advertisement Low Profile Synchronous, 2-Phase Boost Converter Produces 200W with 98% Efficiency – Design Note 455 by Victor Khasiev Introduction Automotive audio ampliﬁers require a high power boost converter that is both efﬁcient and compact. High efﬁciency is essential to keep dissipated heat low and avoid bulky and expensive heat sinks. The LT3782A is a 2- phase synchronous PWM controller, making it possible to produce a low proﬁle, high power boost supply that achieves 98% efﬁciency. Output power is 200W continuous and 250W for short pulse loads, corresponding to 8.5A continuous current and 10.5A pulsed current. This circuit comprises three major sections. Two are the phase-interleaved power trains, and the third is the control circuit. Each power train includes an inductor, two switching MOSFETs, a synchronous MOSFET and an output capacitive ﬁlter. The output ﬁlters are connected together in parallel. Schottky diodes D1 and D2 increase efﬁciency during the dead time. A 24V Output Boost Converter at 8.5A (Continuous), 10.5A (Peak) from a Car Battery Figure 1 shows a boost converter that generates 24V from an input voltage range of 8.5V to 18V. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 5V 8.5V to 18V VIN 1 1μF Q7 2 5.6V SGATE1 10μF ×5 SGATE2 SGATE1 2 SGATE1 3 GND 4 GND 10Ω 5 SYNC 6 DELAY 7 DCL 8 SENSE1+ 2.2nF SENSE1– 9 SENSE1– 10 SLOPE 11 RSET – SENSE2 12 SENSE2 – 23.7k 13 SENSE2+ 14 SS 2.2nF 60.4k 56.2k 0.1μF 330μF ×2 BST GND TG IN TS 5 VCC1 27 26 GND 25 GND 24 VEE1 23 BGATE1 22 GBIAS1 GBIAS2 21 BGATE2 20 19 VEE2 18 GND 17 RUN FB 16 VC D1 330μF ×2 1μF 4 LTC4440-5 VOUT 24V AT 8.5A BG1 BG1 Q1 HAT2172H Q2 0.006Ω 0.006Ω SENSE1+ VIN GBIAS Q3 + 10μF ×3 6 28 2.2μF SENSE1– 5V HAT2166H BG1 10μF ×3 Q6 2.2pF 1μF BG2 U2 1 2 SGATE2 3 VCC BST GND TG IN TS 1μF 6 D2 330μF ×2 + 5 4 LTC4440-5 RUN VOUT VIN BG2 Q4 HAT2172H 15 475k LT3782A 825k RUN 15k 220pF 10Ω VCC L2 8.3μH U1 SGATE2 1 22pF 3 L1 8.3μH + SENSE1+ HAT2166H U3 2k 6.8nF 53.6k 402k 1nF SENSE2 BG2 Q5 + 0.006Ω 0.006Ω SENSE2 – SENSE2 + dn4vk F01 Figure 1. Synchronous Boost Converter Based on the LT3782A (VOUT = 24V at 8.5A, VIN = 8.5V to 18V) 11/08/455 The centerpiece of the control circuit is the LT3782A. This 2-phase PWM controller features low side gate signals and corresponding synchronous signals for high side gate control. Control signals are interleaved by running the two stages 180° out-of-phase. The 2-phase approach with accurate channel-to-channel current sharing minimizes electrical and thermal stress on power train components, and reduces EMI. Using the LTC4440-5 as a high side driver allows for high frequency switching. Performance Results This converter aims for high efﬁciency and low proﬁle, and it succeeds in reaching both goals with efﬁciency reaching 98% (Figure 2) and a maximum component height of 10.5mm. Output voltage regulation over the full input voltage and output current ranges is better than 2%. Figure 3 shows the transient response with a 3A step load. Basic Calculations and Component Selection This section shows how to make a preliminary selection of inductors and MOSFETs. Detailed calculations of the losses and converter efﬁciency evaluation can be found in Robert W. Erickson’s Fundamentals of Power Electronics, 2nd edition. For CCM operation, the maximum duty cycle at low line can be found from the following expression: DMAX = VOUT − VIN(MIN) VOUT Average inductor current and peak current can be calculated as follows: IL(AVG) = IOUT ΔI ;I =I + 2 •(1− DMAX )• η L(PEAK) L(AVG) 2 Data Sheet Download www.linear.com Linear Technology Corporation The peak current through the switching MOSFET is equal to IL(PEAK), and the RMS value of the MOSFET current is: 2 1 I ISW(RMS) = IL(AVG) • DMAX • 1+ • 3 IL(AVG) The peak current through the synchronous MOSFET is equal to IL(PEAK), and the RMS MOSFET current value is: ISR(RMS) = IL(AVG) • 1 DMAX 2 1 I • 1+ • 3 IL(AVG) The MOSFETs should be rated to handle the output voltage plus 20% to 30% of headroom. 98.5 18VIN 98.0 15VIN 97.5 EFFICIENCY (%) The control circuitry can be further divided into three parts: the PWM functionality based on the LTC3782A (U1) and two high side drivers around the LTC44405 (U2 and U3). A linear pre regulator based on Q7 generates the required bias voltage for U2 and U3. This approach allows the use of logic-level MOSFETs to minimize gate losses. 12VIN 97.0 10VIN 96.5 96.0 95.5 8.5VIN 95.0 94.5 2 3 4 5 6 IOUT (A) 7 8 9 dn4vk F02 Figure 2. Efﬁciency vs IOUT (from Circuit in Figure 1) IOUT 2A/DIV VOUT 50mV/DIV VIN = 14V, VOUT = 24V IOUT = 2A To 5A dn4vk F03 Figure 3. Transient Response of the Circuit in Figure 1 for a 3A Loadstep Conclusion The LT3782A based 2-phase synchronous boost converter provides high efﬁciency, excellent transient response and excellent line and load regulation over a wide input voltage range. High power, high efﬁciency and a low component proﬁle allow this converter to ﬁt into tight spaces commonly found in automotive environments. For applications help, call (408) 432-1900, Ext. 3161 dn455 LT/TP 1108 246K REV A • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008

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