AN1317: Single-Stage 5V USB Voltage Regulator

```Single-Stage 5V USB Voltage Regulator
®
Application Note
May 8, 2007
AN1317.0
By Tamara Papalias and Mike Wong
A stable 5V supply is an integral part of many systems, most
notably USB applications. A common solution is to use a
boost converter with an LDO as shown in Figure 1. The
boost converter is configured to accept voltages ranging
from 3V to 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 the EL7515 offers higher efficiency.
This simple solution with a PFET transistor is presented in
detail and compared to the common solution.
When using a boost converter, the output has to be set
above the highest voltage to avoid significant losses.
Therefore, the output of the EL7515 in Figure 1 has been set
to 6.25V. An LP3961 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 in Equation 1:
η System = η Boost∗ η LDO
(EQ. 1)
If both components have an efficiency of 80%, the overall
efficiency is 64%. The efficiency of the boost circuit is
determined by the quality of the design. The efficiency of the
LDO is a direct function of the voltage difference between its
input and output. 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.
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 Shottky 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
VIN = 3V TO 6V
L1
D1
10µH
C1
10µF
1
R4
1.4kΩ
PGND
LX 10
2 SGND
VDD 9
C4
0.1µF
R3
3
RT
FB 8
C3
100kΩ
4
EN
5 LBI
SS 7
LBO
6
value at the output of the EL7515 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.
The PFET acts like a linear resistor. It is fully on when the
input is below 5V. For these voltages, the EL7515 handily
boosts the system output to 5V with no loss through the
transistor. When the input is greater than 5V, pin 10 of the
boost converter needs to raise 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.
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
drops by about 1% per 100mV.
A load regulation curve (Figure 6) is included to show the
precision of the output voltage versus current. The EL7515
has been optimized for currents up to 500mA, with no point
exceeding 0.22%.
VBOOST = 6.25V
68µF
VOUT = 5V
VIN
VOUT
SD
ERR
10k
10k
GND
33µF
R2
37k
R1
10kΩ
C10
4.7nF
LP3961-5
LDO
20nF
EL7515
BOOST REGULATOR
FIGURE 1. TYPICAL 5V REGULATION WITH BOOST CONVERTER AND LDO
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 1317
L1
10µH
VIN (3V TO 6V)
R5
1kΩ
VOUT
(5V)
Q1
BSS84LT1
D1
MBR0520
C1
10µF
R4
1kΩ
R1
27.4k
C5
22µF
U1
R6
22kΩ
R3
71.5k
LX
PGND
SGND VDD
FB
RT
EN
SS
LBI
LBO
C3
22nF
VIN_GOOD
EL7515IY
C4
0.1µF
R2
10k
C2
4.7nF
EN
FIGURE 2. 5V REGULATION WITH BOOST CONVERTER AND FET
80
80
78
79
78
BOOST + FET
EFFICIENCY (%)
EFFICIENCY (%)
76
74
72
70
68
BOOST + LDO
BOOST + FET
76
75
BOOST + LDO
74
73
66
64
77
72
71
0
100
200
300
400
500
600
0
100
200
IOUT (mA)
FIGURE 3. EFFICIENCY OF BOOST/LDO COMBO AND
BOOST/FET COMBO FOR VIN = 3.3V, VOUT = 5V
VIN = 3.7
EFFICIENCY (%)
85
VIN = 4.5V
75
VIN = 5V
VIN = 5.5V
VIN = 6V
60
55
50
600
0.20
VIN = 3.5V
VIN = 3V
70
65
500
0.25
VIN = 3.3
90
80
400
FIGURE 4. EFFICIENCY OF BOOST/LDO COMBO AND
BOOST/FET COMBO FOR VIN = 5V, VOUT = 5V
95
300
IOUT (mA)
0.15
0.10
0.05
0
-0.05
-0.10
-0.15
-0.20
0
100
200
300
400
500
IOUT (mA)
FIGURE 5. EFFICIENCY OF BOOST/FET COMBO FOR
VOUT = 5V, VIN VARIED
600
-0.25
0
100
200
300
400
500
600
IOUT (mA)
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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AN1317.0
May 8, 2007
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