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 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. 2006. All Rights Reserved All other trademarks mentioned are the property of their respective owners. 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 drops by about 1% per 100mV. A load regulation curve (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 LOAD REGULATION (%) 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 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. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN1254.0 May 8, 2006