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DESIGN SOLUTIONS
POWER
Choosing the Right Step-Up/Down Voltage Regulator for
Portable Applications
Introduction
VBAT
A popular power source for portable devices is a single
lithium-ion cell with 4.2V at full charge and 2.8V at end of
discharge. However, some functions within portable electronics, such as a SIM card and DSP, require 2.8V and 3.3V. These
are normally provided by low noise LDOs. The LDOs inputs
(VCC) must be at a slightly higher voltage than the highest
LDO output. Hence, VCC ends up right in the middle of the
lithium-ion battery’s range of operation. The use of a step-up/
down voltage regulator, capable of operating from an input that
can be higher or lower than the output, becomes necessary.
Figure 1 shows the battery voltage (VBAT) as the power source
for a typical portable design.
VBAT
2.8-4.2V
STEP
UP/DOWN
VOUT1 = 3.3V
VCC = 3.4V
LDO
VOUT2 = 2.8V
LDO
L
T3
VCC
Bypass
OFF
OFF
ON
Step-Up
SWITCH
SWITCH/
OFF
Fig 2. Bypass-Boost power train and operation table
This architecture can only regulate VBAT voltages lower than the
set VCC = 3.4V. For VBAT > 3.4V the boost converter stops regulating and the pass transistor turns on, directly connecting VBAT
to VCC. Figure 3 shows the battery profile discharging over time
and the LDO input voltage for the bypass-boost architecture.
Voltage
LDOs Input VCC with Bypass-Boost
VBAT
Bypass Mode
One way to solve the problem is to use a bypass-boost
converter, namely a boost converter with an extra “pass”
transistor integrated between the power source, VBAT, and the
LDO input, VCC. Figure 2 shows the bypass-boost power train
architecture and its operation table. Here the bypass transistor
T3 accomplishes a “poor man’s” step-down operation.
T3
T2
Fig 1. LDOs input voltage set by a Step-Up/Down converter
Bypass-Boost
T2
T1
4.2V
In portable applications the voltage regulator efficiency is of
the utmost importance, since higher efficiency translates into
longer untethered operation. In this Power Design Solution we
will review the available options, compare their performance,
and determine the most efficient solution.
T1
VCC (Bypass-Boost)
3.4V
VBAT
Boost Mode
2.8V
Time
Fig 3. LDOs input voltage profile with Bypass-Boost
For the majority of the time (VBAT > 3.4V) the pass transistor
in the bypass-boost architecture literally “passes the buck” to
the LDOs downstream. The LDOs bear the task of regulating
the high VBAT value down to their output set values. Since this
regulation is linear the result is high power dissipation inside
the LDO. This results in greater energy consumption and also
requires a board design and IC selection capable of dissipating
this energy.
1
Buck-Boost
Case Study
In contrast to the bypass-boost architecture, a buck-boost
converter used in this circuit will never stop regulating its
output to 3.4V. In addition, the regulation is entirely switch
mode, which provides high efficiency operation. Figure 4 shows
the buck-boost power train architecture and its operation table.
In this case study we compare the system efficiency (from
VBAT to VOUT) of Maxim’s MAX77801 buck-boost IC to a
competitor’s bypass-boost IC. Each regulator feeds a single
3.3V LDO loaded with 500mA.
VCC
T3
T1
L
T1
T2
T3
T4
StepDown
SWITCH
SWITCH/
ON
OFF
StepUp
ON
OFF
SWITCH/
SWITCH
T4
T2
Fig 4. Buck-Boost power train and operation table
For VBAT > VCC the IC regulates in buck (step-down) mode,
while for VBAT < VCC it seamlessly transitions to boost
(step-up) operation. The entire battery voltage range is covered
in a switch-mode, high efficiency fashion. Figure 5 shows the
battery profile discharging over time and the LDO input voltage
for the buck-boost architecture.
VBAT
2.8-4.2V
Figure 8 shows the result of the comparison. Solid lines indicate
efficiency and dotted lines show battery current consumption
for each solution. As expected, the efficiency of the two architectures is similar when VBAT is below or near the LDO output voltage. Outside this range, and for the entire time VBAT is
above the LDO output voltage, the efficiency of the buck-boost
(above 90%) is far superior to that of the bypass-boost (as low
as 67% with full battery). This superior performance is due to
the ability of the buck-boost IC to supply power to the LDO in
switch mode across the entire range of operation.
Bypass
-Boost
80
0.9
0.8
Efficien
cy
0.7
70
0.6
Bypass-Boost Current Consumption
60
0.5
Buck-Boost Curre
nt Consumption
50
0.4
0.3
Boost Mode
30
VCC = 3.4V
3.3
3.4
3.5
0.2
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
0.1
4.4
VBAT (V)
2.8V
Time
Fig 5. LDOs input voltage profile with Buck-Boost
Figure 6 superimposes the two modes of operation and highlights
the section where the buck-boost has a clear advantage in terms
of power dissipation. The shaded triangle represents the power
lost in linear regulation by the bypass-boost operation.
Voltage
4.2V
LDOs Input VCC with Bypass-Boost
LDOs Input VCC with Buck-Boost
VBAT
Extra Power Loss
Fig 8. Buck-Boost vs Bypass-Boost efficiency comparison
Conclusion
The comparison of the buck-boost architecture to the bypass-boost architecture shows that in principle the buck-boost
operates with superior efficiency. A practical comparison of the
MAX77801 buck-boost solution vs a competitor’s bypass-boost
architecture shows that in operation, there is an advantage of up
to 25% efficiency for the Maxim device. Thus, the buck-boost IC
is an ideal solution for power-stingy portable applications.
Learn more: MAX77801 High-Efficiency Buck-Boost Regulator
VCC (Bypass-Boost)
Design Solutions No. 1
VCC (Buck-Boost)
Need Design Support?
Call 888 MAXIM-IC (888 629-4642)
VBAT
Time
Fig 6. LDOs input voltage profile with Buck-Boost vs Bypass-Boost
2
1
Buck-Boost Efficiency
40
VCC (Buck-Boost)
VBAT
2.8V
BuckBoost Vs BypassBoost
100
Efficiency (%)
Buck Mode
3.4V
3.3V, 500mA
90
LDOs Input VCC with Buck-Boost
VBAT
3.4V
LDO
Fig 7. Efficiency test setup
Voltage
4.2V
VOUT
VCC = 3.4V
STEPUP/DOWN
IBAT (A)
VBAT
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