LINEAR Bootstrap Biasing of High Input Voltage Step-Down Controller Increases Converter Efficiency Design Note 493 Goran Perica Victor Khasiev
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1. DESIGN NOTES Bootstrap Biasing of High Input Voltage Step Down Controller Increases Converter Efficiency Design Note 493 Goran Perica and Victor Khasiev Introduction High voltage buck DC DC controllers suchas the LTC3890 dual output and LTC3891 single output are popular in automotive applications due to their extremely wide 4V to 60V input voltage range eliminating the need for a snub ber and voltage suppression circuitry These controllers are also well suited for 48V telecom applications where no galvanic isolation is required In a typical application for these controllers the IC s supply voltage INTV c is provided by the on chip LDO This LDO produces 5V from input voltages up to 60V to bias control circuitry and provide power FET gate drive Although simple this built in biasing scheme can be inef ficient Power losses can be significant in applications where the input voltage is consistently high such as in 48V telecom applications Reducing the power losses in the bias conversion can increase efficiency and also reduce the controller case operating temperature Employing EXTVcc to Improve Efficiency One of the attractive features of the LTC3890 andLTC3891 controllers is the external power input EXTV c This is a second on chip LDO which can be used to bias the chip When the input voltage is consistently high it is more efficient to produce the biasing volta2. ge by step ping down the converter s output voltage which is fed into EXTV g rather than generating 5V INTV c from the high input voltage Figure 1 shows a block diagram for this scheme The output can be directly connected to the EXTVoc pin of the chip as long as the output voltage is above 4 7V However extra circuitry described in the following section is required for outputs below 4 7V Vin 4 5V to 60V LTC3890 LTC3891 Vout lt 4 7V VOLTAGE DOUBLER OR BOOST FIGURE 2 OR FIGURE 3 Figure 1 Block Diagram Showing External Bias Voltage Doubler for Output Voltages Below 4 7V When the controller s output is below 4 7V it must be stepped up to allow the built in LDO to work A simple voltage doubler solves this problem as long as the output is higher than 2 5V Below 2 5V output a multivibrator based circuit can be used 4J LT LTC LTM Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation All other trademarks are the property of their respective owners 08 11 493 Figure 2 shows a simple low cost solution for output voltages between 2 5V and 4 7V This is a voltage dou bler scheme based on small P channel and N channel MOSFETs Q1 and Q2 The gates of these transistors are controlled by the bottom gate driver BG of the control ler When BG is high Q2 is on Q1 is off and capacitor C1 charges from output voltage Vout through D1 When BG is lo
3. re realized when a voltage doubler circuit is used for a 3 3V at 5A output see Figure 4 Similar efficiency im provements are shown for a 1 8V at 7A converter with a multivibrator based circuit 92 g CHARGE PUMP 90 89 E 88 87 STANDARD SOLUTION 86 85 Vin 50V Vout 3 3V 84 1 15 2 25 3 35 4 45 5 A DN493 F04 Figure 4 LTC3890 LTC3891 Efficiency Improvement For applications help call 408 432 1900 Ext 2134 dn493f LT AP 0811 226K PRINTED IN THE USA LF MYR LINEAR TECHNOLOGY CORPORATION 2011
4. w Q2 is off Q1 is on and capacitor C1 delivers a voltage close to 2 Voyz to EXT Voc D2 D1 E3 MBR0520 MBR0520 E2 Q1 FDN340P Vout Figure 2 Voltage Doubler Allows External Bias from Voyt in the Range of 2 5V to 4 7V Figure 3 shows a solution for voltages below 2 5V The circuit consists of an astable multivibrator based on transistors Q1 and Q2 and a boost based on N channel Q3 and inductor L1 Q1 and Q2 are biased from INTVcc E1 E2 Li LPS5030 334ML 3304H Rc2 Vout D1 MBR0520 E3 FMMT2222ATA Q1 FMMT2222ATA DN493 F03 Figure 3 Boost Controlled By Astable Multivibrator Is Used for Voyr Lower Than 2 5V Data Sheet Download www linear com Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 www linearcom and output voltage Vgyt is stepped up to 5V which feeds EXTVoc The multivibrator frequency is set at 50kHz to minimize the EMI signature The pulse width is defined by the ratio of resistors R1 and R2 as per the following expressions T 1 D Risa 0 7 C1 _ TeD 0 7 C2 _ EXTV Vout EXTVo R2 Conclusion The efficiency of high input voltage DC DC controllers can be significantly improved by using the controller s output voltage to power the IC instead of allowing the internal LDO to produce the bias voltage For input volt ages above 30V efficiency improvements of 2 to 3 a
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