Singel State Transistor

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. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2007. All Rights Reserved All other trademarks mentioned are the property of their respective owners.

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

LOAD REGULATION (%)

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

Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding.

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AN1317.0 May 8, 2007