ IIN.
Since the voltage of the fully charged Li-ion
battery starts at 4.2 V, the LDO’s efficiency starts
at 3.3 V/4.2 V = 78% and increases as the battery
voltage drops. The average discharge voltage is
approximately 3.7 V, so the LDO’s average
efficiency is 3.3 V/3.7 V = 89.2%.
Like the single buck converter, the LDO is not
capable of utilizing the entire battery capacity
because it maintains regulation only when its
input voltage is higher than the output voltage.
When the input voltage drops near the output
voltage, the LDO enters dropout. Please see
Reference [1] for more information on this
subject. As an example, if the LDO has a dropout
voltage of 0.15 V, the 3.3-V output voltage starts
to drop when the battery voltage falls below
3.3 V + 0.15 V = 3.45 V. Even with this drawback,
the LDO’s small size and low price make it an
attractive solution with the right system
requirements.
D. Solution 4: SEPIC
The single-ended primary-inductance converter
(SEPIC) provides a buck and boost function from
a single circuit. This converter topology resembles
a flyback converter that uses two inductors and a
flying capacitor to provide a regulated output
voltage from input voltages that are above and
below the output voltage. Like the cascaded buck
and boost topology, the SEPIC is capable of
utilizing the battery’s full capacity but also suffers
from relatively low efficiency. Many designers do
not use a SEPIC due to its solution size, cost, and
complexity of design.
Workbook 4-3
Topic 4
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VIN
VIN
+
–
L2
L1
CIN
VOUT
C1
+
D1
COUT
+
–
VOUT
RL
+
Q1
D
Topic 4
Fig. 6. SEPIC circuit.
VIN
Fig. 6 shows the SEPIC circuit topology and
Fig 7a shows current flow for the following
discussion. During the power switch’s on time, Q1
is on and current builds up in the two inductors.
L1 charges like a boost converter while L2
charges though the flying capacitor, C1. During
this time, D1 is off and the output capacitor, COUT,
supplies the load current. During the power
switch’s off time, D1 is on, and stored energy from
L1 and L2 is transferred to the load and output
capacitor through the flying capacitor. Also,
similar to the flyback converter, the SEPIC
transfers current to the load during the power
switch’s off time. Close examination of the current
waveforms in Fig. 7b reveals that the SEPIC’s
switching currents are significantly higher than
those of a classical buck converter. These higher
currents reduce overall efficiency; however, the
SEPIC’s switching currents are lower than those
of a classical buck-boost converter, which makes
the SEPIC slightly more efficient than a classical
buck-boost converter.
E. Solution 5: Classical Buck-Boost Converter
Fig. 8 shows the classical buck-boost converter that uses four switches and a single inductor
for power conversion. Many designers consider
using this solution to generate 3.3 V from a Li-ion
battery. It is a relatively simple design capable of
regulating the 3.3-V output voltage from a Li-ion
battery’s full discharge curve. The main disadvantage is the four switches, which increase
switching losses and reduce overall efficiency.
The two additional switches increase the board
area and cost of discrete power-supply designs.
+
–
VOUT
+
1-D
Fig. 7a. Current flow in SEPIC circuit.
VIN
D
Q1
Voltage
1-D
Q1
Current
0 mA
Diode
Current
0 mA
L1
Current
L2
Current
I L1- A = I L1-B = I OUT •
VOUT
VBAT • 0.8
Fig. 7b. SEPIC converter waveforms.
Q1
Q3
+
–
Q2
Q4
Fig. 8. Classical buck-boost converter.
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Q1
Q3
Q1
+
Q3
+
Q2
–
Q4
–
Q2
Q4
Fig. 9. Classical buck-boost switching and current flow.
F. Solution 6: Modified Buck-Boost Converters
Not all buck-boost converters suffer from
lower efficiency. Fig. 10 shows a modified buckboost converter that overcomes the deficiencies of
a classical buck-boost converter. The separate
buck and boost control circuitry allows the
converter to operate in buck mode when the input
voltage is above the output voltage, and in boost
mode when the input voltage is below the output
voltage. In buck or boost mode, only two switches
operate at any given time, which reduces
switching losses. The RMS currents are identical
to those of a stand-alone buck or boost converter
and are lower than those of the classical buckboost converter, further reducing switching and
Q1
Q3
+
–
Buck
Control
Boost
Control
Q2
Q4
Fig. 10. Buck-boost power stage.
conduction losses. The modified buck-boost
converter is a good solution for producing 3.3 V
from a Li-ion battery because it utilizes 100% of
the battery capacity at a fairly high efficiency.
Comparison of Modified Buck-Boost Converters
Buck-boost converters in portable applications
have been around for a long time. Silicon and
packaging technologies have advanced to the
point where it is feasible to integrate the four
MOSFET switches into a small package with a
suitable control loop. Several integrated buckboost converters are available, each with different
control topologies and operating characteristics.
The buck-boost power stage shown in Fig. 10 can
be operated in three distinct modes, depending on
the input-to-output voltage ratio: buck mode,
buck-boost mode, and boost mode. A specific IC’s
mode of operation is a function of the input-tooutput voltage ratio and the IC’s control topology.
Although different buck-boost solutions have
the same power-stage topology, they have vastly
different control circuitry, of which there are three
main types: the classical buck-boost converter, a
buck-boost control topology that operates only
two MOSFETs per switching cycle, and a buckboost control topology that eliminates the
transition region between buck and boost modes.
The first type, the classical buck-boost
converter, operates all four MOSFETs during each
switching cycle and generates the classical buckboost waveforms. Careful analysis of these
waveforms reveals that the RMS current through
the inductor and MOSFETs is significantly higher
than that of a standard buck or boost converter,
resulting in increased conduction and switching
losses. Operating all four switches simultaneously
also increases gate-drive losses, which can
significantly lower efficiency at lower output
currents.
Workbook 4-5
Topic 4
Fig. 9 shows the two switching states of the
buck-boost converter. One pair of switches is used
to charge the inductor, and a second pair is then
used to transfer the inductor current to the output
capacitor. This switching topology has relatively
high root-mean-square (RMS) switching currents
that increase switching and conduction losses,
resulting in lower efficiencies than those of a
stand-alone buck or boost converter. The worstcase situation occurs when the battery output is
near the 3.3-V operating point. When VIN = VOUT,
the duty cycle is 50%, which results in a peak
inductor current equal to twice the output current.
The inductor current flows through all four
switches during each switching cycle, resulting in
higher switching losses and reduced efficiency.
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Page 4-6
Q3
Q1
+
–
Buck
Control
Q2
Q4
VIN > VOUT : Buck Mode
Q1 and Q2 Switching, Q3 On, Q4 Off
Q1
Q3
+
Boost
Control
–
Topic 4
Q2
Q4
VIN < VOUT : Boost Mode
Q3 and Q4 Switching, Q1 On, Q2 Off
Fig. 11. Buck-boost switch configuration.
The second buck-boost control topology is
newer and reduces losses by operating only two
MOSFETs per switching cycle. This control topology operates in three distinct modes: buck, boost,
and the transition mode between the two (see Fig.
11). When VIN is greater than VOUT, the converter
operates in buck mode by opening Q4 and closing
Q3. Q1 and Q2 switch to form a classical buck
converter. When VIN is less than VOUT , the
converter operates in boost mode by opening Q2
and closing Q1. Q3 and Q4 switch to form a
classical boost converter. In the past, many
designers had a problem when the input voltage
approached the transition region. The transition
region is the operational point where the converter
transitions from buck mode to boost mode in the
case of a falling input voltage, or where the
converter transitions from boost mode to buck
mode in the case of a rising input voltage. The
transition point creates discontinuous boundaries
in the power stage’s operating point between buck
and boost modes. It also creates discontinuities in
the feedback control loop. Unable to cope with
these issues, IC designers opted to insert the
classical buck-boost mode of operation into the
transition point. As already discussed, this
operating mode is less than ideal due to the fourswitch operation and reduced efficiency.
Unfortunately, the transition region falls near the
battery voltage where most of the energy is
available. Therefore, converters using this control
topology operate in the inefficient buck-boost
mode for much of the battery’s discharge time.
The third buck-boost control topology
provides a significant improvement in performance and efficiency by eliminating the transition
region between buck and boost modes. TI’s
TPS63000 buck-boost converter contains an
advanced control topology that eliminates the
traditional buck-boost issues while also eliminating the transition-region issues. In all operating
conditions, the TPS63000 operates only two
switches per switching cycle and stays in either
buck mode or boost mode. This results in reduced
power losses and maintains high efficiency across
the full battery-discharge curve. Fig. 12 shows
that, unlike some solutions, the TPS63000
integrates all compensation circuitry and requires
only three external components for operation,
which minimizes the solution size.
2.2 µH
L1
VIN
+
–
10 µF
L2
VOUT
VIN A
FB
EN
3.3 V
10 µF
PS/SYNC
GND PGND
Fig. 12. Typical TPS63000 buck-boost application
requires only three external components.
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VIN (1 V/div)
VIN (2 V/div)
VOUT (2 V/div)
VOUT (10 mV/div)
Inductor Buck Node
(5 V/div)
Inductor Buck Node
(5 V/div)
Inductor Boost Node
(5 V/div)
Time - 1 µs/div
Time - 20 ms/div
Fig. 13 shows the TPS63000 switching waveforms as the input voltage drops through the
transition region. The waveforms clearly show the
transition from buck mode to boost mode as the
input voltage drops from 4.2 V down to 1.7 V.
With VIN above VOUT, the supply is in buck mode
and Q1 and Q2 are switching. In buck mode, the
left side of the inductor (L1 in Fig. 12) is
switching, and the right side (L2 in Fig. 12) is
fixed at the output voltage. Fig. 14 shows the buck
node, L1, switching between VIN and ground
while the boost node, L2, is connected to VOUT.
The L1 switching waveform is identical to that of
a buck converter.
When VIN drops below VOUT, the converter
transitions to boost mode and Q3 and Q4 are
switching. Fig. 15 shows the converter switching
in boost mode with the right side of the inductor,
L2, switching between VOUT and ground, and the
left side of the inductor, L1, fixed at the input
voltage.
Fig. 16 shows the switching waveforms with
VIN equal to VOUT. During this transition mode,
the TPS63000 switches back and forth between
buck and boost modes. Even though the converter
switches between buck mode and boost mode
every other cycle, only two of the four power
switches operate during each switching cycle. The
switching between buck and boost modes ensures
a well-regulated output voltage during this
transition region.
VIN (1 V/div)
VOUT (10 mV/div)
Inductor Buck Node
(5 V/div)
Time - 1 µs/div
Fig. 15. TPS63000 boost mode with VIN = 3 V
and VOUT ripple shown at 10 mV/div (AC-coupled
probe).
VIN (1 V/div)
VOUT (10 mV/div)
Inductor Buck Node
(5 V/div)
Time - 1 µs/div
Fig. 16. TPS63000 buck and boost modes with
VIN = VOUT and VOUT ripple shown at 10 mV/div
(AC-coupled probe).
Workbook 4-7
Topic 4
Fig. 14. TPS63000 buck mode with VIN = 3.7 V
and VOUT ripple shown at 10 mV/div (AC-coupled
probe).
Fig. 13. TPS63000 transition from buck to
boost mode.
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Page 4-8
4.3
4.1
Cascaded Buck and Boost
TPS62040 and TPS61031
Voltage - V
3.9
3.7
VBAT
3.5
3.3
VOUT
3.1
2.9
175 min
2.7
2.5
4.3
Voltage - V
4.1
LDO – TPS73633
3.9
VBAT
3.7
3.5
3.3
VOUT
3.1
188 min
2.9
2.7
2.5
4.3
4.1
Buck – TPS62040
3.9
Voltage - V
Fig. 17 shows a side-by-side comparison
of the battery-discharge curves and runtimes
for four solutions generating 3.3 V from a
Li-ion battery. These solutions are the
cascaded buck and boost converter, the
LDO, the single buck converter, and the
TPS63000 buck-boost converter. The setup
uses a fully charged 18650 Li-ion battery
with 1650-mAh capacity. The load current is
set at 500 mA, and system shutdown is
defined as the point where the 3.3-V rail
drops 5% below the initial setpoint. Each
setup uses the same battery to eliminate
variations in data due to differing battery
capacities. As anticipated, the cascaded buck
and boost, TPS62040 and TPS61031,
achieve the shortest runtime with only 175
minutes. The LDO, TPS73633, has the nextshortest runtime at 188 minutes. The buck
converter, TPS62040, operates only 2 minutes
longer than the LDO. The TPS63000 buckboost converter achieves the longest runtime
with 203 minutes. Table 1 shows a comparison between several key areas of concern
for these four solutions.
3.7
VBAT
3.5
3.3
VOUT
3.1
2.9
190 min
2.7
2.5
III. OTHER CONSIDERATIONS
4.3
The data in Fig. 17 are taken with a
constant DC load. This is typical of bench
testing but not of real applications. To
maximize runtime in portable applications,
loads are switched on only as long as
required, then switched off. Displays,
processors, and power amplifiers are
examples of loads that produce significant
transients on the system battery. Their load
steps result in voltage drops on the battery
bus because of the battery’s internal source
3.9
4.1
Voltage - V
Topic 4
II. SOLUTION COMPARISONS
Buck-Boost – TPS63000
VBAT
3.7
3.5
3.3
VOUT
3.1
2.9
203 min
2.7
2.5
–5
15
35
55
75
95
115
135
155
175 195
215
Time - min
Fig. 17. Runtimes to generate 3.3 V from a 1650-mAh
Li-ion battery with a 500-mA load.
TABLE 1. COMPARISON OF THE FOUR SOLUTIONS IN FIG . 17.
Topology
Size
Cost
Efficiency/Runtime
Cascaded Buck and Boost
Large
High
Low
LDO
Small
Low
Medium
Buck
Medium
Medium
Medium
Buck-Boost
Medium
Medium
High
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Page 4-9
resistance, protection circuitry, and distribution
bus impedance. When these load steps occur near
the end of the discharge cycle, they can pull the
battery voltage below 3.3 V. With the single-buck
and LDO solutions, this results in early system
shutdown. The buck-boost solution continues to
operate through these transients, thereby
extending the system’s operating time.
Load transients that appear insignificant in lab
testing get much worse under real-world
conditions. A Li-ion battery’s internal resistance
doubles with only 150 charge/discharge cycles.
Internal resistance also doubles when the battery
is operated at 0ºC versus 25ºC. Fig. 18 compares
how the TPS62040 buck converter and the
TPS63000 buck-boost converter perform with a
load transient on the battery. When the partially
discharged battery is lightly loaded, its output
voltage is 3.4 V, which is high enough for the buck
converter to operate normally. When the battery’s
load current increases, the battery voltage drops to
3.3 V, which is too low for the buck converter to
maintain regulation. The buck converter enters
Lightly Loaded Battery
Voltage = 3.4 V
(500 mV/div)
100% duty-cycle mode and its output drops out of
regulation, which could cause a system shutdown.
The TPS63000 buck-boost converter operates
through the transients with no change in output
voltage. The TPS63000’s ability to regulate its
output voltage through system-level transient
loads is critical to the product’s runtime and data
integrity.
IV. CONCLUSION
Many solutions are available to provide a 3.3-V
bus from a Li-ion battery, but there is no single
solution that optimizes all system-level requirements. All the circuits analyzed in this topic have
advantages and disadvantages. The TPS63000
integrated buck-boost converter’s modified
control topology provides the best compromise
between runtime, battery utilization, and solution
size and cost. This topology maintains high
efficiency over the full battery-discharge curve.
Its high switching frequency and fully integrated
power FETs and compensation circuitry minimize
solution size and design time.
Heavily Loaded Battery
Voltage = 3.3 V
Buck Output
Voltage = 3.3 V
(500 mV/div)
TPS63000 Output Voltage = 3.3 V
(500 mV/div)
Battery Current = 600 mA to 1150 mA
(500 mA/div)
Fig. 18. Performance of buck versus buck-boost converter with pulsed
load on Li-ion battery.
Workbook 4-9
Topic 4
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V. REFERENCES
[1] Understanding LDO Dropout, Application
Note, TI Literature No. SLVA207
[2] TPS63000 Datasheet, TI Literature No.
SLVS520
[3] TPS62040 Datasheet, TI Literature No.
SLVS463
[4] TPS61031 Datasheet, TI Literature No.
SLUS534
Topic 4
[5] TPS73633 Datasheet, TI Literature No.
SBVS038
Workbook 4-10