### AN1254: A Novel Approach to Creating a High-Efficiency, Simple Buck-Boost Converter

```A Novel Approach to Creating a High-Efficiency,
Simple Buck-Boost Converter
®
Application Note
May 8, 2006
AN1254.0
Authors: Tamara Papalias and Mike Wong
been set to 6.25V. A low drop-out voltage regulator is then
used to acquire the desired final voltage of 5V. The efficiency
of this set-up can be calculated from the efficiency of each of
the components by this formula:
A stable voltage from an input supply that is higher and lower
than the output is often required. A common solution is to
use a boost converter followed by an LDO as shown in
Figure 1. The boost converter is configured to accept
voltages ranging from 3V-6V and to produce an output of
6.25V. The 6.25V is then regulated to 5V by the LDO. Both
stages of this solution exhibit some conversion losses. A
simpler, one-stage solution with a boost converter offers
higher efficiency. This simple solution with a PFET transistor
is presented in detail and compared to the common solution.
η system = η boost × η LDO
The efficiency of the boost converter ηboost is typically 90%;
however, the efficiency of the LDO ηLDO is ratio of the output
and input (5V/6.25V), 77%. The overall efficiency is 69%.
That difference is a trade-off among the voltage needed at
the output of the boost converter, the drop-out voltage of the
LDO, and margin for the system.
When using a boost converter, the output has to be set
above the highest input voltage to avoid significant losses.
Therefore, the output of the boost converter in Figure 1 has
L1
D1
VBOOST = 6.25V
VOUT = 5V
VIN = 3V TO 6V
10µF
1 PGND
LX 10
2 SGND
VDD 9
0.1µF
C3
4 EN
SS 7
5 LBI
LBO 6
VOUT
10k
10k
SD
FB 8
100kΩ
68µF
GND
C4
R3
3 RT
VIN
R4
1.4kΩ
10µH
C1
R2
37k
R1
10kΩ
33µF
ERR
LP3961-5
LDO
C10
4.7nF
20nF
EL7515
BOOST REGULATOR
FIGURE 1. TYPICAL 5V REGULATION WITH BOOST CONVERTER AND LDO
L1
VIN (3V TO 6V)
VOUT (5V)
10µH
R5
D1
MBR052D
Q1
BSS84LT1
C1
1kΩ
R4
10µF
1kΩ
U1
1 PGND
2 SGND
3 RT
R6
22kΩ
R3
71.5k
R1
4 EN
5
LBI
X
27.4k
C5
22µF
LX 10
VDD 9
8
FB
7
SS
6
LBO
C3
C4
22µF
0.1µF
R2
10k
C2
4.7µF
VIN_GOOD
EL7515IY
EN
FIGURE 2. 5V REGULATION WITH BOOST CONVERTER AND FET
1
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Application Note 1254
To achieve a sizable increase in the efficiency of this system,
a single-stage solution is needed. A single-stage boost
circuit is given in Figure 2. The Schottky diode in series with
the PFET keeps the output of the regulator at a voltage
greater than the system output voltage—keeping the boost
converter in its high-efficiency operating mode. The voltage
value at the output of the boost converter is set by the
combination of the turn-on voltage of the PFET and the
result of the voltage divider attached to its gate.
R5 + R6
1k + 22k
V IN = ( V OUT – V GS ) • ---------------------- = ( 5 – 1 ) • ----------------------- = 4.18V
R6
22k
A comparison of the efficiency of each system is presented in
Figure 3. With a low voltage input (3.3V) being converted to
5V, the boost converter with PFET consistently provides 8%
higher efficiency over the boost/LDO combo shown in
Figure 1. With the circuits under greater stress (as with
VIN = 5V), the boost/FET circuit remains more efficient, about
+4% at low current and +2% for high current uses (Figure 4).
To examine the efficiency for the spectrum of input voltages,
Figure 5 is provided. The efficiency of the system is well
above 80% for input voltages less than or equal to the output
voltage. When the input voltage is increased, the efficiency
(Figure 6) is included to show the precision of the output
voltage versus current.
80
80
78
79
76
EFFICIENCY (%)
EFFICIENCY (%)
The PFET acts like a linear resistor with 1.0V VGS threshold.
It is fully on when the input is below 4.2V. When the input is
greater than 4.2V, pin 10 of the boost converter needs to rise
above the input voltage. Therefore, the voltage divider of R5
and R6 begins to turn the transistor off to increase channel
resistance. This inserted resistance further isolates the
output of the boost regulator from the output of the system
and adds the voltage drop of the channel resistance,
allowing the boost regulator to remain efficient and stable.
The combination of PFET VGS threshold and the R5 and R6
resistor divider ratio determines the input voltage level that
PFET is turned fully on.
Boost+FET
74
72
70
Boost+LDO
68
Boost+FET
76
75
74
Boost+LDO
72
0
100
200
300
IOUT (mA)
400
500
71
600
FIGURE 3. EFFICIENCY OF BOOST/LDO COMBO AND
BOOST/FET COMBO FOR VIN = 3.3V, VOUT = 5V
100
200
300
IOUT (mA)
400
500
600
0.25
VIN = 3.7V
0.2
85
80
VIN = 4.5V
75
VIN = 5V
70 VIN = 5.5V
65
VIN = 6V
VIN = 3.5V
VIN = 3V
60
VIN = 3.3V
55
0
100
200
300
IOUT (mA)
400
500
FIGURE 5. EFFICIENCY OF BOOST/FET COMBO FOR
VOUT = 5V, VIN VARIED
600
90
50
0
FIGURE 4. EFFICIENCY OF BOOST/LDO COMBO AND
BOOST/FET COMBO FOR VIN = 5V, VOUT = 5V
95
EFFICIENCY (%)
77
73
66
64
78
0.15
0.1
0.05
0
-0.05
-0.1
-0.15
-0.2
-0.25
0
100
200
300
IOUT (mA)
400
500
600
FIGURE 6. LOAD REGULATION OF BOOST/FET COMBO
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verify that the Application Note or Technical Brief is current before proceeding.
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2
AN1254.0
May 8, 2006
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